Fig. 6: Peakflow Trends in the Post-Dam Construction Period (1981–2021).

a, c, and e display the Regional Average Mann-Kendall Z values for all the USGS sites, reference-only sites, and non-reference sites, respectively. These Z values indicate the statistical significance of the trends across the regions, with values < −1.96 or > 1.96 representing statistically significant trends. Higher magnitudes of Z correspond to greater confidence in the statistical significance (see Methods). b, d, and f illustrate the short-term (1981–2021) observed peak flow trends for all the USGS sites, reference-only sites, and non-reference sites, respectively. Open circles indicate insignificant trends with color representing the direction of the trend slope (blue for positive, red for negative, and grey for zero), while upward and downward arrows represent statistically significant (p-value < 0.05) increasing and decreasing trends, respectively, based on a non-parametric method. Regional trends are quantified using the Regional Average Mann–Kendall (RAMK) normalized Z-score, which is computed from the sum of site-level Mann–Kendall statistics (including both significant and non-significant sites) across the region (See Method). The regional trend significance thus reflects the net balance of increasing and decreasing site-level trends. Magnitude of trends are shown as percentages of standard deviation change over 40 years. The reference sites include both the HCDN and GAGES-II reference sites, and the non-reference sites include the GAGES-II non-reference and other USGS sites.