MODIFICATION OF FLUVIAL GRAVEL SIZE BY SPAWNING SALMONIDS

Salmonids (salmon and trout) winnow fine sediment from streambed grave ls during construction of the nest or ''redd'' used for spawning and i ncubation of fertilized eggs. The gravels and interstitial fine sedime nts excavated during this process are exposed to currents and differen tially transported: gravels move a short distance, while the fine sedi ments are swept further downstream from the redd. To quantify the resu ltant modification of particle size distributions in redds, we sampled redds and adjacent undisturbed gravels to document changes in size di stributions. These data were compiled with previously published observ ations to analyze the general nature of size modification during spawn ing. The final percentage finer than 1 mm in the gravels, P1f, is rela ted to the initial percentage finer than 1 mm, P1i, by the equation P1 f = 0.63 P1i. Hydraulic variables (water surface slope, mean column ve locity, depth, shear stress, unit stream power) explained little of th e variance and did not appear in the optimal models. Because fisheries biologists are called upon to evaluate gravels as potential spawning sites, these findings should prove useful in such evaluations.