West Fork Environmental assessed the quality and stage-related connectivity and suitability of redband trout spawning habitat within a reach of the Spokane River where normal sediment transport processes have been altered by dams and impoundments over the past century. Within the heavily armored study reach, this study focused on the discontinuous patches of smaller substrate lateral to the thalweg that could potentially be used by spawning redband trout.
The extent, elevation, and topography of each patch were mapped using GIS tools. Suitable spawning areas were calculated according to preferential depth and velocity boundaries documented at 5 different discharges ranging from 1,500 to 18,000 cubic feet per second. This information was then used to develop a relationship between river stage and amount of habitat available.
Survival to emergence experiments were conducted on a subset of the patches to assess the quality of spawning habitat over a range of site-specific characteristics. These experiments employed a novel approach for assessing intra gravel flow rates by pairing each artificial redd with a gypsum cylinder of known mass and dissolution rates which are related to flow and temperature.
Additional assessment of the intra gravel environment were made by measuring dissolved oxygen concentrations in water withdrawn from the artificial redds through plastic tubing affixed within the center of the Whitlock-Vibert boxes which held the developing embryos. Prior to emergence, boxes were retrieved and survival assessed. During the period of active spawning, surveys were conducted weekly to count and mark redds.
At each redd location, depth and velocity were measured in order to develop habitat preference criteria based on the unique spawning conditions redband trout experience in the Spokane River. It was determined that a substantial number of fish were spawning at previously unrecorded depths.