The Influence of Stream Crossing Structures on the Distribution of Rearing Juvenile Pacific Salmon

Roads and railroads often cause barriers to fish migration at stream crossing locations by modifying channel geometry, causing rapid vertical drops or increased water velocities.  Vertical drops often exceed the leaping ability, and increased water velocities can exceed the sustained or burst swimming speeds of juvenile anadromous salmon and resident fish.  Adult salmon migration barriers can result in the absence of anadromous fish from stream systems and dams can influence the survival of migrating smolt.  However, the effects of migration barriers on juvenile salmon distribution during fresh-water residency have not been thoroughly evaluated.  We tested for differences in juvenile Pacific salmon relative abundance upstream and downstream of crossing structures in five moderate sloped and five low-sloped streams.  Stream channel width, bed slope, and substrate were measured within the natural channel and compared to the physical characteristics within the crossing structures.  We measured water velocity during base-flow conditions at the inlet, outlet, and middle of culverts using velocity meters and obtained maximum and minimum velocities using the flow time of dissolved solutes.  Water velocities were compared to the sustained and burst swimming speed of rearing juvenile salmon to evaluate fish passage.  In the moderate-sloped streams that contained spawning adults, relative catch per unit trap of juvenile coho salmon was often 2 to 3 times greater upstream of the crossing.  Within these stream reaches, downstream emigration of salmon fry from spawning locations likely was limited by avoidance of water velocities that exceeded the burst swimming speeds of juvenile coho salmon.  Replacement of two crossing structures with bottomless arch pipes designed and installed to replicate natural channel slopes, widths, and substrate, reduced water velocities and eliminated differences in coho salmon CPUT.  In the low sloped wetland streams, with adult spawning reduced or absent above the crossing, CPUT of coho salmon juveniles was greater below the crossing structure.  Therefore, high water velocities at crossing locations affected both upstream and downstream fish movement and altered the distribution of rearing fish within stream systems.  These changes in the relative abundance of juvenile salmon could influence rearing salmon fitness and survival, and could be used to evaluate the influence of rearing juvenile salmon on invertebrate community composition and abundance.