A number of small tributaries drain directly into the South Fork Boise River, but their contribution for spawning and rearing habitat is poorly understood. Potential tributary enhancement projects could be effective, but the first step is learning whether fish are presently using the tributaries and the water conditions, whether the culverts inhibit or prevent fish passage, and habitat present in those tributaries.
The Boise National Forest has identified the culverts on four South Fork tributaries as barriers to Aquatic Organism Passage, typically referring to fish but can be other biologically important species. Culverts installed typically prior to the 1990’s like those on the SFB tributaries, are not designed for fish passage since they typically move water faster than the stream itself. While fish swim against a current at higher speed in shallow water, their passage through the culvert when moving upstream from the river to the tributary can be restricted due to water velocity, length of the culvert and depth of water.
Research by Idaho Fish and Game between 1977-1979 established that rainbow trout were accessing several tributaries during the spring to spawn. No recent information exists, however, to determine the role of these tributaries as contributors to spawning activity.
Volunteers are needed to survey South Fork Boise tributaries to monitor flows throughout May and into the summer, and to conduct a redd count on Saturday June 7 for evidence of spawning activity. Volunteers will be instructed in redd identification by a USFS Fisheries Biologist.
Instructions for volunteers working on their own for the flow measurement and fish activity assessments are available here: MS Word Doc version and Adobe PDF version.
Later in the summer, a Boise National Forest summer work crew will electro-shock portions of these tributaries to look for evidence of fish presence and to collect tissue samples to compliment the genetic study allowing a connection to be drawn between river residents and any headwater resident populations.
By monitoring the tributary flows and presence of fish at various locations, an assessment and prioritization can be made for future remediation projects such as culvert replacement to improve access for spawning and rearing habitat.
