Fig. 1: Wetland hydrologic connectivity classification.

a, Four hydrologic connectivity classes: Riparian wetlands have an outlet within one 30 m pixel from a stream and bidirectional flows. The three non-riparian classes are greater than one pixel from a stream and all have unidirectional flows. NRShw have permeable and poorly drained soils on the flowpath between the wetland and downstream water. Owing to poor drainage, subsurface flows are shallow and surface flows can occur relatively frequently through saturation excess overland flow¹⁷. NRMid have permeable and well-drained soils on the flowpath. Owing to good drainage, subsurface flows are deeper (mid-depth), but surface flows can occur occasionally through infiltration excess overland flow¹⁷. NRDeep have impermeable soils on the flowpath. Non-channelized surface flows can occur when the wetland basin is filled with water and additional water input causes the wetland to either spill over or merge into downstream waters¹⁸, but this is limited to rare and episodic flooding events. Water transport is more common via deep subsurface flowpaths from the bottom of the wetland to downstream waters. Note that depth in the non-riparian class name refers to flowpath and not wetland depth. b, Flow chart summarizing classification of wetland hydrologic connectivity classes. Note wetlands are defined on the basis of 2011 NLCD³³ classes 90 (woody wetland) and 95 (emergent herbaceous wetland); however, woody versus emergent herbaceous type is not incorporated into the resulting classification. For details, see Methods.