During the summer, when streamflows are naturally low and the demand for water is high, water users are in competition for a limited supply of water. This report describes preliminary streamflow requirements necessary to maintain aquatic habitat at six critical riffle sites in the Assabet and Charles Rivers. The report describes summer low-flow-period streamflow requirements determined by the R2Cross and Wetted-Perimeter methods and compares the requirements to a target flow regime recommended by the Range of Variability Approach. The study area includes a main-stem reach of the Charles River and reaches on tributaries to the Assabet and Charles Rivers in Massachusetts. The Assabet River tributary reaches were on Danforth Brook, Great Brook, and Elizabeth Brook. The Charles River Basin reaches were on Mine Brook and on the main stem of the Charles River at Medway.
The R2Cross and Wetted-Perimeter methods require collection of site-specific physical and hydraulic data, such as channel geometry, average velocity, and mean depth at riffle sites. An advantage of the R2Cross and Wetted-Perimeter methods is that they are based on field observations and do not require data from a streamgaging station, so the flow values obtained by these methods can be applied in hydrologically disturbed drainage basins and at gaged or ungaged sites. Care must be taken, however, to choose appropriate study sites. Streamflow requirements determined for natural riffle sites may not be sufficient to protect habitat at sites in a widened channel, and flow requirements estimated at sites with a narrowed channel may not provide sufficient flows for habitat protection in unaltered stream reaches.
A water surface profile model, the U.S. Army Corps of Engineers' River Analysis System (HEC-RAS), was used in this study to simulate the water-surface profile for each riffle site and to determine the hydraulic parameters required for application of both the R2Cross and Wetted-Perimeter methods. HEC-RAS does one-dimensional hydraulic calculations for a network of natural or constructed channels under steady or gradually varying flow. Results from the calibrated HEC-RAS models for a riffle reach were summarized in tables of hydraulic parameters required by the R2Cross and Wetted-Perimeter methods for determining summer low-flow requirements. Of the six reaches modeled, one reach (Great Brook in the Assabet River basin) was affected by backwater at low to moderate discharges and thus could not be considered as a riffle habitat for determining streamflow requirements by means of R2Cross and Wetted-Perimeter methods.
The median flow requirements defined by the R2Cross and Wetted-Perimeter methods for the five-riffle reaches were 0.75 ft3/s/mi2 and 0.13 ft3/s/mi2, respectively. The discharge required for each riffle site was calculated by averaging the streamflow requirements for the appropriate cross sections of each reach.
The median R2Cross and Wetted-Perimeter streamflow requirements for the reaches were compared to observed daily summer-flow records for the streamgaging station at Nashoba Brook near Acton for the (1963 to 2001) period of record. The median R2Cross streamflow requirement is generally greater than the 75th percentile of daily discharges for the July, August, and September as determined from the Nashoba Brook station records. The median R2Cross requirement is generally between the 75th percentile and the median of the daily discharges for June and October. The median Wetted-Perimeter streamflow requirement for the reaches is between the 25th and 50th percentile of historic daily discharges observed at the Nashoba Brook station for the summer low-flow period (July through October). The median Wetted-Perimeter streamflow requirement is usually below the 25th percentile of daily discharge during the month of June for this station.
The RVA diagnostic method was applied to records from five gaging stations with largely unaltered flow in or around the Assabet River and Charles River basins, for the common period of 1969 to 1998 so that regional and seasonal comparisons with the R2Cross and Wetted Perimeter results for the five ungaged riffle reaches could be made. The interquartile ranges of the mean monthly discharges from all five gaging stations, normalized for drainage area, are mostly bracketed by the median R2Cross and Wetted-Perimeter streamflow requirements for July through September. The Wetted-Perimeter median flow requirement is close to the 10th-percentile flow for all five months shown; this result indicates that the Wetted-Perimeter flow requirement is violated about 10 percent of the time.
The Tennant standard-setting method was applied to the records for the same five regional gaging stations used in the RVA analysis. The Tennant method commonly defines minimum streamflows for small streams during summer low-low periods as 40, 30, and 10 percent of the mean annual discharge (QMA); these flows represent good, fair, and poor habitat conditions, respectively. The median R2cross streamflow requirement for the five riffles compares very closely to the median Tennant 0.4 QMA definition for good habitat. The median Wetted-Perimeter streamflow requirement compares closely to the median Tennant 0.1 QMA definition for poor habitat condition requirement. These results correspond well with the R2Cross and Wetted-Perimeter streamflow requirements, which, in turn, bracket the RVA summer-period monthly inter-quartile ranges.
The range of median discharges for the five ungaged riffle reaches defined by the median R2Cross and Wetted Perimeter streamflow requirements--0.75 ft3/s/mi2 and 0.13 ft3/s/mi2, respectively--brackets the interquartile range of monthly mean flows for July, August, and September. The interquartile range is the flow management target identified by the RVA record analysis for the five gaging stations that measure mostly unaltered flow. This discharge range is similar to the streamflow requirements determined from the Tennant analysis of the same five gaging stations.
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