Erosion and Storage Capacity
New York City’s drinking water supply has understood the benefits of long- term planning for nearly two centuries. The engineers who designed the reservoir system studied river valleys, precipitation trends, and population projections as they looked decades into the future of a growing city. Today, we are looking 50–100 years into the future as we plan rehabilitation work on dams, upgrades to aqueducts, and modifications to treatment facilities. We must also account for the changes that nature and time will bring to our watershed and reservoirs.
That’s why the City funded a three- year study by the U.S. Geological Survey (USGS) that examined how sediment deposition has affected storage capacity in our six reservoirs in the Catskill Mountains. The study, Bathymetry of Ashokan, Cannonsville, Neversink, Pepacton, Rondout, and Schoharie Reservoirs, New York, 2013–15, found that natural erosion of watershed mountains and streams has reduced total storage capacity in the reservoirs by approximately 2.4 percent. The new storage numbers are the first comprehensive data collected by scientists and engineers since the reservoirs were constructed from 1913–1964.
The results come as no surprise. Rain and melting snow are slowly eroding the Catskill Mountains and the fine sediments, pebbles and relatively large stones — collectively known as “bed load” — that were once picked up by mountainside streams and carried toward the ocean by larger rivers are now winding up in New York City’s reservoirs, which act as huge catch basins.
The USGS study, which gathered data from 2013–2015, used sonar to measure the depths of the reservoirs. Guided by a GPS system, the boat used for the survey traveled almost 700 miles as it traced dozens of shore-to-shore cross sections at each reservoir. Overall, the capacity of the six reservoirs decreased from 489.66 billion gallons to 478.06 billion gallons. The decrease in storage capacity by percent at each reservoir varied from 9 percent at Schoharie Reservoir to 0.7 percent at Pepacton Reservoir. USGS is currently working on a similar study of reservoirs in the Croton System.
The findings come with some good news. The storage capacity lost in the reservoirs is relatively small, and it does not affect water supply operations or reliability. The study found that sediment is primarily collecting in areas that are deeper than the lowest intakes that send water to New York City, making them inaccessible for water-supply purposes. Computer modeling using the new storage numbers found no impact on the system’s safe yield — a measurement of the maximum amount of drinking water the reservoirs could provide under the most severe drought.
In short, the sediment that has collected in our reservoirs will not have an effect on the quantity or quality of water we provide to New York City. But that will not be true forever. We must continue the good habits of long-term research and planning that have always guided our water supply. Using these new storage data as a baseline, we will seek to build expertise within our science and engineering staff to conduct similar studies more frequently in the future. This will allow us to understand the rate at which sediment collects in the reservoirs so that we can plan for action that might be needed decades or centuries from now.
Tracking reservoir storage is one of many scientific efforts that we have undertaken as we look into the distant future. We are gathering and analyzing similar data on the effects of climate change, numerous water quality parameters, and more.
We are fortunate to work on New York City’s water supply for just a few decades. During that time it’s our duty to keep this system of reservoirs and aqueducts in good condition by understanding all the forces that affect it now and perhaps longer into the future than we can imagine.
