LAUSR

10 Groundwater flow regimes in the seasonally-thawed soils in areas of continuous permafrost are relatively unknown despite their potential role in delivering water, carbon, and nutrients to 12 streams. Using numerical groundwater flow models informed by observations from a headwater catchment in arctic Alaska, USA, we identify several mechanisms that result in substantial surface14 subsurface water exchanges across the land surface during downslope movement and create a primary control on dissolved organic carbon (DOC) loading to streams and rivers. The models 16 indicate that surface water flowing downslope has a substantial groundwater component due to rapid surface-subsurface exchanges across a range of hydrologic states, from unsaturated to 18 flooded. Field-based measurements corroborate the high groundwater contributions, and river DOC concentrations are similar to that of groundwater across large discharge ranges. The persistence of 20 these groundwater contributions in arctic watersheds will influence carbon export to rivers if thaw depth increases in a warmer climate. 22