Introduction

Global warming remains the world’s greatest environmental challenge due to its numerous adverse effects, including rising biodiversity loss, intensification of tropical storms, unpredictable weather patterns, droughts, wildfires, glacial melt, and rising sea levels. International efforts to limit temperature increases to below 2 °C above pre-industrial levels1 focus on reducing atmospheric greenhouse gas levels, primarily carbon dioxide and methane, through two main strategies: reducing fossil fuel combustion and sequestering previously released carbon. Among sequestration methods, biological carbon capture in wetlands has gained prominence due to its high efficiency and long-term storage potential compared to forest ecosystems. Wetlands typically store twice as much carbon per unit area as tropical forests due to their low oxygen diffusion rates, which slow organic matter decomposition2. This makes wetlands essential for global carbon management, yet they are often underrepresented in sequestration efforts.

South Korea’s geographic and climatic characteristics, including its ~ 15,000 km of coastline and complex topography, host extensive riverine and coastal ecosystems. These systems are vital for carbon sequestration, particularly within the four major rivers: Han, Yeongsan, Geum, and Nakdong. These rivers exhibit shared geomorphological and hydrological features, including monsoon-driven flow regimes, sediment transport dynamics, and floodplain development3,4. However, over the past 50 years, substantial wetland loss due to land-use changes and agricultural conversion has significantly reduced carbon reservoirs4. Current wetland management plans aim to reverse these losses, yet comprehensive carbon stock assessments and geomorphological surveys remain lacking, limiting effective conservation and restoration strategies.

Recent advancements in understanding soil carbon dynamics in wetlands have highlighted the importance of geomorphological features such as grainsize distribution, hydrological variability, and the influence of extreme climatic events on sediment and organic matter transport5,6,7. These studies emphasize the role of fine-grained sediments in stabilizing organic carbon and the importance of managing sediment dynamics to enhance sequestration potential. Addressing these gaps requires integrating high-resolution topographical data with multiproxy geological analyses to evaluate the impacts of geomorphological and hydrological processes on carbon storage.

This study evaluates carbon stock levels in a riverine wetland environment approximately 50 km from South Korea’s western coastline. It examines the environmental factors influencing carbon sequestration, with a particular focus on the impacts of extreme climatic events such as floods and tropical cyclones. Using geological and 3D topographical surveys produced through multiproxy records and UAV (unmanned aerial vehicle) technology, we assess grainsize distribution, vegetation cover, and landscape changes to estimate biological carbon storage. By integrating these analyses, the study provides critical insights into the mechanisms driving carbon sequestration in riverine wetlands and offers guidance for their effective management and restoration.

Study area

Geographical and geological aspects

South Korea is located on the southern half of the Korean Peninsula in East Asia. It shares its northern border with North Korea, while its eastern and western coastlines are bounded by the East Sea (Sea of Japan) and the West Sea (Yellow Sea: China) respectively. The southern tip of the peninsula is surrounded by the Korea Strait (Fig. 1). The country’s approximate geographic coordinates are 36.5°N latitude and 127.5°E longitude. The terrain of South Korea is characterized by a mix of mountainous regions, valleys, plains, and coastal areas. The eastern part of the country is dominated by the Taebaek Mountain (Fig. 1B-a), while the western side features the smaller Sobaek Mountain (Fig. 1B-b). Flat inland areas in South Korea are mainly associated with the four major rivers; Han, Yeongsan, Geum, Nakdong rivers, which, respectively have area coverage of 34,674 km2, 2798 km2, 9900 km2, and 23,384 km8 (Fig. 1C). The Yeongsan River originates in the Noryeong Mountain (Fig. 1B-c) and flows westward, eventually emptying into the West Sea (Yellow Sea). It passes through or near cities such as Gwangju, Naju, and Mokpo (Fig. 1A). The river has significant cultural, economic, and ecological importance.

Fig. 1
figure 1

Maps of study area. A The terrain of the Korean peninsula showing the location of the cities of Gwangju, Naju, and Mokpo. The eastern, western, and southern coastlines are respectively bounded by the East Sea (Sea of Japan), the West Sea (Yellow Sea), and Korea Strait (South Sea). B Map of mountain ranges marked by Taebaek-a, Sobaek-b, Noryeong mountains-c. C Map showing the rivers Han (a), Geum (b), Yeongsan (c), and Nakdong River (d). The satellite imagery A was obtained from © Google Earth and subsequently modified using Adobe Illustrator software. The maps B, C were modified from Choe et al., 2009 (A, B) and Srivastava et al., 2015 (C).

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Located at the convergent boundary of several tectonic plates South Korea’s geology is complex and diverse9. The eastern part of the country, bordering the East Sea, experiences frequent significant seismic activity due to the subduction of the Pacific Plate beneath the Eurasian Plate, leading to the creation of mountainous terrain (Fig. 1B). The western part of South Korea is characterized by sedimentary plains and coastal areas formed by the deposition of materials eroded from the higher elevations in the east. The southern region features rocky coastlines and numerous islands.

Study site: Yeongsan River and Dam-Yang Stream Wetland (DYSW)

Yeongsan River, one of the major four rivers in S. Korea, runs for approximately 129 km through the North Jeolla and South Jeolla Provinces (Fig. 2) before reaching the West Sea. The river has played a crucial role in shaping the landscape of southwestern S. Korea and it has been an important source of water for irrigation, transportation, and industrial activities near the city of Gwangju, located in South Jeolla Province. Over the years, the river has been managed for various purposes, including flood control, water supply, and environmental conservation.

Fig. 2
figure 2

Maps of study area. A Study area (DYSW) is marked in a yellow star. B Blue lines indicate Yeongsan River and a yellow circle marks the city of Gwangju. C The DYSW consists of three sections (Upper, Middle, Lower), surrounded by agricultural fields, small tributaries (blue lines), and man-made structures (drainage gate, bamboo forest, and embankment). The satellite imageries (A–C) were obtained from © Google Earth and subsequently modified using Adobe Illustrator software.

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Dam-Yang stream wetland (DYSW) is located on the mainstream of the Yeongsan River in South Jeolla Province (Fig. 2). The DYSW is approximately 1 km2 in area10 and is divided into three parts (Upper, Middle, and Lower), having a stream bed with an overall slope of approximately 4.16 degrees. The diverse ecosystem in the DYSW is supported by freshwater input from the Yeongsan River and sedimentary and nutrient input from the surrounding mountains and agricultural fields. Freshwater input is controlled by man-made embankments and levees.

Extrapolating insights across South Korea’s major river systems

The Han, Yeongsan, Geum, and Nakdong Rivers collectively hold significant cultural, economic, and ecological importance for South Korea. The Han River serves as a critical water source for South Korea’s capital, the Nakdong River supports agriculture, industry, and biodiversity as the nation’s longest river, and the Geum River contributes to agricultural productivity and ecological diversity. These rivers share key geomorphological and hydrological features, such as monsoon-driven flow regimes, sediment transport dynamics, and floodplain development11. These shared characteristics, coupled with similar responses to climatic events and human interventions, make the findings applicable across all four river systems, allowing insights from one river system, such as the Yeongsan River, to be reasonably extrapolated to the others.

Understanding the impacts of climate events, such as tropical cyclones and floods, alongside geomorphological changes on carbon storage, is essential for mitigating land loss and enhancing carbon sequestration. Wetland surveys identifying the effects of these events and analyzing geological (grainsize) and geomorphological changes are vital for maintaining sediment transport regimes for both organic and inorganic materials. While the limitations of extrapolating findings from a single river are acknowledged, underscoring the need for further validation across multiple river systems to strengthen generalizability, this knowledge plays a critical role in preventing land loss and enhancing carbon sequestration across these interconnected river systems.

Weather and climate

Climate types of the study area vary from “humid continental” (Dwa) to “humid subtropical” (Cwa) on the Koppen scale11. The summers are hot and humid, while the winters are cold and dry. Annual precipitation is between ~ 1000 and ~ 2000 mm12. The climate is controlled by the East Asian Monsoon, which drives dramatic changes in precipitation and temperature across Asia. The summer monsoon, known as “Changma” in Korea, occurs between late June and July, and brings warm moist air from the Pacific Ocean12, resulting in a large percentage of the country’s annual rainfall. Additionally, the period from July to August is marked by the occurrence of typhoons, which further contribute to an increase in precipitation during these months. Although annually the average monthly precipitation is ~ 100 mm, monthly averages for June to August are between 150 and 500 mm, respectively13. The daily maximum precipitation during the monsoon season can exceed 300 mm, often resulting in damaging floods, including levee breaches, crop damage, and the destruction of infrastructure, especially housing collapses14. The dry season occurs between October and May with average daily lows < 4 °C, while monthly average precipitation drops to 10–40 mm13.

From June to August 2020, one of the longest and most intense precipitation events in Korean history occurred, resulting in destructive fluvial flooding, thousands of flood victims, and approximately 7 billion U.S. dollars in property damages, including severe agricultural losses15 (Fig. 3). During this event, the daily precipitation exceeded 300 mm, accounting for more than one-fifth of the annual precipitation16. The increased stream power drove channel erosions and stream bank collapses in the DYSW. Older meander belts were abandoned due to channel migration and new waterways were created. Sediments in side channels and inner bends (point bars) were redistributed by lateral bank migration. Large upstream particles (cobble, pebble, sand) were transported downstream and deposited over floodplains as poorly sorted sediments.

Fig. 3
figure 3

Aerial photographs of DYSW. The aerial photographs captured overbank flooding from the Upper, Middle, Lower sections. Massive amounts of sediment and freshwater overflowed the surrounding areas after breaking levees. Entire floodplains were inundated. The photographs were taken during our fieldwork.

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Additional details in supplementary material

Much of the detailed information regarding key topics such as anthropogenic activities and controlled floodplains, vegetation and its role in organic carbon storage, stream dynamics (hydrology and sediment) and their relationship with organic carbon, the role of wetlands in long-term carbon sequestration, and South Korea’s climate initiatives and challenges in wetland carbon management can be found in the supplementary material.

Materials and methods

Drone imagery and aerial surveys primarily provide information on surface characteristics and horizontal spatial distribution as they excel at mapping land cover, vegetation, and surface water bodies, crucial for understanding horizontal carbon storage and large-scale geomorphological features. Our remote sensed imagery adequately captured wetland characteristics by observing large-scale and surface-level environmental features. However, the vertical profile of sediments and soils, including the depth, composition, and stratification of carbon storage, requires direct geological methods. Therefore, we employed coring and in-situ soil sampling to investigate the depth and composition of floodplain sediments. These methods allow for precise measurements of vertical carbon storage, including organic matter content, sediment layers, and soil horizons, which remote sensing cannot capture. The combination of both approaches provided a comprehensive understanding of carbon dynamics (vertical and horizontal carbon storages) in wetland environments, allowing for more effective management and conservation strategies17.

Sediment samples (cores and surface sediments) were collected between May and October 2022 (Table 1). Coring was performed manually using a hand corer (100 cm length, 8 cm diameter), pushed until reaching an impenetrable layer (rock fragment bottom). Surface samples (gravel and pebbles) were collected using a small shovel. Sediment compaction was measured. Collected cores were transported to the Korea Institute of Civil Engineering and Building Technology, Environmental Lab and stored in a refrigerated room (4 °C).

Table 1 Sampling locations.
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Stream morphology and high-resolution mapping

High-resolution (3 cm resolution) images of the wetlands were acquired using a Phantom 4 RTK aircraft, an unmanned aerial vehicle, with a high-precision GNSS receiver, D-RTK 2 Mobile Station, in May and September 2022. A series of high-resolution aerial photos were taken under double-grid missions generating various angles of objects to ensure a comprehensive set of data. The aircraft was deployed for ~ 45 min at each site (Upper, Middle, Lower), taking over 1,000 photos following the operational procedures of Cohen et al.18; i.e. 15% of overlap, 85% of frontal and 75% of lateral superposition for camera angle, and flying at 100 m altitude at maximum velocity. The high-precision GNSS receiver supports all major global satellite navigation systems, allowing direct georeferencing19.

The multiple overlapping photographs were processed using Agisoft PhotoScan to generate photogrammetric digital images of orthorectified orthomosaics, identifying overlapping images and triangulating their positions to create a 3D point cloud, mesh, and model, and then imported to Global Mapper 18 as a GeoTIFF format for spatial analysis18. The Global Mapper processed and analyzed geospatial data by orthorectified orthomosaics georeferenced images, thereby generating precise Digital Elevation Models (DEMs) and Digital Surface Models (DSMs). A supervised classification and 3D elevation profile were visualized. Reconstructed 3D wetland topographies and 3D elevation profiles in a dense point cloud model distinguish wetland geomorphological features, including streams, banks, point bars, and levees.

Sedimentary analyses

Initial sedimentary analyses were conducted by conventional methods, including the description of sedimentary texture by visual inspections20. Subsamples from the cores (DYM-Middle1, -Middle2, -Middle3) were extracted at 2-cm intervals to measure percentages of organics using a Thermolyne Benchtop Muffle Furnace at Korea Agriculture Technology Promotion Agency to produce loss-on-ignition (LOI) data. An Octagon sieve shaker was used to sort materials greater than 2,000 μm, while materials smaller than 2,000 μm were analyzed using a Beckman LS 13 320 particle size analyzer at the Korea Polymer Testing & Research Institute, following the procedure of Howard, et al.21. Particle types and textural groups were reported for each sample using GRADISTAT22, which calculates the mean grainsize, sorting, skewness, and kurtosis (Tables S1–3).

Organic carbon assessment

Since the average organic matter present in this study is above 29%, we applied the Loss on Ignition (LOI) technique to estimate percentages of soil organic carbon (Table S4). The LOI was manually performed20 using ~ 5 g of oven-dried material for each sample. Samples were burned at 550 °C for 4 h and reweighted to calculate the percentage of organic content using Eq. (1). Given that organic matter is composed of approximately 58% carbon (as indicated by the Van Bemmelen factor)23, the percentage of organic matter is multiplied by 0.58 to estimate soil organic carbon, as shown in Eq. (2). The total soil carbon stock within the DYSW was calculated following the methodology of Howard, et al.21, by multiplying the soil organic carbon mass (g) (SOC) within the 50.27 cm3 core cylinder, by area (ha) (Eqs. 3, 4). The SOC values from the short cores (DYM-Middle2 and -Middle3) were normalized by multiplying the averaged organic content in the core by a depth of 20 cm.

$$\:\text{Loss on Ignition}\:\left({\%}\right)\:=\frac{\text{Initial sample (g)}-\:\text{Ignition sample (g)}}{\text{Initial sample (g)}}\:\times\:100$$
(1)
$$\:\text{Organic carbon }(\text{g}/\text{k}\text{g})=\text{\% organic matter }(\text{since }1\%\:=\:10\text{ g/kg})\times\:0.58\:$$
(2)
$$\:\text{C}\:\text{stock density}\:(\text{g}\:\text{C}\:/\:\text{c}\text{m}^2)\:=\text{SOC (g C)}\times\: \text{ BD }\left(\frac{\text{g}}{{\text{cm}}^{3}}\right)\times\:\text{SD}\left(\text{cm}\right)$$
(3)
$$\:\text{C}\:\text{stock}\:(\text{Mg}\:\text{C}\:/\:\text{ha})\:=\left(\text{C}\:\text{stoc g C cm}^{-2}+\text{all sections}\right)\times\:100$$
(4)
$$\:\text{Cylinder Core Bulk Density (BD)}\:=\frac{\text{Mass of sample (g)}}{\text{Volume of the cylinder (\text{cm}}^{3})}$$
(5)
$$\begin{aligned}\text{Conversion factor}& =\hspace{0.17em}1\:\text{ha}\:\left(\text{hectare}\right)\approx\:{10{,}000}{\text{ m}}^{2}\approx\:\:\frac{1\:\text{cm}^{2}}{{100{,}000{,}000}}\approx\:\frac{1\:\text{km}^{2}}{100}\\ & 1\:\text{Mg}\:\left(\text{megagram}\right)\approx\:{1{,}000{,}000}\text{ g}\:\approx\:\frac{\text{g}}{1{,}000{,}000}\end{aligned}$$
(6)

SOC stands for Soil Organic Carbon and represents the amount of carbon (in grams) present in the soil sample. BD stands for Bulk Density and represents the mass of the soil per unit volume (in grams per cubic centimeter) in Eq. (5). Soils with high organic matter content typically have bulk densities ranging from 0.1 to 0.5 g/cm³. Soils with lower organic content typically have higher bulk densities, ranging from 1.2 to 1.6 g/cm³. SD stands for Sampling Depth and represents the depth (in centimeters). This method allows for estimating the carbon stock within a specific soil sample by considering both the carbon content and the physical characteristics of the soil.

Statistical test and grainsize distribution

We computed a regression plot to assess the relationships between mean grainsize, organic percentage, and carbon stock. To better visualize these relationships, we used logarithmic transformation-based regression plots, which effectively display the strength and direction of correlations between variables. The regression plots also include fitted trend lines to illustrate the nature of the relationships and highlight key patterns. Statistical significance was evaluated using p-values, ensuring that only significant correlations (p < 0.05) were emphasized in the analysis. This approach provides a clear and concise view of the relationships, focusing on meaningful trends and significant findings.

When multiple distinct grainsize populations coexist, the median value does not account for the complexity or relative proportions of different grainsizes, relying solely on the median value can be misleading24. While the median value is commonly used, it does not fully account for the complexity or relative proportions of multiple distinct grainsize populations, making it potentially misleading in certain cases. The mean value was therefore utilized as a complementary statistical indicator to provide a more comprehensive understanding of the overall grainsize distribution25. In addition to the mean and median, additional parameters such as mode and skewness were calculated to capture the variability in grainsize distributions (Tables S1–3). A detailed analysis was conducted to examine the relationship between grainsize distribution and organic carbon storage, using these parameters to interpret patterns and identify key contributors to soil carbon dynamics.

Results

Drone sensing and stream morphology

Geospatial models derived from photogrammetry (including 3D point clouds, DEMs, and DSMs) were used to identify stream morphological features across three sites: Upper Sect. (0.807 km2), Middle Sect. (0.878 km2), and Lower Sect. (0.807 km2) (Fig. 4).

Fig. 4
figure 4

Maps of study site and sampling locations. A Digital elevation model displays wetland topography. B Red lines (a to a’, b to b’, c to c’) mark the locations of the sampling transects across the stream. C Vertical profiles of stream wetland along the sampling transects display a lower ground elevation downstream. Red-vertical bars indicate sampling locations. The satellite imagery A was obtained from © Google Earth and subsequently modified using Adobe Illustrator software. The 3D point cloud imageries in (B) and (C) were produced using Global Mapper, version 18 (Blue Marble Geographics).

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The maps in Fig. 4A, color-coded by elevation, show higher areas in red and lower areas in blue. The maps in Fig. 4B highlight specific geomorphological features, such as different vegetation types, water bodies, and landforms, within these sections. The point cloud data visualize a floodplain surrounded by agricultural fields, with red vertical lines indicating transects that were analyzed in the cross-sectional profiles shown in Fig. 4C. The cross-sectional profiles (Fig. 4B: a-a’, b-b’, and c-c’) distinguish elevation changes along each transect bounded by levees. The levees are height in elevation between 40–50 m. Profile a-a’ shows the greatest elevation variation, indicating rugged terrain with prominent geomorphological features. Profile b-b’ depicts moderate variation, signifying a transitional zone with a vegetation canopy, while profile c-c’ reveals a gradual eastward increase in topography and a flatter area westward, where the main river channel crosses. Red bars along the profiles indicate specific measurement stations. The geospatial models mark significant spatial variation within the landscape, driven by changes in elevation, ground composition, and stream levels. The average transect elevation decreases from 29 m above mean sea level (amsl) in the Upper section to 25 m amsl in the Lower section (Table 2). The river gradient exceeds 4° in both the Upper and Lower sections and is 3.8° in the Middle section. Vegetation in the Upper and Lower sections is characterized by a high canopy (> 15 m), while the Middle section has a lower canopy (< 10 m). Containment levees (> 35 m amsl) are covered with pavement and other hard surfaces.

Table 2 Terrain analysis metadata from the DYSW.
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The elevation in the inner floodplain, adjacent to the channel, varies. The outer bank of bends (cutbank) is usually higher (~ 2 m) than the inner bank (point bar), which experiences lateral, rather than vertical sediment accumulation. Relict stream courses, including abandoned channels, ponds, oxbow lakes, and incised channels, are relatively lower (< 2 m) than the inner banks. The sediment bars in the middle of the channel are higher (> 1 m) than the stream bed.

The combination of aerial imagery, close-up views, and cross-sectional profiles offers a comprehensive understanding of the landscape, useful for applications such as flood risk assessment, riverine morphological dynamics, and ecological variation. The clear elevation differences in the point cloud profiles demonstrate that each section (Upper, Middle, Lower) has distinct physical characteristics, influencing land use and stream dynamics.

Sedimentary analyses

Sedimentary analysis identified a variety of types and sizes of sediments across the DYSW (Fig. 5). The riverbed material in the Upper section consists of poorly sorted granules, pebbles, cobbles, and boulders with sizes ranging from 60 to 400 mm (Fig. 5A-a). Sediments in the Middle section consist of sandy mud to organic-rich clays in the inner and outer banks, while pebble and sand mixtures occur in a small pond (Fig. 5B-e). The Lower site consists predominately of poorly sorted granules and pebbles with grainsizes ranging from 2 to 60 mm (Fig. 5C). The hard rock surfaces in the Upper and Lower did not allow for coring.

Fig. 5
figure 5

Types of sediments. Photographs display sedimentary types and depositional environments at each site. (A Upper) Sedimentary rocks cover various stream landscape. Pebble and cobble are dominant in the Upper section. (B Middle) The Middle section consists of sand to clay with plant organics. (C Lower) Pebble and cobble are dominant in the Lower section.

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The LOI and grainsize data from the Middle site (Middle1, Middle2, Middle3) display high percentages of organics (up to 50%), sands (15–88%), and muds (11–85%) with trimodal, bimodal, and polymodal distributions (Tables S1-3). Poorly sorted fine sands to coarse silts (32–128 μm) are dominant in Middle1 with 30–42% organics, 14–42% sands, and 57–85% muds. Middle2 consists of poorly sorted medium sands and coarse silts with mean grainsize ranging from 46 to 118 μm, 24–46% sands and 53–76% muds with 25–32% organics. Middle3 includes 15–89% sands and 11–84% muds with 9–50% organics. Poorly sorted fine sands to medium silts are present throughout the core with mean grainsize varying from 37 to 839 μm.

In general, the stream bed is covered by hard rocks, while high percentages of organic content are found in water-logged, low-velocity locations, such as abandoned channels and oxbow lakes that only experience flow during wet season. The organic content of point bars and sediment bars is low. The pebbles, cobbles, and boulders found in the Upper and Lower sites are heterogeneous.

Organic carbon

Geomorphological factors (elevation and characteristics) heavily influence both the percentages of organics and C stock levels (Table 3). Middle1 (331.5 g C/ha) has the highest C stock level, followed by Middle3 (178.1 g C/ha), while Middle2 has the lowest C stock level (167.2 g C/ha). Elevation is inversely proportional to the level of C stocks as inner banks and abandoned channel (oxbow lakes and ponds; areas < 25 m in elevation) contains high C stocks, while sediment bars and outer banks (> 26 m in elevation) have low C stocks.

Table 3 Carbon stocks in middle (AVE: averaged).
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The variation in carbon stock levels across the study area underscores the critical influence of geomorphological positioning on carbon sequestration. The highest carbon stocks are concentrated in areas with lower elevations, which are prone to organic matter accumulation due to slower water flow and frequent sediment deposition. In contrast, higher elevations, like sediment bars and outer banks, experience more dynamic hydrological conditions that limit organic matter retention, resulting in lower carbon stocks. Areas with dense vegetation in lower elevation exhibit higher carbon sequestration due to the continuous input of organic matter from plant litter and root biomass. In contrast, sparsely vegetated areas, often associated with higher elevations and coarser sediments, show reduced carbon storage capabilities.

Statistical relationships

The relationships between organic percentages and carbon stock levels (g C/cm2) show a perfect positive correlation (r2 = 1.000, p = 0.000, Fig. 6A). This indicates a significant relationship, reflecting the inherent proportionality between these two variables due to the method of estimating organic carbon from organic matter. To address concerns about dependency, we reexamined the calculations and confirmed that organic matter and organic carbon percentages are appropriately treated within the analysis as related but distinct measures, ensuring valid interpretations mean grainsize shows a weak negative correlation with both carbon stock levels (r2 = 0.150, p = 0.018, Fig. 6B) and organic percentages (r2 = 0.150, p = 0.018, Fig. 6C). These correlations are statistically significant at the 5% level, suggesting that finer grainsizes are associated with higher organic carbon and organic matter contents.

Fig. 6
figure 6

Relationships between organics (%), carbon stocks, and grain sizes. A The regression graph of percentages of organics and carbon stocks (g C/cm2) shows a perfect relationship. B Grainsize negatively correlates with carbon stock (g C/cm2) and C organic content (%); as grainsize increases, carbon stock and organic percentage decrease.

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Discussion

The level of soil carbon storage in riverine wetlands is controlled by a number of factors both environmental (e.g., geomorphology, hydrology, and grainsize) and anthropogenic (e.g., levees and embankments), all of which can affect wetland soil composition26. The temporal and spatial variability of organic carbon storage results from heterogeneous hydrological and geomorphological conditions20. Geomorphology, the shape, and structure of the land, plays a significant role in determining the capacity of carbon storage. Grainsize of the surface materials influences the level of soil moisture and the decomposition processes related to carbon storage. For flood control, development, or other purposes, anthropogenic factors alter natural water flow, potentially influencing vegetation growth and sediment deposition27.

In subsequent sections, we explore these dynamics in greater detail. First, we discuss the relationship between organic carbon stock and spatially variable grainsize parameters, emphasizing the importance of fine-grained sediments in stabilizing organic matter and enhancing carbon storage. Next, we examine the effects of the East Asian Monsoon on sediment organic carbon, highlighting how seasonal hydrological patterns and monsoon-driven sediment transport shape organic carbon distribution. The impacts of extreme weather events, such as the 2020 flood in the Dam-Yang Stream Wetland (DYSW), are analyzed to demonstrate how such events reshape landscapes and influence carbon storage. We then evaluate the potential for carbon sequestration in major river systems, illustrating the significant contributions of low-energy wetland environments to regional carbon budgets. The role of anthropogenic activities in carbon control is addressed, with a focus on strategies such as controlled overbank flooding and hydrological interventions to enhance carbon sequestration in riverine wetlands. Implications for carbon sequestration and management strategies are discussed, integrating ecological and geomorphological insights to propose effective conservation approaches. Finally, we acknowledge the limitations of the study and highlight the need for future research to validate and extend the findings. Understanding the complex interplay of these environmental and anthropogenic factors is essential for effective wetland management, conservation efforts, and maximizing carbon storage potential28.

Organic carbon stock under spatially variable grainsize parameters

The relationships between organic carbon stocks and grainsize highlights the importance of fine-grained particles, such as clay and silt, due to their larger surface area, which enhances the adherence of organic carbon and stabilizes organic matter in soils5. This is evident in natural freshwater systems like meandering rivers and lakes in the DYSW, where organic material in sediments is inversely correlated with grainsize.

Fig. 7
figure 7

Point cloud images: A The digital elevation model displays wetland topography and streamlines marked in blue. B The point cloud image in the Upper section shows the concrete embankment and flow direction (blue arrow). C The point cloud image in the Middle section indicates different elevations for various geomorphological features. D The anthropogenically driven suspended sediment inflow from levee construction is marked by the orange arrow. E The point cloud image in the Lower section shows varying stream geomorphology, with gravel deposits aligned with the direction of stream flow. The satellite imagery A was obtained from © Google Earth and subsequently modified using Adobe Illustrator software. The 3D point cloud imageries B-E were produced using Global Mapper, version 18 (Blue Marble Geographics).

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Low-energy environments, such as abandoned channels, accumulate finer sediments like silt and clay, while high-energy areas, such as sand bars, deposit coarser materials like sand and gravel (Fig. 7C, E). This process is closely tied to stream velocity, where larger particles require more energy to remain suspended and settle quickly as velocity decreases, while finer particles remain suspended longer and settle in low-energy environments. Consequently, low-energy areas, such as marshes, swamps, and abandoned channels, retain higher organic carbon levels due to their ability to trap fine sediments, whereas high-energy environments dominated by coarse sediments retain lower carbon levels.

Fast-flowing streams with coarse-grained beds are less effective at retaining carbon, while geomorphologically diverse, low-velocity wetlands nurture higher soil organic carbon due to the deposition of fine sediments. These dynamics illustrate the critical role of stream energy and sediment size in influencing carbon storage in freshwater systems8,26. Such conditions also facilitate the burial of organic material, promoting long-term carbon sequestration. To enhance riverine carbon stocks, floodwaters should be directed into low-energy areas, such as abandoned channels, small isolated wetlands (Fig. 7C), topographical depressions, sunken areas, or seasonally connected channels, where fine sediments can efficiently capture and retain organic material.

The effects of east Asian monsoon on sediment organic carbon

The East Asian Monsoon induces significant seasonal precipitation and river discharge variability, affecting sediment and water dynamics. During summer, torrential monsoonal rainfall, often intensified by tropical cyclones, substantially increases stream volume, velocity, and transport capacity. These conditions result in upstream erosion and downstream sediment deposition, altering river meanders, shifting main channels, and forming new riverine wetland habitats, such as ponds, point bars, and back marshes6. The geomorphological changes in the DYSW enhance landscape heterogeneity, which supports greater biodiversity and increases carbon storage capacity. Monsoon-driven precipitation events establish hydraulic forces as the primary drivers of sediment distribution and geomorphological change, thereby playing a critical role in shaping sedimentary riverine carbon storage7.

Seasonal hydrological patterns driven by the monsoon also play a crucial role in sediment transport and organic carbon distribution. During periods of high precipitation, increased streamflow facilitates the mobilization and deposition of organic-rich fine sediments in low-energy environments such as floodplains and back marshes, enhancing soil carbon storage. Conversely, during drier seasons, reduced discharge slows sediment transport, allowing accumulated organic material to stabilize in these depositional zones. This seasonal variability creates a dynamic equilibrium that influences both the spatial distribution and long-term sequestration of organic carbon in riverine systems.

Effects of the 2020 flood events and organic carbon

Typhoon landfalls, often associated with coastal flooding, have dual impacts on the environment. While they can negatively affect coastal land and habitats by causing erosion, salt intrusion, and habitat degradation, they also play a vital role in sediment distribution across coastal plains. The overtopping of coarse materials during these events promotes sediment deposition, particularly fine-grained particles beneath the coarse layers, enriching coastal soils and enhancing soil carbon storage. The creation of new microhabitats through sediment redistribution also increases biodiversity and strengthens ecosystem resilience27, demonstrating the complex interplay between destructive and constructive processes in coastal systems. Hence, capturing organic material transported by extreme events such as typhoon landfalls and floods is critical for enhancing riverine carbon stocks.

A particularly large flood event occurred in August 2020 (Fig. 3). Record-breaking rainfall of over 650 mm in a week, with daily maximum precipitation exceeding 250 mm, caused massive freshwater discharge, triggering significant watershed erosion and deposition of large-grained clastic materials (> 63 mm) downstream (Figs. 3 and 5). This reshaped the wetland landscape, leaving a distinctive sedimentary structure consisting of a large-grain substrate (pebbles, cobbles, and boulders) overlaid by fine-grained sediments deposited as floodwater velocity decreased. This sedimentary layering provides clear evidence of the dynamic nature of fluvial systems during extreme flood events, illustrating their environmental impact in the DYSW region.

Potential for carbon sequestration in major river systems

The sequestration of 3.31 megagrams of carbon per hectare at elevations below 25 m above sea level, as demonstrated by point cloud images in Fig. 8, highlights the precision of carbon capture rate analysis. This precision is further enhanced by digital elevation models, providing unparalleled detail in examining wetland geomorphology. Point cloud imagery reveals the diverse stream morphologies, emphasizing areas at the crucial elevation of 25 m above sea level. This elevation serves as baseline data for current organic carbon storage, underscoring the importance of accurate terrain representation in environmental research.

Fig. 8
figure 8

Point cloud images correlated with the carbon stock levels at each site. A Point cloud images display areas of 0.807 km2 in the Upper site, 0.878 km2 in the Middle site, and 0.807 km2 in the Lower site. B Point cloud images reveal the levels of carbon stock in megagrams for areas below 25 m above sea level. C Metadata elevation histograms display elevation distributions each site. The 3D point cloud imageries in A were produced using Global Mapper, version 18 (Blue Marble Geographics).

Full size image

These detailed insights are essential for understanding the carbon sequestration potential across South Korea’s major river systems. Current analysis shows carbon capture rates of 1.3 megagrams at the Upper site, 47.9 megagrams at the Middle site, and 46.7 megagrams at the Lower site. With effective management of hydrological systems to achieve 3.31 megagrams per hectare, carbon capture could increase significantly: 267.1 megagrams at the Upper site, 291.3 megagrams at the Middle site, and 267.1 megagrams at the Lower site.

When extrapolated to the entirety of the four major rivers (Han, Yeongsan, Geum, Nakdong), covering 7,075,600 hectares (70,756 km2), the potential carbon storage is approximately 23.42 million megagrams (metric tons). Based on the equation below, this represents 3.9% of South Korea’s annual carbon emissions of 600 million megagrams (Our World in Data CO2and Greenhouse Gas Emissions database)29,30.

$$\text{Percentage of carbon budget}=\left(\frac{\text{Potential carbon storage}}{\text{Total annual carbon emissions}}\right)\times\:100$$

The significance of these river ecosystems lies not only in their contributions to wetland productivity but also in their vital role in the regional carbon cycle, offering substantial potential to mitigate carbon emissions.

Anthropogenic carbon control

Human infrastructure (levees, drainage gates, and embankments) has significantly altered the dynamics of stream hydrology and landscape patterns (Fig. 7A–D), with large effect on the downstream transport of organic materials. However, water management strategies have not been aimed at the retention of organic carbon, much of which will be remobilized farther downstream or delivered to the sea. To effectively address this challenge, the annual floods and natural flow of organic materials should be deliberately managed in an effort to capture and retain fine -grained sediments and organic materials. Due to the permanency of the East Asian monsoon any strategy for increasing riverine carbon stocks should be based on leveraging this climatic reality.

Considering the existent network of dams and hydrological infrastructure (Fig. 7B), the deployment of controlled overbank flooding during seasonal flooding events is feasible. Management strategies should be aimed at depositing organic-rich fine-grained material waters into low-lying, low-energy locations such as sunken grounds, abandoned channels, and wetlands situated at the edges of rivers (Fig. 7A, C). Blind channels, baffles, sills, and deflection ridges can be developed to reduce water velocity in the desired areas. These low-velocity areas become repositories for fine materials, which can then capture, retain, and sequester organic material. The timing and intensity of controlled releases can be fine-tuned to meet specific ecological objectives while minimizing the risks associated with unregulated flood events (Krauss et al., 2021). The integration of dam operations with managed flood releases presents a modern pathway to capitalize on the proven ecological benefits of overbank flooding in a rapidly evolving landscape.

Implications for carbon sequestration and management strategies

This study emphasizes the complex interplay between geomorphology, hydrology, and anthropogenic factors in influencing organic carbon storage across riverine wetlands in South Korea. The findings highlight the importance of fine-grained sediments, which provide higher organic carbon retention due to their increased surface area and capacity to stabilize organic matter. Low-velocity environments, such as abandoned channels and wetlands, are particularly effective at sequestering carbon, contrasting sharply with high-velocity zones where coarse-grained sediments dominate and carbon retention is minimal. This underscores the critical role of stream energy and grainsize distribution in shaping carbon sequestration potential.

Seasonal variability driven by the East Asian Monsoon adds another layer of complexity. Monsoon-driven floods, coupled with tropical cyclones, significantly influence sediment transport and deposition, creating dynamic wetland environments that enhance carbon storage. For instance, the 2020 flood event reshaped the landscape of the DYSW and deposited fine-grained, nutrient-rich sediments that increased organic carbon content in low-energy areas. These seasonal processes highlight the importance of leveraging natural hydrological cycles for effective carbon management strategies.

On a broader scale, the study demonstrates the immense carbon sequestration potential of South Korea’s four major rivers. With a capture rate of 3.31 megagrams of carbon per hectare, these rivers collectively could store up to 23.42 million megagrams of carbon annually, representing approximately 3.9% of the nation’s carbon budget. This finding underscores the strategic importance of these ecosystems not only in mitigating carbon emissions but also in contributing to regional and global carbon cycles.

However, the study also reveals the significant impact of human infrastructure, such as levees and embankments, on sediment dynamics and organic carbon retention. These structures disrupt natural flow patterns and sediment deposition processes, often resulting in reduced carbon storage downstream. To address this, the study advocates for adaptive water management strategies, including controlled overbank flooding. Such strategies, inspired by practices on the Mississippi River, could deposit organic-rich, fine-grained sediments in low-energy environments, enhancing carbon sequestration while supporting ecological and hydrological functions.

Ultimately, these findings highlight the need for an integrated, multidisciplinary approach to wetland management that balances ecological objectives with community and developmental needs. Incorporating hydrological models, seasonal discharge data, and engineered structures (e.g., diversion canals, baffles, and sills) could optimize sediment retention and carbon storage while mitigating the risks of unregulated flooding. By aligning wetland conservation efforts with broader climate mitigation strategies, this study provides a framework for harnessing the carbon sequestration potential of riverine wetlands in South Korea and beyond.

Controlled overbank flooding is one of the main management strategies currently being employed along the lower Mississippi River in the North America in order to reduce the rate of oceanic transgression28,31. A combination of internal and external stresses including land subsidence, sea level rise, and anthropogenic activities (deforestation, levee, dredging) has led to the degradation of coastal wetlands and rapid shoreline retreats6,20. Several diversion canals have been built to transport water from the river channel into the adjacent wetlands in an effort to enhance marsh accretion rates28,32. Diversion canals along the South Korean rivers, designed to discharge water and sediments into low-lying areas containing a variety of engineered structures (sills, blind channels, oxbows, etc.) would potentially significantly increase riverine carbon storage levels by increasing fine sediments capable of trapping long-term capture of organic material. Hydrological models and seasonal discharge data can be used to develop optimal release schedules, including the timing and intensity of these releases to optimize sediment deposition during periods most beneficial for marsh growth. Such a management strategy naturally requires a collaborative, multidisciplinary approach balancing ecological objectives with the needs of local communities and industries.

Limitations

This study highlights the importance of the four river ecosystems as dynamic contributors to wetland productivity and crucial components of the regional carbon cycle, with significant potential to mitigate carbon emissions. However, despite these promising findings, the study acknowledges several limitations. Field observations were conducted exclusively within the Yeongsan River system, and while findings were extrapolated to the Han, Geum, and Nakdong Rivers based on shared hydrological and geomorphological features, direct sampling validation for these systems is lacking. The dataset is temporally limited to a specific period (May–October 2022), potentially missing seasonal variability outside the monsoon and dry seasons. Surface-level and horizontal spatial data were effectively captured using drone sensing and 3D topography surveys, but vertical sedimentary profiles were assessed through limited core sampling, which may not fully represent subsurface carbon dynamics. Although anthropogenic structures such as levees and embankments were identified as influencing factors, their long-term impacts on carbon sequestration were not comprehensively analyzed. Additionally, the use of certain conversion factors (e.g., Eq. 6) and the potential interdependence of variables yielding perfect correlation coefficients require further validation to ensure robust interpretations. Finally, while the study emphasizes physical and geomorphological drivers, ecological factors such as biodiversity and plant-soil interactions were not extensively explored, leaving room for future research to build a more comprehensive understanding of wetland carbon dynamics.

Conclusions

In an effort to retard global warming, the S. Korean government is actively attempting to reach carbon neutrality through a variety of means, including carbon sequestration. One approach is the burial of organic matter in the country’s four major rivers. East Asian river discharge rates are highly seasonal, peaking during the summer and often resulting in highly damaging floods. These floods also resuspend sediments and organic material, transporting both downstream. Effective carbon sequestration requires both the capture and retention of organics. This process is most effective in areas with fine-grained sediments, which, due to hydrodynamics, mainly occur in low-energy areas.

The deliberate use of controlled discharge events designed to introduce the top layer of flood water (carrying the suspended load) over the bank of the main channel into overflow areas containing a variety of energy-reducing structures (blind channels, baffles, sills, and deflection ridges) during the high-volume summertime discharge events is a method for retaining finer sediments, and subsequently, organic material. Extended across S. Korea’s four major rivers, the amount potentially sequestered is a significant percentage of the nation’s carbon budget.

By adjusting the timing and intensity of these releases, managers can also achieve specific ecological goals and reduce flood risk. This adaptive management approach enables the preservation of downstream wetland habitats and balances the needs of human infrastructure with those of natural ecosystems.

The relationship between organic content and grainsize, the impacts of 2020 flood events, and the seasonal aspect of carbon dynamics under the East Asian monsoon highlight the intricate interplay between the geological, climatic, and ecological factors that shape wetland environments. This comprehensive understanding holds promise for informed wetland management and the conservation of these valuable ecosystems in the face of evolving conditions.