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Recent efforts to advance river connectivity for the Millstone River watershed in New Jersey have led to the evaluation of a low-flow gauging weir that spans the full width of the river. The methods and results of a desktop modelling exercise were used to evaluate the potential ability of three anadromous fish species (Alosa sapidissima [American shad], Alosa pseudoharengus [alewife], and Alosa aestivalis [blueback herring]) to pass upstream over the U.S. Geological Survey Blackwells Mills streamgage (01402000) and weir on the Millstone River, New Jersey, at various streamflows, and to estimate the probability that the weir will be passable during the spring migratory season. Based on data from daily fishway counts downstream from the Blackwells Mills streamgage and weir between 1996 and 2014, the general migratory period was defined as April 14 to May 28. Recorded water levels and flow data were used to theoretically estimate water depths and velocities over the weir, as well as flow exceedances occurring during the migratory period. Results indicate that the weir is a potential depth barrier to fish passage when streamflows are below 200 cubic feet per second using a 1-body-depth criterion for American shad (the largest fish among the target species). Streamflows in that range occur on average 35 percent of the time during the migratory period. An increase of the depth criterion to 2 body depths causes the weir to become a possible barrier to passage when flows are below 400 cubic feet per second. Streamflows in that range occur on average 73 percent of the time during the migration season. Average cross-sectional velocities at several points along the weir do not seem to be limiting to the fish migration, but maximum theoretical velocities estimated without friction loss over the face of the weir could be potentially limiting. Introduction The U.S. Geological Survey (USGS) Blackwells Mills streamgage (01402000) located on the Millstone River in Somerset County, New Jersey (U.S. Geological Survey, 2016) has a period of record that dates back to 1921. The Blackwells Mills streamgage is situated immediately upstream of a lowhead weir (fig. 1) that spans the full width of the river. The weir has been functioning to provide a stable low-flow control for the computation of streamflow at the streamgage since about 1937. Together, the streamgage and weir have been essential to the management of water resources and the protection of the aquatic ecosystem in the Millstone and Raritan River systems. The streamflow data computed at the Blackwells Mills streamgage are used by the New Jersey Water Supply Authority to monitor State-mandated minimum streamflows in the downstream river reach and to manage releases from State water supply reservoirs to provide public drinking water. The streamgage needs to measure streamflow in the Millstone River upstream from the confluence of the Raritan and Millstone Rivers and downstream from the Six Mile Run tributary to preserve the historic dataset and statistics. The weir provides reliable and accurate streamflow data during low-flow conditions when the natural stream channel is insensitive to changes and subject to scour and fill shifting. Scour and fill shifting reduces the accuracy of computed low flows; therefore, the suggested alternatives of removing the weir, backwatering the weir, or notching the weir would each likely result in an insensitive or unstable streamflow control that would be subject to increased shifting and error in the computed streamflow. The need to calculate and provide accurate streamflow data for public water supply at low-flow conditions and the lack of suitable alternative methods of providing a stable and reliable low-flow control supports retention of the weir. 2 Hydraulic and Biological Analysis of the Passability of Select Fish Species at Blackwells Mills, New Jersey

scientific investigations report

Hydraulic and biological analysis of the passability of select fish species at the U.S. Geological Survey streamgaging weir at Blackwells Mills, New Jersey