What is a Watershed?

A watershed is a landform shaped by geologic forces and defined by ridgelines (connected high points within a given terrain). A watershed serves as a basin that carries water "shed" from the land after rain falls and snow melts. Drop by drop, water is channeled into creeks and streams, making its way to larger rivers and, eventually, to the ocean.

Macro-scale
Ridgelines separate one watershed from another. The largest watershed division in the continental United States occurs at its highest and longest ridgeline, the Continental Divide in the Rocky Mountains. Precipitation falling on the eastern slopes of the Rockies contributes to the flow of the Mississippi River. The easternmost watershed boundary, or ridgeline, of the Mississippi basin lies in the Allegheny Mountains, and extends south to the Applachians. The Mississippi is a vast interior watershed draining most of the landmass of the United States. The headwaters of the Allegheny Watershed is one of the headwaters of that great basin.

The Allegheny Watershed encompasses most of western Pennsylvania and a portion of southwest New York. It is bounded by the Allegheny Mountains to the east, the Erie Escarpment to the northwest, and ridgelines to the west and south (including one ridgeline that bisects the City of Pittsburgh). Its total area is 11,770 square miles, and it includes all or parts of 19 counties (3 in New York, 16 in Pennsylvania). An estimated two million people live within the Allegheny Watershed. The extent and range of the Watershed's headwaters, creeks, and streams makes it difficult to count their many miles. But daily, the mean annual discharge rate for the Allegheny River is 620 tons of water per second at Pittsburgh, or 6.4 million tons per day.[1] At the confluence, the Allegheny mixes with the Monongahela and forms the Ohio River. From there, our once local waters travel west and south, converge with the Mississippi at Cairo, Ohio, and continue south where they drain into the Gulf of Mexico.

If you were to travel upstream, on the Mississippi River starting at the Gulf of Mexico and at every fork take the stream that has the larger volume of water, you'd end up on the Allegheny River. Continuing this journey in search of the Allegheny's headwater, you would travel 325 river miles and end up in Coudersport, Pennsylvania. Ten miles to the east behind family run dairy farms in a forest dominated by beech trees, you would find the spring-fed headwaters of the Allegheny: the ultimate headwaters of the Mississippi.[2] If you drove too far and passed over the ridgeline, you'd descend into the watershed of the Susquehanna River, whose waters drain into the Chesapeake Bay.

Micro-scale
Watersheds come in all sizes; they can span continents, or occur in backyards. Say there is a puddle in your yard; that puddle is a watershed because it's holding all the water draining into it from higher points above it.

Processes
Watersheds are associated with creeks, streams, rivers, and lakes; but they are much more. A watershed is a highly evolved series of processes that convey, store, distribute, filter, and utilize water to sustain terrestrial and aquatic life in obvious and unseen ways. Water is constantly moving through the world we live in; it falls as rain, travels through soil, is pulled up into trees and released as water vapor. It travels underground feeding forests and may surface as a spring, or be tapped by a well. We don't even see most of the water that supports the natural systems we're part of. Yet we depend on clean water everyday. In modern western nations, a person typically uses between 80-100 gallons of water per day for drinking, cooking, washing, and flushing. In medieval times, a person used only three to five gallons a day.

Significance
An appreciation of watersheds requires a holistic perspective where one seeks to understand the relationships between natural systems, land use, water quality, and the demand for clean water. Historically, agricultural lands and suburban sprawl have displaced forests, wetlands, and floodplains as a result of tilling and grazing, filling and paving. Between 1780 and 1980 alone, Pennsylvania lost 56 percent of its wetlands according to conservative estimates.[3] The disruption of integrated natural systems lessens nature's abilities to clean water, store it, recharge aquifers, feed forests, and reduce flooding.

In western Pennsylvania, we've begun to successfully address human impacts on our natural systems. Watershed conservation groups exist in many sub-basins within the Allegheny Watershed. Their works are specific to water quality issues where they live. Examples of diverse projects include: reducing agricultural runoff by planting riparian vegetation; remediating acid mine drainage; re-introducing river otters and paddlefish; removing invasive, exotic plants from streambanks; and monitoring water quality to determine inputs from land uses and industrial discharges. (See Conservation Efforts under the Allegheny Watershed in Explore the Watershed).

We all live in a watershed, and are likely to travel between two or more everyday. How we live on the land—whether we grow things on it, or pave it, what we discharge from our homes and lawns, and how we process water in our industries—affects water quality for aquatic species and human populations downstream. And the fact is, we all live downstream.

[1] Phone Interview with Mike Koryak, limnologist, Army Corps of Engineers, Pittsburgh District

[2] Of all the rivers feeding into the Mississippi, the Ohio River contributes the greatest volume of water. And the Allegheny is the biggest tributary of the Ohio. Some consider this to be the determining factor in naming the source of the great river. In fact, the mean annual flow of the Allegheny is 37% greater than the Monongahela. Some say, due to sheer volume of water, that the Ohio River should be called the Allegheny River, with the Monongahela River as one of its tributaries.

[3] Pennsylvania Dept. of Environmental Protection

This document is part of the Resource Center of the Watershed Atlas of the Allegheny River.