Photos of the Long Island Sound

Research & Monitoring

Status & Trends

LISS Ecosystem Targets and Supporting Indicators

Impervious Cover

Through green infrastructure, low impact development, and stormwater disconnections, decrease by 10 percent the effective area of impervious cover in the Connecticut and New York portions of the watershed by 2035 relative to 2010 baseline.

Map shows amount of impervious cover (IC) by percentage for each basin in NY and CT. Thin lines show basin boundaries, while thick lines show major drainage basins. A key to the names of the major drainage basins is above the Learn More section. Basins shaded in green are below the 10% IC threshold, yellow basins are between the 10% and 25% thresholds and red basins exceed 25%.
* Click labels in legend to hide data and adjust scale

A study that will compare impervious cover in 2015 to the 2010 baseline should be available in 2018. Assessing the scope and effectiveness of green infrastructure is needed needed to estimate changes in the effective area of impervious cover.

Impervious Cover
CT (sq mi)NY (sq mi)Total (sq mi)

Status and Trends

The extent of impervious cover in Long Island Sound can be assessed up to the 2010 baseline. An analysis of 2015 impervious cover is expected to be available in 2018.

From 1985 to 2010, impervious cover in both states increased a combined 10 percent, from 414.9 to 459 square miles. During that period, Connecticut experienced a 12.23 percent increase, while New York experienced a 3.71 percent increase. University of Connecticut’s Center for Land Use Education and Research (CLEAR) is currently analyzing satellite land use imagery from 2015 to add to the dataset.

Connecticut has less impervious surface and more vegetated land in comparison to the Long Island Sound watershed in New York. Based on 2010 data, 7.4 percent of Connecticut land consists of impervious surfaces compared to 17.63 percent in New York.

The total area of land in Connecticut that lies in the Long Island Sound watershed is 5,150 square miles. In New York, the total area that lies in the Long Island Sound watershed is 478 square miles.

Many Long Island Sound communities are encouraging green infrastructure projects,  but there is no adequate system yet to track their impact on reducing impervious cover in the Long Island Sound watershed (see challenges).



Municipalities will need to work with property owners and their own public works departments to encourage green infrastructure projects that will most effectively reduce the impact of impervious surfaces. Municipalities also will need to keep track of these projects as well as conduct site visits to identify existing developments where stormwater is directed to the ground for infiltration and treatment instead of draining directly on hard surfaces to storm drains and streams. The Long Island Sound Study Watersheds and Embayments Work Group is looking to develop a database that will keep track of all green infrastructure projects.  

How is This Target Measured?

Impervious cover is   tracked about every five years  using satellite imagery with a 30 meter (approximately 100 feet ) resolution.

The analysis is based on CLEAR’s land use dataset.  ( (Wilson and Arnold 2008) The study area is defined by the TMDL, and the study area boundary can be found here: .


The degree of impervious cover, particularly near waterbodies, has been shown to be associated with degradation of water quality in rivers and streams.

Rainfall carries pollutants from these hard surfaces to storm drains and tributaries of rivers that flow into Long Island Sound.

Green infrastructure and low impact development such as green roofs and permeable parking lots  can filter and treat pollutants from stormwater runoff before the water drains carried into storm sewers and tributaries, effectively reducing the impact of impervious cover. Stormwater disconnections involve directing stormwater into the ground instead of a hard surface leading to a storm drain, such as when a roof drain pipe is directed to grass instead of a driveway.

Additional Information

Based on hundreds of studies, the Center for Watershed Protection has developed a general watershed planning model that uses percent watershed impervious cover (IC) to predict various stream quality indicators.

The model predicts expected stream quality declines when watershed impervious cover exceeds 10 percent and severe degradation beyond 25 percent impervious cover. These thresholds can be characterized as good, fair, or poor. In Long Island Sound, a majority of the Sound’s 194 small drainage basins  (about 71%) are predicted to have streams with good water quality because they contain less than 10 percent impervious cover. About 27 percent of basins are predicted to show signs of stream degradation because they exceed the 10 percent threshold. About two percent of basins show signs of severe degradation because they exceed the 25 percent impervious cover threshold. The basins with the most impervious cover (and predicted poor stream quality) are in the urbanized areas of  the Western Sound in New York City, Nassau County, and Westchester County. While the Western Sound has more impervious surfaces,communities in central and eastern Connecticut and eastern Long Island are adding impervious surfaces at a higher rate than the rest of the region from 1985 to 2010 . During that period, five basins exceeded  the 10 percent threshold suggesting that their streams are beginning to see signs of degradation. These basins are  Freshwater Brook  and  Upper Mattabesset River basins, both part of the Connecticut River major drainage basin, Fenger Brook in the Southeast Connecticut Coast drainage basin, Byram River in the Southwest Connecticut Coast drainage basin, and a small island off of New York City.


Robert Burg, New England Interstate Water Pollution Control Commission

Source of Data



Data Notes

  • The technical explanation on how the target was selected is found in Appendix B of the Comprehensive Conservation and Management Plan.
  • The atlas to track LID projects described in the technical explanation of the CCMP does not provide project details  to calculate effective pervious cover for every project listed. It also does not provide a comprehensive list of projects. As a result, the Long Island Sound Study is looking at alternatives that would provide the information needed to track LID projects for this target.
  • The CCMP states that there is 463 square miles and not 459 because it looked at statewide totals, which includes a very small area of Connecticut that is not part of the Long Island Sound watershed.
  • The impervious surface estimates were derived through a process known generically as spectral un-mixing by UConnn CLEAR. The methodology is described at the UConn CLEAR website.
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