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Nonpoint source pollution is polluted storm water runoff containing sediment, fertilizers and pesticides, gasoline, heavy metals, and wastewater from degraded septic systems. These pollutants may be carried by storm water runoff to streams, reservoirs and Long Island Sound. 

Plants native to the region are key to the success of natural filtration systems. Plant roots, stems and leaves filter out pollutants, including nutrients and sediments, before they reach streams and water bodies. The roots also reduce soil erosion by binding the soil. Plants improve habitat for fish and wildlife and act as sponges, soaking up excess water after storms.

 Bronx River
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Stage 1
The shore of Bronxville Lake, formed by a dam on the Bronx River, was the subject of a bank stabilization project using a variety of techniques. Here, native perennial­ grasses, sedges and wildflowers that die back each year and send out new growth in the spring ­await planting along the bank. This strip of perennials will help stabilize the bank, curb geese traffic, filter out pollutants before they reach the river, and provide habitat for beneficial animals.
 
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Stage 2
In addition to perennials, the bank stabilization also relied on biodegradable mesh blankets to temporarily stabilize the soil of river banks with cuttings from shrubs and small trees. Here, cuttings from red-twig dogwood (foreground) and willow and silky dogwood (background) are tied into bundles and layered on top of each other in a terracing effect that is temporarily stabilized by a mesh blanket until the roots of the shrubs permanently bind the soil.
 Beaver Swamp
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Before
A marsh next to Beaver Swamp Brook in Harrison was completely dominated by the highly invasive and aggressive common reeds (Phragmites). It was further degraded by past dumping, including a derelict automobile, a shed, steel drums and car parts. The first task in restoration was to remove the reeds and junk.
 
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After
After the reeds and junk were removed, the reeds were allowed to grow back again before the site was sprayed with an herbicide developed for aquatic conditions. The site was eventually planted with native vegetation that included thousands of grasses, rushes, sedges, wildflowers, shrubs and trees.