page_title: Saw Mill River
section_title: Hydrology
hierarchical_section_title: Saw Mill River / Hydrology
caption_attribution_description: English: Saw Mill River Sewer Marker that notifies people that the stormwater flows into the Saw Mill River
context_page_description: The Saw Mill River is a 23.5-mile tributary of the Hudson River in Westchester County, New York, United States. It flows from an unnamed pond north of Chappaqua to Getty Square in Yonkers, where it empties into the Hudson as that river's southernmost tributary. It is the only major stream in southern Westchester County to drain into the Hudson instead of Long Island Sound. It drains an area of 26.5 square miles, most of it heavily developed suburbia. For 16 miles, it flows parallel to the Saw Mill River Parkway, a commuter artery, an association that has been said to give the river an "identity crisis."
  The watershed was settled by the Dutch in the 17th century and was the site of Philipse Manor Hall, seat of Philipsburg Manor. The land was owned by Frederick Philipse I and subsequent generations until the family lost it at the end of the American Revolution. The land along the river was later divided into multiple towns. Industry in Yonkers developed along the Saw Mill, so polluting the river by the end of the 19th century that a local poet called it a "snake-like yellow scrawl of scum".
context_section_description: The USGS maintains a stream gauge on the Saw Mill just above the river's mouth in Yonkers. Mean discharge since 1944 has been 32 cubic feet (0.91 m³) per second, with extremes of 1,840 cubic feet (52 m³) during the April 2007 nor'easter and 0.11 cubic feet (3,100 cm³). Average annual precipitation in the watershed is 46.2 inches (1,170 mm).
  The Saw Mill River's water quality varies, reflecting its history and surroundings. Its headwaters in the town of New Castle are considered "relatively healthy". There the river is less disturbed, and its ecosystem supports a diversity of organisms. In Yonkers, where it flows through a concrete-lined channel, there is less life in the water and it is considered to be environmentally impaired. A 1983 United States Geological Survey (USGS) study found that concentrations of heavy metals in the water increased further downstream, a phenomenon observed with many other pollutants in the river and correlated with the urbanization around and above its mouth. DDT was detected in the streambed sediments throughout the river. In its final 6 miles (9.7 km), more than 50 micrograms of PCBs were found per kilogram of water. In the 1990s, the USGS found that of the 35 Hudson tributaries it tested, the Saw Mill had the worst levels of cadmium, copper, mercury, nickel and zinc in the sediments near its mouth, and among the worst nationwide (however, only the river's manganese levels were found to exceed federal standards). It is believed to add more pollution to the Hudson than any other single tributary.
  Unusually for a river, the Saw Mill's waters have consistently had a slightly alkaline pH, suggesting it has not been as affected by acid rain as other Hudson tributaries. In 1951, a state Department of Health survey reported pH between 7.25 and 9.1. Four decades later, another study found pH readings rising steadily from 7.59 in Chappaqua to 8.24 in Yonkers. Similarly, a 2007 Manhattan College study done for the New York State Water Resources Institute found a median low of 7.36 in Chappaqua and a median high of 7.81 near Torre Road in Yonkers, with a drop to 7.67 at the tunnel, for a total median for the river of 7.59. The lowest recorded pH in the year-long study was 7.1 at Chappaqua with the highest reading, 8.17, at Torre Road. All results were between 6.5 and 8.5, the range required by state regulations.
  The 1983 USGS study also classified the water quality of the entire river. The first 14.5 miles (23.3 km) from the river's source in Chappaqua was classified as suitable for any purpose besides drinking. The next 6.0 miles (9.7 km) was classified as being safe to drink. The last 3.0 miles (4.8 km) of the river from the sewage treatment plant to the Hudson was determined to be unsafe to drink, bathe in or fish in. The water was only safe for agricultural and industrial use.
  In regulations adopted in 1985 and amended in 2008, New York's Department of Environmental Conservation (DEC) divides the river into four water-quality regions similar to those in the 1983 USGS study. The first 1,100 feet (340 m) from the Saw Mill's mouth is affected by the Hudson's tides and thus is often salty like the river at that point. It is considered Saline Class B surface water, to be kept suitable for primary and secondary contact recreation such as swimming, boating and fishing, and capable of supporting "fish, shellfish and wildlife propagation and survival." The next section extends to the tailwater at the Yonkers sewage plant impoundment, and is Class C fresh water, with the same purposes, to the extent that "other factors" do not limit them. From there to the Woodlands Lake inlet is the third section, designated as Class A fresh water, to be kept clean enough for drinking. The remainder to the source is the fourth section, designated Class B, or fresh water kept to the same standards as the salt water above the river's mouth. Tributaries, named and unnamed, and subtributaries are generally held to the same standards as the section into which they drain.
  A 1991 study by Irene Gruenfeld, a Williams College under
caption: Saw Mill River Sewer Marker
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