ABSTRACT
A bedload trap located on the East Fork River, western Wyoming, has provided direct measurements of bedload transport and a unique opportunity to study the hydraulic adjustment of an alluvial stream to the particular combination of sediment and water it carries. The mountainous parts of the East Fork River basin have large water and small sediment yields. Conversely, the lowlands underlain by the erodible Wasatch Formation have small water and large sediment yields. Irrigation of the hay meadows along Muddy Creek, an East Fork tributary, has greatly accelerated erosion of the stream channel and banks, and, thus, has recently increased the sediment discharge of Muddy Creek into the East Fork River. Due primarily to the sediment contribution of Muddy Creek, the bedload-sediment discharge of the East Fork River in 1975 increased from approximately 200 t upstream from the confluence of Muddy Creek to more than 3,200 t at the bedload trap downstream.
The East Fork River slope decreases from 0.0011 immediately upstream from the confluence of Muddy Creek to 0.0084 in the vicinity of the bedload trap. There is no evidence to indicate that slope has adjusted to the increased sediment load. The bankfull-channel area of the East Fork River, however, increases downstream by approximately 30 percent, although the total available flood discharge is unchanged. As a result, the bankfull discharge is greater and a larger part of the available flood discharge is conveyed within the channel downstream of Muddy Creek. Consequently, at the respective bankfull discharges, the stream power per unit length and per unit area is approximately the same in both East Fork River reaches.
The downstream hydraulic geometry between the East Fork River study reaches differs considerably from the observed mean condition of other rivers in the region. Mean velocity and depth increase downstream more rapidly, while width increases less rapidly, than the corresponding mean values for other streams in the region. With the constraints that slope is independent and the concentration of sediment per unit width of active bed is constant, the measured hydraulic exponents for the East Fork River agree exactly with exponents computed by minimizing the variance of velocity, width, depth, and stream power per unit length and per unit area. Therefore, the hydraulic characteristics of the East Fork River, with the exception of slope, have adjusted mutually to transport the sediment contributed by Muddy Creek.
