Effects of hydraulic roughness on surface textures of gravel-bed rivers

Field studies of forest gravel-bed rivers in northwestern Washington and so utheastern Alaska demonstrate that bed-surface grain size is responsive to hydraulic roughness caused by bank irregularities, bars, and wood debris. W e evaluate textural response by comparing reach-average median grain size ( D-50) to that predicted from the total bank-full boundary shear stress (tau (0bf)), representing a hypothetical reference condition of low hydraulic ro ughness. For a given tau(0bf), channels with progressively greater hydrauli c roughness have systematically finer bed surfaces, presumably due to reduc ed bed shear stress, resulting in lower channel competence and diminished b ed load transport capacity, both of which promote textural fining. In chann els with significant hydraulic roughness, observed values of D-50 can be up to 90% smaller than those predicted from tau(0bf). We find that wood debri s plays an important role at our study sites, not only providing hydraulic roughness but also influencing pool spacing, frequency of textural patches, and the amplitude and wavelength of bank and bar topography and their cons equent roughness. Our observations also have biological implications. We fi nd that textural fining due to hydraulic roughness can create usable salmon id spawning gravels in channels that otherwise would be too coarse.