ABSTRACT

A straight, four-mile tract of the Donjek River, Yukon, Canada is braided throughout, discharging about 50,000 cfs in flood.

The active part of the tract shows upper regime flow in the main channels, and lacks vegetation. Higher, older levels are former river courses, partly or completely vegetated, with continuous flow (principally lower regime) only in the main channels. Comparing channel (width:depth) and topographic indices, the younger surfaces are more strongly dissected because of active channel cutting, whereas channels on older levels are subject to infilling.

The sediments vary from clays to coarse gravels; most are poorly sorted. CM plots permit division into three main groups: silt and mud of low energy environments such as abandoned channels, gravel of high energy channel-bar complexes, and a variable intermediate group which fills channels under medium-energy conditions. Seven facies are distinguished on textural, floral, and sedimentary structural characteristics. Facies relations are mostly gradational within channels, commonly fining upwards. They are more complex between channels, and invariably erosional.

The origin and directional significance of hierarchical orders of sedimentary structures were studied. Six hundred and sixty measurements of small-scale structures (mostly ripples) show moderate within-bar and between-bar variation, and give a grand vector mean 22 degrees from the river trend. Bisectors of channel directional arcs (maximum and average ranges) approximate the river trend within 5 degrees.