Channel morphology was measured along a 3.6km reach of the proglacial Sunwapta River, Alberta, with constant discharge and significant downstream fining. The decrease in sediment load relative to discharge as the result of the formation of a proglacial lake, in addition to modifications to the reach by highway construction, prompted various forms of channel adjustment over the previous 55 years. The disequilibrium in stream capacity caused a decline in slope upstream resulting in the currently rectilinear form of the long profile. Consequently, the reduction in the braiding index and degradation of the channel bed in the upper reach contributed to aggradation and an increase in braiding in the lower reach. The downstream decline of the present-day grain size distribution is also significant in causing a downstream increase in channel width and braiding, as predicted by rationally-derived hydraulic geometry equations.