The gravel‐sand transition: Sediment dynamics in a diffuse extension

As gravel‐bedded rivers fine in the downstream direction, they characteristically exhibit an abrupt transition from gravel‐ to sand‐bedded conditions. The prevailing theory for why abrupt gravel‐sand transitions emerge is based on bed load sorting of a bimodal sediment. The abruptness is thought to be a consequence of sand overwhelming the gravel‐sand mixture once it reaches a critical coverage on the bed. The role suspension plays in the development of gravel‐sand transitions has not been fully appreciated. The Fraser River, British Columbia, is an archetypical abrupt gravel‐sand transition with a “diffuse extension” composed of a sand bed with some patches of gravel. We examine flow, shear stress, and suspended sediment flux in the diffuse extension to better understand sediment dynamics where the sand bed emerges. Sand is carried in suspension upstream of the primary abrupt gravel‐sand transition, but in the diffuse extension, sand is moved as both bed load and suspended load. We do not observe downstream gradients in shear stress or suspended sand flux through the diffuse extension that would suggest a gradual “rain out” of sand moving downstream, which raises the question, how is the sand bed formed? Sediment advection length scales indicate that with the exception of very fine sand that moves as wash load in the diffuse extension, fractions coarser than the median sand size cannot be carried in suspension for more than one channel width. This suggests that sand is deposited en masse at the beginning of the diffuse extension, forming a sediment slug at low flood flows that is smeared downstream at high flood flows to form the sand reach.