The Effects of Longitudinal Differences in Gravel Mobility on the Downstream Fining Pattern in the Cosumnes River, California

Downstream fining in the Cosumnes River is partially controlled by longitudinal variation in sediment mobility linked to changes in cross-sectional morphology. Strong fining occurs where the channel is self-formed with section-averaged bankfull dimensionless shear stress (⁠$τ*$⁠) near the threshold of motion (ca. 0.031), allowing for size-selective transport. In contrast, fining is minimal in confined reaches where $τ*$ is generally greater than twice the threshold value and transport is nonselective. Downstream fining is best described by a model that depicts fining in discrete intervals separated by a segment in which equal mobility of bed material accounts for the lack of diminishing grain size.