Low Flow Inventory

Neponset River Basin

Observations

Baseflow on the Neponset River at the USGS Norwood gaging station, as estimated from the 7-day annual minimum flow and adjusted for annual daily average flow, has declined significantly since the gage was installed in 1940 (1). A local resident reported that Spring Brook in Walpole has problems with low flows, but no other information is available at this time (7).

Suspected Causes

Water Withdrawals The Neponset River Watershed Association reports that the interbasin transfers of water out of the Neponset River Watershed exceed 20% of the river’s total annual freshwater discharge (2). Water withdrawals have also been noted as a possible factor causing low flows which affect smelt spawning in the Neponset River and Gullivar Brook in Milton (6).

Beaver Meadow Brook from Stoughton to Canton is known to have low baseflow problems, as noted by both Neponset River Watershed Association members and Division of Watershed Management staff. Watershed Association members report that local homes and an apartment building pump water directly out of the Brook for lawn and garden watering (6).

The Town of Sharon's municipal groundwater wells near Lake Massapoag are the suspected cause of low flows in the unnamed tributary that flows from the outlet of Lake Massapoag to the inlet of Hammer Shop Pond (6).

Dam Management Pequid Brook in Canton has an overall high quality benthic macroinvertebrate community and high species diversity, but the presence of low flow-tolerant species (Parametriocnemus sp.) suggests that the brook may experience periodic low flow events. A local resident noted that the brook was dry on August 19, 2002 (8), and the Army Corps of Engineers reported that low flows (less than 10 cubic feet per second) were common in Pequid Brook. The management of several upstream impoundments may affect the frequency and severity of these low flow events (6).

Bypassed Reaches / Diversions Below Hammer Shop Pond in Sharon, flow in Massapoag Brook is diverted during low flow conditions to Clay Meadow Brook, which drains to Steep Hill Brook and ultimately into Bolivar Pond (6).

Unquity Brook in Milton is an urban stream that runs underground for several stretches. Downstream of Milton Cemetery, where Unquity Brook resurfaces, the streambed is completely dry (6).

Consequences

Aquatic Flora and Fauna Between 1991 and 1994, the Neponset’s cold water fishery declined. During low flow events in early summer, smelt spawning habitat is often exposed and the entire season’s eggs are desiccated. The photograph below shows the Neponset River just downstream of smelt spawning areas. Temperatures in the river are also elevated during extreme low flow periods, causing further stress to aquatic communities (2).

Dry Neponset River Smelt Spawning Habitat

The benthic macroinvertebrate community in Mill Brook in Medfield is dominated by chironomid midge larvae that are associated with low flow conditions. The fish community includes typical coldwater species that are tolerant of pollution. These two characteristics of the aquatic community indicate that low flow conditions limit the biological potential of Mill Brook. During field visits for the 1999 water quality assessment, DWM staff observed that only about half of the channel bed was covered with water despite heavy rain the week before (6).

Beaver Brook in Sharon was also home to several species of chironomids tolerant of low flow conditions. Additionally, the low density of fish in Beaver Brook may be attributable to periodic low flow events. The Boston Harbor 1999 Water Quality Assessment Report suggests that possible sources of reduced baseflow be investigated including the Sharon Water Department's withdrawals (0.55 mgd registered and 0.42 mgd permitted), private water use, and sewer inflow and infiltration (6). This Brook has also been observed to have low flow problems by local residents, who believe that the Brook supports a small but declining population of brook trout, a species that requires cold, flowing water to survive (9).

Human Uses of the River One business that previously used the Neponset River for cooling water converted to refrigeration because the river has become too warm due to low baseflow conditions and heated runoff from impervious surfaces (2).

Pollution

During low flow events, pollutants become more concentrated and can cause health problems for humans and aquatic organisms. Activities such as swimming, fishing and boating can be restricted and aquatic organisms may become trapped in ever-shrinking pools with pollutants that become toxic when concentrated (2).

References & Resources

1. Neponset River Watershed Association Newsletter, March-April 2000. Has Streamflow Really Changed?
2. Neponset River Watershed Association: The Streamflow Squeeze
3. USGS real-time stream gage data for the Neponset River at Norwood.
4. USGS real-time stream gage data for the East Branch Neponset River at Canton.
5. USGS real-time stream gage data for the Neponset River at Milton Village.
6. O’Brien, Katie, Weinstein, Mollie, and Richard McVoy. October 2002. Boston Harbor 1999 Water Quality Assessment Report. MA Department of Environmental Protection, Division of Watershed Management.
7. Turner, Roger. Personal communication. Neponset Watershed Initiative Team Meeting, September 12, 2002.
8. Lavin, Carl. Personal communication. Neponset Watershed Initiative Team Meeting, September 12, 2002.
9. Lauenstein, Paul. Sharon resident. Personal communication, February 25, 2004.