Photos of the Long Island Sound

Research & Monitoring

Status and Trends: LISS Environmental Indicators

Impervious Cover and Stream Health in Long Island Sound Basins

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%.

Map shows the change in impervious cover from 1985 to 2006 for each basin in NY and CT. Thin lines depict basin boundaries, and thick lines depict major drainage basins. A key to the names of the major drainage basins is above the Learn More section.

 Impervious Cover  in CT & NY Portion of Watershed (sq. mi.)
1985 341.7 73.2
1990 357.6 73.9
1995 364.5 74.2
2002 374.7 74.6
2006 380.5 74.8
2010 383.5 75.5
Percentage of  Impervious Cover in CT/NY Portion of Watershed
1985 6.36 17
1990 6.65 17.26
1995 6.78 17.33
2002 6.97 17.43
2006 7.07 17.47
2010  7.14  17.63
0-10% (good) 10-25% (fair) >25% (poor) Total
# of basins         132          37        0.0        169
IC (Acres)   136,829 108,583        0.0  245,412
IC (Sq MI)            214         170        0.0          384
Total Basin  (acres) 2,683,118 756,614        0.0 3,439,732
Total Basin (Sq Mi)         4,192       1,182        0.0        5,375
# of basins        5          16           4         25
IC (acres)     2,555   31,223  14,526  48,304
IC (Sq Mi)          4           49         23          76
Total Basin (Acres)   39,155 183,021 51,813 273,989
Total Basin (Sq Mi)          61         286          81          428


Impervious cover is any surface in the  landscape that cannot effectively absorb or infiltrate rainfall. These surfaces include sidewalks, roads, parking lots, and roof tops. Rainfall carries pollutants from these hard surfaces to storm drains and tributaries of  rivers that flow into coastal waters such as Long Island Sound.


Basins (or watersheds) are all the lands in a geographical area that drain into a body of water. Small basins drain into the tributaries of larger rivers that flow into coastal waters such as Long Island Sound. In New York and Connecticut, there are 194 basins (shown on map). These basins are grouped into 10 major drainage basins  (areas between the thicker black lines) such as the Connecticut River drainage basin that contributes 70 percent of all the fresh water flowing into Long Island Sound. The map to the right includes a legend with the names of the major drainage basins.


Based on hundreds of studies the Center for Watershed Protection in Maryland has developed a general watershed planning model that uses percent watershed impervious cover (IC) to predict various stream quality indicators. It predicts expected stream quality declines when watershed impervious cover exceeds 10 percent and severe degradation beyond 25 percent impervious cover. These thresholds are some times characterized as good, fair, and poor.


A majority of basins (about 71 percent) in the Long Island Sound watershed in New York and Connecticut 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 (see change in impervious surface map). During that period, five basins exceeded  the 10 percent threshold suggesting that their streams are beginning to see signs of degradation (see the hashmarked areas in the above map). 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..


The impervious surface estimates were derived through a process known generically as spectral un-mixing by the University of Connecticut’s Connecticut Land Use and Education Research (UConn CLEAR program). The methodology is described at the UConn CLEAR website.

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