Reservoir Operations & Watershed Monitoring

Forecasting reservoir levels in New York City’s Water Supply System is one of the most important and most difficult tasks we face in the operation of the water supply. Reservoir levels are primarily determined by the balance between streamflow into the reservoirs, diversions (withdrawals) for water supply, and releases to maintain appropriate flows in the rivers below the dams. Streamflow is affected by rain and snowmelt, both of which are hard to predict accurately.

View the Reservoir Levels in our watershed and our History of Drought and Water Consumption for additional information.

Flexible Flow Management Program

Water in the Delaware River Basin is jointly managed pursuant to a 1954 Supreme Court decree that governs water allocations for New York City and the states of New York, Pennsylvania, New Jersey, and Delaware. The Flexible Flow Management Program (FFMP) is intended to provide a more adaptive means for managing releases from Cannonsville, Pepacton, and Neversink reservoirs by providing for releases based solely on the time of year and reservoir storage conditions.

The Operations Support Tool (OST) FFMP Summary is a set of graphs and tables that summarize the model output data supporting each release decision for Cannonsville, Pepacton and Neversink Reservoirs.

Download the OST-FFMP Summary

Operations Support Tool

Operations Support Tool (OST) is a computerized decision support system that guides the managers that operate the New York City Water Supply System. OST was built through a joint effort of DEP staff, consultants, leading scientists, and the National Weather Service. It is one of the most advanced and complex support tools for water supply operations in the world.

OST is able to:

  • join together computer models of reservoir operations and water quality,
  • ingest near-real-time data on streamflow, water quality, reservoir levels, diversions, and releases, and
  • incorporate cutting-edge ensemble streamflow forecasts from the National Weather Service.

OST then applies all the operating rules and laws governing reservoir operations on the data it collects, and predicts a range of possible future reservoir levels and water quality conditions (as well as their individual probabilities). OST can make projections of reservoir water quantity and quality up to one year in the future. This approach, known as Forecast Informed Reservoir Operations (FIRO), allows managers to make risk-informed decisions regarding water supply operations. Since OST is run frequently, it also allows managers to adjust reservoir operations in response to the latest environmental conditions and evolving weather and streamflow forecasts.

A panel of experts from the National Academies of Sciences, Engineering and Medicine strongly endorsed OST for guiding the operation of New York City’s drinking water supply, managing risks such as droughts and turbidity events, and planning for the future effects of climate change. Read the review.

Download the OST Fact Sheet

Mapping & Modeling Technology

We use a series of linked computer models to aid in short-term operational decisions about the water supply and long-term planning and assessment of the City’s water supply system and watershed management programs. Our models can simulate the transport of water and contaminants within the watershed and reservoirs, as well as the generation and transport of pollution from the land surface to the reservoirs. In this capacity, our comprehensive computer modeling system is used to explore alternative future scenarios and examine how the water supply system and its components may behave in response to changes in land use, population, climate, watershed or reservoir management, and system operations.

Geographic Information System

We manage a comprehensive Geographic Information System (GIS) that allows watershed staff to analyze and evaluate information and programs in a unique spatial and temporal context (space and time). We regularly acquire, update, and develop new GIS data layers, create watershed program databases, perform GIS analyses and research, and produce maps and statistical reports. On average, our staff generate over 500 GIS watershed maps per month and support dozens of watershed data requests from City, State and Federal agencies.

Our GIS staff train and support other staff, interns, and local government agents in the use of Global Positioning Systems (GPS) for project-specific data gathering efforts such as field inventories or facilities mapping. They also provide remote sensing support in the analysis of satellite or aerial imagery for land use mapping.

Of particular importance to watershed protection is our Watershed Lands Information System (WaLIS)—a custom database application that manages information about watershed lands and resources owned by New York City and its neighbors. WaLIS integrates GIS data analyses, relational database management, document management, and reporting capabilities to specifically support natural resource management and land acquisition projects, as well as other watershed programs, applications, and decisions.

Other Resources

View the reservoir elevation forecasts from the Advanced Hydrologic Prediction Service of the National Weather Service:

You can also visit the Office of the Delaware River Master to access their data.