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

Author: Tyelyn Brigino, University of South Florida; Kai Rains, Edgar Guerron-Orejuela, Jacob Argueta, Syverine Bentz, and Mark Rains.

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Abstract

Groundwater discharge plays a critical role in the hydrologic and ecologic functioning of rivers, including modulation of stream flows and stream temperatures. This becomes increasingly important in the Kenai Peninsula Lowlands, Alaska, where salmonids must overwinter in freshwater streams and rely on groundwater inputs for cold water refugia in the summer and warm water refugia in the winter. Simultaneously, groundwater supports the increasing demands of people as population continues to grow in south-central Alaska. 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 used a geochemical approach to determine the relative contribution of groundwater to streamflow. Over six years, we collected 287 samples of precipitation (i.e., both rain and snow), ephemeral overland flow (i.e., snowmelt and rainfall runoff), groundwater (i.e., seeps, springs, and wells), and streamflow in six non-glacial mainstem salmon-bearing rivers. We analyzed these samples for major ion chemistry and stable isotopes and then used a mass-balance mixing model to calculate the relative contribution of groundwater to instantaneous and annual streamflow. 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 ranging ±20 percentage points between the six major watersheds. 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. Our work is incorporated into ongoing local and regional decision making and engagement, helping to translate science into management outcomes regarding groundwater source protection.

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