Salmonids (salmon and trout) winnow fine sediment from streambed gravels during construction of the nest or ''redd'' used for spawning and incubation of fertilized eggs. The gravels and interstitial fine sediments excavated during this process are exposed to currents and differentially transported: gravels move a short distance, while the fine sediments are swept further downstream from the redd. To quantify the resultant modification of particle size distributions in redds, we sampled redds and adjacent undisturbed gravels to document changes in size distributions. These data were compiled with previously published observations to analyze the general nature of size modification during spawning. The final percentage finer than 1 mm in the gravels, P1f, is related to the initial percentage finer than 1 mm, P1i, by the equation P1f=0.63P1i. Hydraulic variables (water surface slope, mean column velocity, depth, shear stress, unit stream power) explained little of the 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. ¿ American Geophysical Union 1993