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Channel adjustments to changing discharges, Powder River, Montana

¹ Department of Geology, University of Delaware, Newark, Delaware 19716

Sixteen years of annual surveys reveal how Powder River responds to varying discharges. During 1978, the second largest recorded daily mean discharge occurred. Cutbank migration, bed degradation, net bank erosion, and overbank deposition all contributed to increase the channel area at 12 cross sections by an average of 62%. During the ensuing years, the channel area decreased as sediment was stored in low-lying benches adjacent to the active bed of the channel.

The survey data indicate that the balance between bank erosion and deposition varies with discharge. In years when the annual maximum daily mean discharge is <60 m³/s (a flow of 60 m³/s has a recurrence interval of 1.1 yr), bank erosion and deposition are approximately equal. In years when the annual maximum daily mean discharge is between 60 and 150 m³/s (a discharge of 150 m³/s has a recurrence interval of 2.7 yr), bank deposition exceeds bank erosion, and the channel contracts, often by developing benches. In years with higher discharges, the channel expands through net bank erosion. These results demonstrate that the channel of Powder River is influenced by a wide variety of formative discharges. Powder River's recent history of expansion and contraction and the development of prominent benches cannot be explained by equilibrium models based on a single, channel-forming discharge.

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