Morphology and controls on the position of a gravel-sand transition: Fraser River, British Columbia

Alluvial river channels often exhibit a relatively abrupt transition from gravel- to sand-bedded conditions. The phenomenon is well documented, but few prior studies have analyzed the spatial variability through reaches where transitions occur. The downstream fining pattern observed in the Fraser River is cited as a classic example of an abrupt gravel-sand transition in a large alluvial channel. However, important questions regarding the exact location of the transition, its sedimentology and morphology, and what controls its location remain unanswered. Here we present observations of the downstream change in bed material grain size, river bed topography, and channel hydraulics through the reach within which the transition occurs. These observations indicate that the gravel-sand transition is characterized by a terminating gravel wedge, but there are patches of gravel downstream of the wedge forming a diffuse extension. We show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility. We argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed. We also find that sand in suspension declines rapidly at the downstream end of the wedge, suggesting that sand is delivered to the bed, completing the sedimentary conditions for a gravel-sand transition. We propose that the break in river slope associated with the transition is a consequential feature of the transition.