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Surface Water and Watersheds

Precipitation that falls over land and snowmelt as the seasons change finds its way to bodies of surface water as surface runoff. If the ground is saturated, frozen, or otherwise impervious to infiltration, liquid water flows downhill forming larger and larger rivulets until reaching creeks, streams, or rivers heading toward the ocean.

Where or when rainfall or runoff from snowmelt will eventually reach the ocean depends on its watershed. All of the water that falls in a particular watershed goes to the same place. For example, all of the water that falls in the Susquehanna River Basin in New York, Pennsylvania, or Maryland enters the Chesapeake Bay. Watersheds and river basins are important because everything that happens upstream, both human induced and natural, affects the water quality of a river midstream and at the outflow point. Watersheds help conceptualize flows of potential pollutants and how even localized incidents in the energy industry can have widespread effects. One of the first steps in mitigating any kind of environmental incident or problem is a thorough understanding of the associated watershed.

Strahler Stream Order Representation of Rivers of the Contiguous United States. The stream order indicates the level of branching in a river system, with higher stream orders indicated as thicker lines. The stream order map above reflects the morphology of various watersheds. Note the massive watershed for the Mississippi, Missouri, and Arkansas rivers at the center of the map, shown in pink.

Again considering the Susquehanna River, in 2005 and 2011, American Rivers, an environmental group, named the Susquehanna River the #1 most endangered river in the United States. Pollution enters the river from a variety of sources including agriculture, urban and suburban runoff, sewage, and extractive industries, such as mining and hydraulic fracturing. By 2016, this ranking was down to the #3 position as most endangered, and by 2022, the river was off the list of the top 10 endangered rivers in America.1American Rivers, 2016, “Susquehanna River Named #3 ‘Most Endangered,” Rivers, 2022, “America’s Most Endangered Rivers,”, accessed 12/18/2022. The river’s improved health is making progress under Pennsylvania’s Clean Water Blueprint to reduce stormwater runoff, soil erosion, and pollution from many sources.

Images: “Watershed” by phichak/; “Strahler Stream Order Representation of Rivers” by Robert Szucs