No Groundwater, No Fish: The Critical Role of Groundwater in Supporting Non‐Glacial, Salmon‐Bearing Rivers in South‐Central Alaska

ABSTRACT Groundwater discharge plays an important role in the hydrologic and ecologic functioning of rivers including sustaining streamflow and related habitat year‐round. Simultaneously, groundwater supports the increasing demands of people as the global population continues to grow. Balancing the needs of users becomes increasingly important as climate change introduces greater uncertainty in water resources and fisheries, especially for economically important anadromous species that depend on freshwater resources. We investigated the seasonal and regional variability of groundwater contributions to six non‐glacial mainstem salmon‐bearing rivers in south‐central Alaska. We hypothesized that groundwater contributes more than half of the annual streamflow, and nearly all of the streamflow during late summer and mid‐winter periods. Our results show that groundwater discharge is the dominant source of streamflow, with an annual relative contribution of 70%. Groundwater contribution to streamflow varies seasonally, ranging from approximately 50%–70% during peak flow to 70%–80% during low flow. Groundwater contribution to streamflow also varies spatially, with maximum differences in groundwater contribution between watersheds ranging ±20 percentage points. However, all rivers in the study area followed similar trends, with higher groundwater contributions in summer and winter and lower contributions in spring and fall.