Impervious Surface and Stream Health in Long Island Sound Basins


 Impervious Surfaces  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
Percentage of  Impervious Surface in CT & NY Portion of Watershed
1985 6.36 15.8
1990 6.66 15.9
1995 6.79 16.0
2002 6.98 16.1
2006 7.10 16.1


0-10% (good) 10-25% (fair) >25% (poor) Total
# of basins 132 37 0 169
IC (Acres) 135694.1 107853.5 0.0 243547.5
IC (Sq MI) 212.0 168.5 0.0 380.5
Total Basin  (acres) 2683118.2 756614.1 0.0 3439732.3
Total Basin (Sq Mi) 4192.4 1182.2 0.0 5374.6
# of basins 6 15 4 25
IC (acres) 2477.8 30934.4 14437.0 47849.3
IC (Sq Mi) 3.9 48.3 22.6 74.8
Total Basin (Acres) 39346.6 182829.4 51812.6 273988.6
Total Basin (Sq Mi) 61.5 285.7 81 428.1

 What is impervious cover?

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.

what are basins?

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.

What does this indicate?

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 2006 (see change in impervious surface map). During that period, four 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, and Byram River in the Southwest Connecticut Coast drainage basin.

data notes

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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