Moving Sediment into Ferron Creek, Downstream from Millsite Dam, Utah: Case Study

Millsite Dam, closed in 1971, is typical of the hundreds of NRCS dams constructed in the United States. Built for water storage, the original 18,000 acre-ft of storage has been reduced by about 3,000 acre-feet due to reservoir sedimentation. The nature of sediment deposition in reservoirs is such that most sediments deposit in the active pool rather than in the dead pool. Deposition in the active pool reduces water availability for irrigation and municipal and industrial water supply. The least expensive method of maintaining reservoir storage at Millisite Dam involves dredging deposited sediment and either storing it offsite or passing it downstream to Ferron Creek. Permits (401 and 404) were requested for the operation. The permits, approved after about 18 months, require that the sediment concentration in the outflow not exceed that in the inflow on an average daily basis. Downstream dissolved oxygen must exceed 3 mg/L during the operation, and sediment deposition in excess of 1.5 ft at any of the seven surveyed cross sections downstream much be removed and stored offsite. The permits allow sediment to be reintroduced into the downstream Ferron Creek, where turbidity will increase for some distance and for the duration of the dredge and removal operation. Operations were carried out for the first time in 2015, but for only three days, as the freely spilling water from Millsite Dam was in decline. A request for a change in operation is under consideration that would allow higher concentrations of sediment to be passed downstream during the operation subject to similar constraints.