River Flow

Each day, billions of gallons of fresh water flow through thousands of streams and rivers that eventually empty into the Bay. That fresh water also carries polluted runoff from the Bay's 64,000 square-mile watershed. The amount of pollution in the Bay each year is largely determined by a combination of the amount of pollution on the land and the amount of water flowing into the Bay from its many tributaries.

Precipitation increases river flow because the water washes off the land and into streams and rivers. In addition, some water seeps into the soil and into groundwater. It can take years, even decades, for these waters — and the pollutants they may carry — to slowly travel through underground systems until they reach the streams that drain into the Bay.

How does river flow impact the Bay?

River flow impacts:

• The amount of nutrients, sediment and other pollutants delivered to the Bay from its watershed. As river flow increases, so does its potential to carry more pollutants from the watershed.
• The salinity of Bay waters. Under normal weather conditions, fresh water flowing from rivers and streams makes up about half the Bay's entire water volume. (The other half comes from sea water from the Atlantic Ocean.) Higher river flows cause more fresh water to reach the Bay, resulting in lower salinity levels; lower flows produce the opposite effect.
• Mixing of oxygen into the water. River flow is generally turbulent and fast-moving, which mixes in oxygen from the air. All Bay creature — from fish to crabs to worms — need oxygen to survive.