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About Delaware Basin Reservoir Releases

Flexible Flow Management Program and Flood Control

A photo of the Canonsville Reservoir spilling water
Canonsville Reservoir spilling water

The FFMP does not guarantee empty reservoir space to capture major runoff events for flood control. Structural limitations of water supply reservoir release mechanisms, the potential that large releases in advance of a storm will exacerbate downstream flooding, and uncertainty in long range weather forecasts severely constrain the ability of water supply reservoirs to act as flood control structures. The FFMP does include a reservoir spill mitigation component, which should help to reduce flooding impacts. This component reduces the probability and duration of flow through the reservoir spillway, with limited risk to water storage, by maximizing releases when reservoir levels and refill probability are high.

Benefits of Flood Mitigation

The U.S. Army Corps of Engineers, the National Weather Service, and the U.S. Geological Survey presented a Flood Analysis Model tool to the DRBC in 2009. This new tool helps the Decree Parties and DRBC understand the role water supply reservoirs and flood control dams throughout the Basin play during floods. The DRBC link located in the right sidebar has information about this model, including the DRBC analysis using it to study historical flooding events and the peer review performed on it by an outside consultant. Future consideration of flood mitigation benefits will need to be in balance with other needs and uses.

graph showing discharge times for inflow and outflow through the spillway

Flow through the Spillway

Flow through the spillway of a full reservoir does not increase flood levels downstream. When a major runoff event occurs, rivers without reservoirs convey runoff in a free flowing, unobstructed manner. On the other hand, if a river enters a reservoir, river flow then spreads out over the wider width and greater depth of the reservoir. This acts to slow the flow of water over the length of the reservoir. Even though the total amount of water passing through the reservoir is the same, this delaying mechanism reduces flood heights below a reservoir in almost all circumstances.

The Flexible Flow Management Program

Neither NYS nor DRBC can require NYC to release additional water. Any change that affects the conditions established by the 1954 US Supreme Court Decree must be approved by all the Decree Parties, including NYC. However, as established in the Decree, the USGS River Master can require additional releases from the New York City reservoirs for the purpose of maintaining the Montague, New Jersey, flow objective. In addition, the FFMP subscribes to an adaptive management framework. Accordingly, release modifications are considered by the Decree Parties as emerging issues arise or as other conditions warrant an immediate response.

Controlling Stream Temperatures

The FFMP does not directly manage stream temperatures at downstream sites. Instead, seasonal release rates are established to provide fixed volumes of cold water sufficient to protect against thermal stress to the cold water fishery. The seasonal schedules set fixed release volumes year round that are predictable and sustainable and provide a greater level of public understanding. Previous versions of reservoir release plans included "banks" of cold water reserved for thermal control. Yet, the sustainable and productive implementation of these banks proved to have several insurmountable obstacles, including capacity to anticipate air and water temperatures on both an hourly and long term basis. This resulted in either premature depletion of storage banks or storage banks left unused. In concert with the adaptive management frame work of the FFMP, the Decree Parties occasionally implements temporary programs to provide additional water when specific air and water emergency thermal criteria were met.

Conservation and Loss Reduction

New York City has substantially reduced its per capita consumption through an aggressive program of conservation and loss reduction. Although the result is that a portion of NYC's authorized diversion is not directly utilized, NYC remains authorized to divert 800 mgd. Potential future events and changing conditions, such as the unavailability of water from other parts of the NYC water supply system or increased demand, mean that NYC may need to use their full authorization at any point in time. In any event, NYS supports the continued development of sustainable programs to make best use of all water available while maintaining NYC's rights under the Supreme Court Decree.

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