Frequent surveys of eight cross sections located in self-formed reaches of the East Fork River, Wyoming, during the 1974 snowmelt flood showed a close relation between channel morphology and scour and fill. Those cross sections narrower than the mean reach width filled at discharges less than bankfull and scoured at discharges greater than bankfull. Those cross sections wider than the mean reach width scoured at discharges less than bankfull and filled at discharges greater than bankfull. The accumulation and depletion of sand-sized bed material in a cross section was concentrated in the near-bank parts of the stream channel and thus significantly influenced bank stability and retreat. In those cross sections that scour at discharges greater than bankfull, the basal bank material is eroded and the banks become undercut and unstable. Conversely, in those cross sections that fill at discharges greater than bankfull, the basal bank material is covered by the accumulated sand-size material and is not eroded. Streambanks in these cross sections are moderately inclined and stable. A resurvey in the summer of 1980 of the cross sections located in straight reaches showed that those cross sections which scoured at discharges greater than bankfull had become 2–4 feet wider, whereas those cross sections which filled at discharges greater than bankfull were unchanged. Thus bank stability and to some extent the adjustment of stream channel width in the East Fork River study reach appears to be controlled by the processes of scour and fill.