A Sediment Budget for a Semi-arid Reservoir

Sediment accumulation within reservoirs poses a continued threat to reservoir storage capacity and decreases the operating efficiency and expected life of a dam. Numerous small irrigation reservoirs built during the last century in the semi-arid western U.S. are undergoing siltation at rates that make active sediment management plans imperative. Managers of Halligan Reservoir on the North Fork Cache la Poudre River in northern Colorado are incorporating a sediment budget into sediment management practices. Data collection for the sediment budget began during the spring 2002 snowmelt. Sediment and water discharge were measured upstream of the reservoir during the months April-June, with the same measurements collected downstream from the reservoir during the fall draw-down in early September. Because of the abnormally low snowpack during Spring 2002, peak discharge upstream never exceeded 1.5 m³/s (one order of magnitude lower than during an average year) over the course of the field measurements, and maximum suspended and bedload transport rates were 9.7E-07 and 0.17 metric tons/day, respectively. As such, the delivery of sediment into Halligan Reservoir during 2002 was severely limited. Fall releases of sediment have historically been used to regain storage capacity in the reservoir. Suspended sediment transport rates downstream from the dam, measured during a two-day step-down in September, were significantly higher than inflow rates measured in the spring. An important control on sediment influx into the reservoir is beaver dam storage. One beaver dam within the upstream study reach currently stores 3.6 m³ of fine-grained sediment behind the beaver dam. Bathymetric surveys of the sediment accumulation within Halligan Reservoir indicate that maximum accumulation occurs near the dam, up to thicknesses of 4.5 m. Similar measurements will be repeated during the spring 2003 snowmelt to further quantify the sediment flux into and out of the reservoir over a range of discharges. Ultimately, a reservoir operating regime will be developed from the sediment budget data that allows for the release of sediment without threatening the downstream ecosystem.