Many sections of our 3 rivers still fall short of meeting “fishable and swimmable”  standards, as envisioned by the Clean Water Act.

Nutrients

Excess nutrients, overabundant aquatic plants, municipal and industrial effluent, and polluted stormwater runoff have impaired the rivers. Thanks to new stringent wastewater discharge limits, we have achieved tremendous improvement. However, the nutrients in sediments still hinder achieving the Clean Water Act goal.

Eutrophication

An excess of aquatic plants caused by an over-abundance of nutrients (mainly phosphorus and nitrogen) degrades the habitat for fish and other wildlife, makes boating nearly impossible, and looks and smells terrible as the plants decompose. In the summer, parts of the Assabet River used to be a green blanket of floating duckweed and algae, with a forest of aquatic plants rooted in the thick sediments below. An in-depth study of the Assabet River (a TMDL) identified the sources of phosphorus and laid out how much needed to be reduced from two sources: effluent from four municipal wastewater treatment plants and nutrients recycled from river sediments trapped behind dams. Advocates successfully used this study to press for more phosphorus removal from wastewater.

Contaminants of Emerging Concern

Chemicals that are harmful in very small concentrations have been found in our rivers, such as pharmaceuticals, other endocrine-disrupting chemicals, and PFAS, the “forever chemical.” These can affect wildlife and human health, but few of them are regulated, underscoring the urgent need for effective regulation to protect river health.

Mercury

All 3 rivers in the watershed carry mercury contamination, largely from atmospheric deposition and a Superfund site near Ashland on the Sudbury River, posing a long-term health risk for fish and those who consume them.

Salt

Chloride levels in our rivers and streams are rising due in large part to road salt. This poses a threat to aquatic life and to human health when it enters drinking water.

Heat

Warm water may not seem like pollution, but it is. Water temperature is critical to the availability of the dissolved oxygen that fish and other aquatic wildlife need to breathe.

Read more about

• How we measure water pollution
• Our Assabet River TMDL, a nutrient loading, study

The 161 dams along the Sudbury, Assabet, and Concord rivers and thier tributaries create a series of pond-like impoundments, slowing the flow of phosphorus-rich water, allowing the proliferation of aquatic weeds, and trapping soft sediments behind each dam. During the growing season, aquatic plants take the phosphorus from the sediments and water column to grow profusely. As the plants die back in the fall, the phosphorus and other nutrients settle into the sediments, ready to feed another year’s growth. This phosphorus recycling comprises a major non-point source of pollution.

Dams can have many negative effects on the environment and people, including

• Habitat destruction: Dams block fish from moving between feeding and spawning grounds, limiting their ability to reproduce and find refuge from drought. They also hold back sediments, causing nutrient buildup, leading to algae blooms, proliferation of invasive aquatic plants, and reduced nutrient supply to downstream ecosystems.
• Flooding: Dams can cause flooding, which can destroy habitats and lead to the loss of life, property, flora, and fauna. Few of the dams in this watershed are used for flood control.
• Water temperature: The large water surface and slowed water raise summer water temperatures, approaching or exceeding the mortality threshold for temperature-sensitive coldwater species, like wild eastern brook trout.
• Eutrophication: Warmer nutrient-rich waters cause eutrophic conditions, which degrade wildlife habitat and can cause harmful algal blooms, dead zones, and fish kills.
• Recreation: Dams make it difficult for boaters to navigate long stretches of river.

LEARN MORE ABOUT DAMS AND OARS’ WORK TO RESTORE RIVER CONTINUITY

Read more about

• What OARS has been doing to restore stream and river continuity
• What OARS is doing to understand the and minimize the impact of dams
• Solutions being studied

Baseflow is critical

Rivers require a certain amount of water, or flow, to sustain aquatic life, wildlife habitat and recreational activities. Precipitation, both rain and snow, contribute to streamflow throughout the year. During summer and early fall, rivers depend on groundwater seeping slowly into the streams (baseflow) to provide relatively cool, clean water to sustain streamflow. This summertime baseflow is critical to maintaining a suitable habitat for fish and other aquatic creatures and is essential to keeping the streams from drying up in the summer. For a description of how the water cycle affects our rivers, read Early Fall on the Sudbury, Assabet, and Concord rivers by Ingeborg Hegemann.

In the SuAsCo rivers and Hop Brook, baseflow and flow from the tributaries dilute the nutrients and other pollutants in the wastewater effluent being discharged. In drier summers, the proportion of effluent to baseflow in the Assabet River gets very high—as much as 80 to 90% effluent, depending on the river section.

Unfortunately, the watershed’s tributary streams have already lost much of their baseflow because of existing groundwater withdrawals, sewering, and extensive paved and otherwise impervious surfaces where water can’t infiltrate back into the aquifers. For this reason, the impacts of new or increased groundwater withdrawals from the watershed must be carefully evaluated to ensure that they will not reduce groundwater or baseflow contributions to the rivers and streams. Additional losses of these flows will exacerbate the rivers’ existing eutrophication problems and undermine existing and future investments in nutrient control.

OARS maintains gauges to measure streamflow at nine tributary sites in the SuAsCo watershed. Streamflow recordings are taken # times a year during water quality sample collection. USGS collects streamflow data with four continuous flow gauges in our watershed: the Sudbury River in Saxonville/Framingham, the Assabet River in Maynard, the Concord River in Lowell, and the Nashoba Brook in Acton.

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