* See also: information about flooding, flora and fauna, types of invasive vines, and municipalities in the watershed.

The Saw Mill River

Quick Facts:

Headwaters/source: Town of New Castle, 1.75 acre pond

Mouth: Yonkers into the Hudson River at Dock Street

Length: About 20 miles

Watershed basin: 26.5 square miles

Average width of Watershed Basin: 1.4 miles

Average width of River: 8-12 feet

Average slope of the river: 22 feet/mile

Population in watershed: Approx. 110,000

Population density: 4,151 per square mile, nearly twice the density in all of Westchester and 10 times that of New York State

Original name origin: The river originally was called the Nepperhan, from the Native American word for “rapid little stream.”

Current name origin: The Saw Mill River got its present name from the saw mill that was built near its mouth at Philipse Manor Hall in Yonkers in the mid-1600s.

Hudson River tributary: The Saw Mill River is one of 65 tributaries in the Hudson River watershed and lies completely within Westchester County.

Saw Mill River Parkway parallels river for about 16 miles (entire length of parkway is 29.8 miles.) Parkway was constructed between 1925 and 1954.

Tributaries: Tertia Brook, Nannyhagen Brook, Mine Brook, Rum Brook, other numerous unnamed tributaries.


Summary description: Extending twenty miles from Chappaqua to Yonkers, the Saw Mill River is a major natural resource in Westchester and a critical riparian corridor. It is the County’s southernmost tributary to the Hudson River and provides some of the only remaining habitat in this part of the County for a wide range of plants and animals. The swath of green land around the river is one of the few extended open space corridors below interstate 287, and it is used by thousands of bikers, joggers, rollerbladers, and fishermen. Yet, years of heavy land use, miles and miles of roadways, and high population density present serious challenges to the river, including flooding, pollution, streambank erosion, habitat loss, and ecosystem degradation. The Saw Mill River is also one of the major pollutants of the Hudson River.

Narrative facts: The Saw Mill River is one of 65 tributaries in the Hudson River watershed. It is just 20 miles in length. The Saw Mill River Basin is 26.5 square miles, with an average width of 1.4 miles. It lies entirely within the county of Westchester. From New Castle, the river flows through Mount Pleasant (including the villages of Pleasantville and Sleepy Hollow, though just marginally), and Greenburgh (including the unincorporated section and the villages of Tarrytown, Elmsford, Irvington, Ardsley, Dobbs Ferry, and Hastings-on Hudson), on its way to Yonkers and the Hudson River.

Headwaters & Mouth: The river begins as a 1.75 acre pond in a sub-division of the affluent suburban hamlet of Chappaqua, NY, population 9,500. It empties into the Hudson in downtown Yonkers, the fourth largest city in New York State, population 196,000.

Multiple land uses can be found along its length: Industrial, commercial, residential, parkland. The condition of the Saw Mill River reflects the surrounding land uses as it flows from suburban to urban areas, resulting in a general decline in environmental quality from north to south. For instance, in New Castle, the river is considered “relatively healthy.” The plants in the riparian zone filter pollutants and provide tree canopy that keeps the water cool and rich in oxygen. The riverbed is lined with gravel and cobble, providing habitat for macroinvertebrates and fish. In contrast, the Saw Mill River in central Yonkers has been modified into a rectangular, concrete-lined channel that was designed for urban flood protection and provides few environmental functions. By the time it gets to the Hudson, it has become an impaired waterbody. A USGS study in the early 1990s found that the Saw Mill River at Yonkers has among the worst concentrations of metals in stream-bottom sediment nationwide. (Of the 35 Hudson River Basin rivers tested, the Saw Mill at Yonkers had the highest levels of cadmium, copper, mercury, nickel, and zinc).

Population density: With the increasing urbanization, comes increasing population density. From the upper reaches, with as few as 1,000 people per square mile, to the river’s mouth, with up to 10,000 people per square mile. Nearly 110,000 people live in the watershed, representing 12 percent of Westchester’s population — on 6 percent of its land mass. In fact, the population density in the Saw Mill River watershed is double that of the county’s — compare 4,151 vs. 2,134 — and ten times that of New York State, which is 402 people per square mile.

Sandwiched between roads: In addition, for much of its length, the river is sandwiched by two parkways, the Saw Mill River Parkway, which continuously crisscrosses the river, and the commercially trafficked Route 9A. The river’s course was significantly altered by the construction of the Saw Mill River Parkway in the 1920s, as well as by construction of other highways, such as the New York State Thruway, which runs along part of the river as well. The river also has a bit of an identity crisis with the parkway of the same name.

Flood projects: Because of severe flooding, the natural stream channel in many reaches of the Saw Mill River has been intensively altered — relocated and channelized — over the years. Four engineering projects since 1981 have been completed (Chappaqua, Ardsley, two in Yonkers) over 25 percent of the stream with another in the planning stages (Elmsford/Greenburgh). These flood control projects have contributed to the degradation of the Saw Mill River Basin’s ecosystem. The methods used, installing concrete or plastic lining in the riverbed, does not allow for turtles to nest or aquatic life to burrow into sediment.

Other challenges:

  • Stormwater runoff
  • Illicit discharges
  • Invasive species (bittersweet, multiflora, mugwort, knotweed, porcelain berry, etc.)
  • Habitat degradation/loss of tree canopy
  • Streambank erosion (leading to siltation and sediment loads)
  • Floatables, aka “trash”

Underground flume: And, the final half mile of the river in Yonkers has been built over, buried in the early part of the 1900s. The river begins its underground journey at Chicken Island in Yonkers and flows in darkness until it reaches the Hudson River at Dock Street, underneath the train station.

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