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Lake Bernard Frank, a multiple-purpose reservoir in Montgomery County, Md., impounds runoff and sediment from 12.5 square miles of the upper North Branch Rock Creek basin. Sediment-trap efficiency was studied by the U.S. Geological Survey between 1967 and 1976. Suspended-sediment inflow to the impoundment for unmeasured storm periods in 1968-76 was estimated from daily suspended-sediment transport curves. Bedload contribution was estimated by the Schoklitsch and Meyer-Peter and Muller bedload-transport equations. The sediment contribution for the 1.76-square-mile ungaged part of the basin was estimated from land use and land cover determinations derived from aerial photographs taken annually and land use and land cover sediment-yield relations. The impoundment attenuated both streamflow peaks and instantaneous sediment loads. An instantaneous streamflow peak on North Branch Rock Creek was measured at 207 cubic feet per second per square mile above the impoundment, but, for the same storm, the peak downstream from the impoundment was only 7.0 cubic feet per second per square mile. For one storm, the maximum instantaneous suspended-sediment load was measured at 4,880 tons per day on North Branch Rock Creek upstream from the impoundment, while the maximum instantaneous suspended-sediment load below the reservoir was 33 tons per day. Discharge-weigh ted percentages of sand, silt, and clay averaged 16,50, and 34 for locations upstream from the impoundment and were zero, 16, and 84 below the reservoir. From 1968 through 1976, about 136,000 tons of sediment was estimated to have entered the impoundment. Over the same years, 5,910 tons of sediment was measured to have left the impoundment. The measured trap efficiency was 96 percent, which closely agrees with the trap efficiency computed from the impoundment's capacity-inflow ratio. INTRODUCTION In the early 1960's, two Public Law 566 structures sponsored by the Montgomery Soil Conservation District, Maryland-National Capital Park and Planning Commission, and Montgomery 2 SEDIMENT-TRAP EFFICIENCY, NORTH BRANCH ROCK CREEK, MD. County were constructed in the Upper Rock Creek watershed. The purposes served by these structures were flood control, sediment reduction, fish and wildlife development, and recreation (Montgomery County and others, 1962). One structure is on Rock Creek, and the second structure, impounding Lake Bernard Frank, is on the North Branch Rock Creek just upstream from its confluence with Rock Creek. The U.S. Geological Survey, in cooperation with the U.S. Soil Conservation Service, began investigations of streamflow and sedimentation in the North Branch Rock Creek basin (fig. 1), Montgomery County, Md., in September 1967. The investigation was part of a national effort to determine the sediment-trap efficiency of floodwater-retarding structures. Daily streamflow and suspended-sediment data were collected immediately downstream from Lake Frank at the North Branch Rock Creek near Rockville gaging station (01647740) (fig. 1). Daily streamflow and storm-period suspended-sediment data were collected at two upstream gaging stations in conjunction with a separate, but concurrent, research project in the same basin. Upstream data were collected for the 9.73 mi2 North Branch Rock Creek near Norbeck basin (station 01647720) and the 1.01 mi2 Manor Run near Norbeck basin (station 01647725) (fig. 1). The investigation was designed to measure suspended-sediment loads leaving the impoundment and to collect the data necessary to estimate sediment inflow to the impoundment accurately. The period of recorded data available at each of the streamflow and recording-precipitation gaging stations is given in table 1. Additional data were collected on the particle-size distribution of fluvial sediments, land use and land cover, and storm precipitation distribution. Similar investigations of trap efficiency have been made in Colorado (Mundorff, 1968), Kentucky (Anttila, 1970), Ohio (Flint, 1972a), and West Virginia (Flint, 1972b). Sedimentation studies have also been made on Loch Raven and Prettyboy Reservoirs in Baltimore County, Md. (Holeman, 1965).

Sediment-trap efficiency of a multiple-purpose impoundment, North Branch Rock Creek basin, Montgomery County, Maryland, 1968-76