Pennsylvania Watersheds

Small, Large and Major Watersheds

As you zoom in on the map, two additional levels of watersheds will appear. Click on any individual watershed to see the watershed outlined and an informational "popup." The popup will name the stream(s) or river(s) at the heart of the outlined watershed, and sideways-pointing triangles will take you to the information about the other levels of watershed surrounding that point.

Data Explanations

A watershed is an area within which all water that enters as precipitation will exit via the same stream or river. The water may reach that river as overland runoff, as groundwater flowing within soil and/or bedrock layers, or as storm sewer flow via urban/suburban stormwater collection systems, piped straight into streamflow. Smaller tributaries also contribute to the watershed's outflow. The watershed's name is that of the outflow stream or river for the small and major watersheds, but may include the names of significant tributaries within the large watersheds.

If you zoom in on this map and examine the watershed boundaries, you will see that the more detailed watersheds do not line up with the more general ones. The reason is that these separate maps came from different sources compiled at different times, and were probably based on different original maps of the streams and elevations.

Comparative Data:

The land uses reported in the table below are based on apparent uses of the land interpreted from aerial photography.

Land area percentages in each watershed may not add to 100% due to rounding of smaller percentages.
Area (sq. mi.) % Forest % Agriculture % Developed % Water % Wetland % Mining % Barren
Lake Erie 511 40% 33% 15% < 1% 4% 0% 5%
Ohio River 15,614 63% 23% 7% < 1% 1% < 1% 5%
Genesee River 99 70% 26% 1% < 1% < 1% 0% < 1%
Susquehanna River 20,960 66% 25% 7% 1% 1% < 1% 1%
Potomac River 1,584 55% 36% 8% < 1% < 1% 0% < 1%
Delaware River 6,422 48% 27% 19% 2% 3% 0% < 1%