Stream gains and losses across a mountain‐to‐valley transition: Impacts on watershed hydrology and stream water chemistry

[1] The mountain to alluvial valley transition is a dominant landscape of the American West, and of mountainous regions around the world, and is crucial to water resources in these regions. We combined stream water and groundwater (GW) hydrometric methods with geochemical hydrograph separations to investigate stream gains and losses across a mountain to alluvial valley transition in southwestern Montana to address the following questions: (1) How do alpine to valley bottom transitions affect stream discharge? (2) How do stream gains and losses change across alpine to valley bottom transitions? There was an annual 23% net loss in stream discharge across the transition zone, which we refer to as the mountain front recharge (MFR) zone. Gross stream gains were minimal in the MFR zone. Gross stream losses across the valley bottom equaled ∼140,000 m3 while gross gains equaled ∼70,000 m3, resulting in a net loss of ∼70,000 m3 in the valley bottom zone. Stream discharge was 97% alpine runoff (AL) in the MFR zone, whereas downstream discharge in the valley bottom was 48% AL and 52% valley groundwater. These large spatial and long temporal scale exchanges of water across the mountain-to-valley transition affected stream discharge magnitude, valley aquifer storage state, and valley stream water chemistry. This work suggests that streams do not simply lose or gain over a particular reach, but rather many streams are both gaining and losing and that net differences yield an incomplete depiction of stream hydrology.