Ask a Scientist: How do heavy spring rainstorms affect the Chesapeake Bay?

Welcome to the latest entry in our "Ask a Scientist" series! Each month, we take a question submitted through our website or Twitter (@chesbayprogram) and have a scientist from the Bay Program partnership answer it here on our blog.

Today’s reader question is about the effect of spring rainstorms on the Chesapeake Bay's health. We asked Scott Phillips, Peter Tango and Joel Blomquist, scientists with the U.S. Geological Survey and members of the Bay Program’s Nontidal Water Quality Workgroup, for their explanation on why heavy rains have such a big effect on the Bay and its local rivers.

They say April showers bring May flowers. But around the Chesapeake Bay, rainstorms bring a whole lot more.

The rain and snow that falls on the Bay watershed, an area of land that stretches from New York to Virginia, drains into local streams and rivers, which eventually flow to the Chesapeake Bay. About half of the water in the Bay comes from its rivers; the other half from the Atlantic Ocean.

The U.S. Geological Survey measures river flow throughout the Bay watershed and estimates the amount of fresh water that enters the Bay each month and year.Typically, 52 billion gallons of water drain into the Chesapeake Bay each day.

The river water that flows into the Bay has a significant impact on the Bay’s water quality, habitats, and fish and shellfish. Spring rains affect the amount of pollution going into the Bay. During periods of higher river flow, more nutrient and sediment pollution enters the Bay. During dry periods, fewer pollutants are washed into streams and carried into the Bay. In general, river flow into the Bay is highest during the spring, when there are more rain storms.

This past March started with noteworthy flooding across the watershed. River flows in March were some of the highest ever recorded. Field crews mobilized to collect nutrient and sediment samples that help determine the amount of pollution that washed into the Bay.

Too many nutrients and sediment contribute to pollution in the Bay and local streams. Elevated nutrient levels in the Bay tend to cause excessive algal growth. As algae decay, dissolved oxygen levels drop. This leads to unhealthy conditions for fish, crabs and other underwater life. Algae and sediment also block out sunlight that underwater grasses need. For these reasons, Chesapeake Bay Program partners are working to reduce these pollutants.

River flow also affects the salinity, or amount of salt, in the Bay’s water. The Bay’s salinity ranges from fresh water near the top of the Bay at Havre de Grace, Maryland, to ocean water near Norfolk, Virginia. In dry years, there is less river flow so saltier water moves further up the Bay. During wet springs, more fresh water enters the Bay, pushing salty water farther south.

Changes in salinity affect fish, oysters and underwater bay grasses. For example, some underwater grasses cannot survive if the water is too salty, while others can only survive in fresh water. Diseases spread to more oysters in saltier waters. Finally, sea nettles are more common in saltier water. So salinity and river flow influence our choice of places to swim to avoid frequent and painful jellyfish stings!

For more information on river flow to the Bay, visit the U.S. Geological Survey’s website. You can also read our March 22 article about Maryland DNR's monitoring after the heavy rains in early March.