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Whitney Environmental Management Plan

To respond to community concerns during the planning of the Lake Whitney Water Treatment Plant, the Authority hired a team of environmental scientists to conduct an environmental evaluation of the project’s potential impacts on areas upstream and downstream of the Lake Whitney dam, the waterfall, and the site of the existing filtration plant. A report summarizing findings and recommendations was completed in January of 1999. The report included recommendations for ongoing environmental monitoring and performance standards to mitigate potential impacts of future water withdrawals.

The Authority operates the Whitney Water Treatment Plant in accordance with the Whitney Management Plan. It is a dynamic document that is sensitive to both water supply needs and environmental issues, and can be changed to reflect new information gained from experience in operating the plant and mitigation measures. The Authority worked cooperatively with a multidisciplinary Environmental Study Team and the CT Department of Energy and Environmental Protection to develop the Management Plan.

In addition to the information regularly updated on these pages, we discuss the current status of Management Plan implementation at our annual public meeting each February. The Authority is fully committed to carrying out the obligations of the Resolution and operating the Lake Whitney Water Treatment Plant in a manner that will mitigate any impacts resulting from the plant operation upon associated environmental resources.

Environmental Management Plan Update

April 2019

The Water Treatment Plant continues to operate in accordance with the Management Plan. Click here for information on lake levels, spillway flows and water withdrawals.

The Whitney Management Plan specifies operating procedures and monitoring measures to implement the performance standards in the 1999 Environmental Study Team Report. Elements of the Management Plan are as follows:

Withdrawal Operating Guidelines

Operating guidelines have been established to minimize the amount of time that there will be no water flowing over the spillway to the Mill River. Flows over the Lake Whitney spillway to the Mill River average about 55 million gallons a day (MGD) and can exceed several hundred MGD during high flow periods that normally occur in winter and spring. In general, as the amount of flow over the spillway is reduced from seasonal changes in water flow, water withdrawals from Lake Whitney are also reduced. When the lake level drops to less than 0.2-feet over the spillway elevation, the withdrawal of water into the water treatment plant is cut back to less than one-third (32.36%) of the maximum daily withdrawal allowed by the Connecticut Department of Energy and Environmental Protection, except as needed for critical operational periods such as summer peak demand periods lasting several days, fire protection needs, very dry conditions such as a drought and water system emergencies such as a major water main failure.

Minimum Release Procedures

Except during extreme dry years or multiyear droughts such as the one in the 1960s, the number of days when Lake Whitney is not overflowing to the Mill River will be infrequent. In some years there will always be flow going over the waterfall. However, when Lake Whitney does not overflow, the Authority will release water from Lake Whitney to the Mill River below the waterfall (spillway) through the use of an artificial waterfall and/or gravity pipe to the spillway plunge pool. Downstream releases to the river are managed to achieve a balance between water supply needs, the downstream Mill River and the upstream lake environment. This minimizes the extent of drawdowns in the shallow areas of upper Lake Whitney while providing water flow for aquatic biota in the lower Mill River.

To ensure annual channel scouring and inundation of the lower Mill River freshwater tidal marshes, the Authority ensures that flows of approximately 100 MGD occur in the river for at least two days between February 15 and March 15. While water can be released through the Lake Whitney dam to accomplish this, it is expected that this will occur naturally even during drought periods.

Dissolved Oxygen

The Authority will work to maintain healthy dissolved oxygen (DO) levels for the aquatic organisms in the plunge pool below the Lake Whitney spillway. DO studies by the Authority have established a DO target level of 7 mg/L in the plunge pool during periods where there is no flow over the spillway and we’re releasing water from Lake Whitney to the Mill River. It is expected under most circumstances that this will result in a surface water DO concentration above 5 mg/L at the Orange Street Bridge during dry weather conditions, although surface water DO concentrations less than this have been observed absent of water withdrawals due to prevailing water quality factors unrelated to the treatment plant such as stormwater runoff. It may also be necessary to make allowances to this target value when DO in Lake Whitney is below 7 mg/L, since the Authority has no control over low DO events in Lake Whitney.

Marsh Vegetation

Using sampling transects established at two locations in the Mill River tidal freshwater marshes in 1998, we have established a multi-year record of marsh plant communities. Tidal freshwater wetlands are a significant biological component of the lower Mill River corridor. Freshwater wetland plant species generally survive where the average annual salinity of soil water is less than 0.5 parts per thousand. An increase in average annual salinity caused by decreased freshwater flows or increased saltwater intrusion through the existing downstream tide gates could adversely affect freshwater marsh plants. However, a review of the literature on salinity tolerance of marsh plants indicates that the species of the Mill River tidal marshes are among the most salt tolerant of freshwater species. In addition, monitoring of soil water salinity in the root zone of the plants has shown no changes in soil water salinity from water withdrawals or nearby river salinities. Most areas of the marsh are naturally inundated only during periods of high freshwater flow and will not be exposed to higher salinity water during summer low flow periods.

Aquatic Habitat Monitoring

We have conducted multiple years of aquatic habitat monitoring includes quantitative assessments of aquatic habitat and the distribution of benthic aquatic invertebrates in the lower Mill River. These organisms are generally the most sensitive biological indicators of changes in aquatic habitat conditions and water quality. No adverse impacts from our operation of the treatment plant have been detected.


The Authority constructed an artificial waterfall that can simulate the sight and sound of a waterfall during periods when water does not flow over the spillway. The artificial waterfall also serves as a means of releasing and aerating water from Lake Whitney to the lower Mill River.

Lake Whitney

Lake Whitney is recognized as an important open space corridor in the Town of Hamden, providing aesthetic and wildlife habitat benefits to the region. Water withdrawals and downstream releases are managed to minimize drawdown impacts to the shallow upper basin of Lake Whitney, which contains the sediment delta that has accumulated since the mid-1800s from the inflowing Mill River. In 2002, we completed a comprehensive study that assessed the physical, biological, and chemical features of the upper basin and provided management recommendations. This was followed up by additional data collection from 2004 to 2009.

Traditionally, copper sulfate is applied to water supply reservoirs to control nuisance levels of algae. While copper sulfate applications can have short-term benefits to drinking water supplies, particularly when the treatment process is inefficient at treating high levels of algae, longer-term water quality effects are usually negative, as a result of increased oxygen demand, accelerated recycling of algal nutrients and toxicity to algae grazers. It is the Authority’s plan not to use copper sulfate in Lake Whitney, as the advanced treatment process at the new filtration plant will be capable of removing high levels of algae and associated taste and odor causing compounds.

The Authority is continuing its ongoing source water protection efforts including watershed inspections, review of watershed development site plans, construction of stormwater treatment wetlands, water quality monitoring and spill response. These efforts help to control nutrient, sediment and chemical inputs from Lake Whitney’s watershed, thereby maintaining and enhancing the lake’s natural environment.

Lake Whitney Studies and Reports

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