Navigating Shallow Streams: Effects of Changing Stream Water Depths on Outmigration of Endangered Coho Salmon

ABSTRACT Declining streamflow from water management and climate change threatens fish migration. In coastal California, USA, we evaluated the effects of receding streamflow on the seaward migration of juvenile Coho Salmon ( Oncorhynchus kisutch ) in four streams over 10 years (2010–2020). We monitored the movement of juveniles tagged with passive integrated transponders and measured depths at riffle crest thalwegs (RCTs), the shallowest channel feature that fish must navigate. Each stream exhibited a unique relationship between RCT depth and flow, relating to its geomorphic characteristics, producing distinct ranges of water depths among streams. During the peak migration period, juveniles moved over a wide range of depths (interquartile range 5.8–19.2 cm), with greater depths occurring in wet years than in dry years. However, the depths at which fish were detected were significantly deeper than continuous, observed depth measurements during the same period. We found the greatest differences between depth‐at‐movement and observed depths when streams were shallowest (< 10th percentile of observed depths). The timing at which stream depths fell below a regional fish passage criterion (9.1 cm) was also significantly correlated with the end of juvenile migration. In dry years, we observed greater variation in migration timing among streams, highlighting stream‐specific variation in the timing at which depths limit fish movement. Collectively, our study indicates that shallow depths impair salmon outmigration and that flow‐RCT depth relationships can be used to assess flow needs for fish migration and to guide streamflow restoration.