Assessing trout spawning habitat quality using sediment in the Bear River of California

Grain-size distributions, bed mobility, and analysis of discharge, are indicators of trout spawning habitat quality. This study assessed brown and rainbow trout spawning habitat quality using grain-size distributions, pebble counts and sieve analyses, bed mobility estimates, painted tracers and shear stress, and secondary discharge data in the Bear River, from Combie Reservoir to the Wolf Creek confluence in the Sierra Nevada Foothills of California. Median diameters of potential spawning sediment in three of seven sediment samples were within the optimal size range for spawning (D50, 15-30 mm), however, spawning sediment was limited in the selected representative study reach. Bed mobility estimates indicated fine sediment transport (10 mm) and immobility of potential spawning sediment (D50, 15-30 mm), suggesting optimal trout spawning habitat conditions. Analysis of average monthly discharge in the Bear River from 2010 to 2013 indicated that flow conditions for brown and rainbow trout spawning are within the optimal range (0.27 to 56 cms), as defined by trout water velocity requirements and study site geometry. For both brown and rainbow trout spawning habitat quality, less than optimal conditions were determined for sediment size and availability. Increasing upstream discharge may increase spawning habitat availability by bank inundation during critical trout spawning months, however, sediment quantity and quality was the limiting issue. In the Bear River, from Combie Reservoir to the Wolf Creek confluence, quality trout spawning habitat was limited.