Scour and fill in a stream channel, East Fork River, western Wyoming

Frequent soundings of 11 cross sections located on the East Fork River, western Wyoming, during a spring flood revealed two sequences of channel scour and fill. All sections either scoured or filled at the flood crests relative to their low flow condition. The sections which scoured at high flow (called scouring sections) generally tended to fill at low flow. Conversely, the sections which filled at high flow (called filling sections) generally tended to scour at low flow. The critical discharge at which the character of a section changed from scouring to filling or vice-versa was approximately the bankfull discharge. Therefore, at any discharge except bankfull, some sections were accumulating bed material (fill), while others were being depleted of bed material (scour).

The mean at-a-station hydraulic geometry of the East Fork River agrees with the theoretical minimum-variance hydraulic geometry. Thus, on the average, the East Fork River accommodates a change in discharge by mutually minimizing the adjustment of velocity, width, and depth. The hydraulic geometry of every cross section, however, deviated from the mean of the reach, and the associated sequence of scour and fill was a consequence of the deviation. The scouring sections had larger velocity and smaller width and roughness hydraulic exponents than the mean of the reach. Consequently, the sediment-transport rate varied more rapidly with discharge in the scouring sections than the mean of the reach. Hence, these sections had relatively large sediment-transport rates and scoured when discharge exceeded bankfull, and relatively small sediment-transport rates and filled when discharge was less than bankfull. Conversely, the filling sections had smaller velocity and larger width and roughness hydraulic exponents than the mean of the reach. Consequently, the sediment-transport rate varied with discharge in the filling sections less rapidly than the mean of the reach. These sections had relatively small sediment-transport rates and filled when discharge exceeded bankfull, and relatively large sediment-transport rates and scoured when discharge was less than bankfull.