Scientists Evaluate Effects of June Rainfall on Bay Health

July 2006 -- Scientists with the Bay Program have found little damage to underwater grass beds in the upper Bay and tidal Potomac River during their initial trips to assess the impacts of the major rainstorms and flooding that took place in the Bay watershed during the end of June.

Intense rainfall events affect water quality by carrying excessive loads of sediments, nutrients and contaminants into the Bay. This runoff has become more intense in recent years, due to the increase in impervious surfaces (such as paved roads, driveways and parking lots) in the Bay watershed. Instead of being absorbed into the ground, the rain flows rapidly and intensely across these surfaces into streams, causing streambank erosion and an excess flow of dirt and pollutants into the water.

The excess flow can cause losses of clams, oysters, underwater grasses and other living resources by blocking sunlight, burying them in sediment or creating oxygen-deprived “dead zones.” The beginning of summer is an especially critical time of the year, because shellfish are spawning and young grasses are trying to grow.

While the flow into the Bay after the June rain event was high, it was not unprecedented. Flows this high or higher occur about once every three years. The flow from Hurricane Agnes, which hit the Bay region in June 1972, was three times higher than this June's rainfall event. However, a flow this high during the early summer period is unusual; 1972 was the only other year this has occurred in June since 1968.

Scientists with the Bay Program will continue to take extra steps to monitor the health of the Bay this summer, including:

• Additional cruises and flyovers to track water quality conditions. These will show scientists the effects of excess nutrients and sediment on water clarity, and allow them to see if harmful algal blooms are forming.
• Visits to oyster beds and underwater grass meadows, which are vulnerable to the excess flow of nutrients, sediment and contaminants caused by the rainfall.
• Using increased technology to pinpoint where excess sediments end up in the Bay.

An immediate concern with the rainfall is the potential for high bacteria counts in some water bodies. People should not swim in the Bay's rivers, creeks and any other area that is not regularly monitored for bacteria. The Bay's swimming beaches are regularly monitored, and swimming there will be restricted if high bacteria levels are found.

For updates on Bay conditions this summer, visit the Bay Program Web site; also, conditions in the Maryland portion of the Bay will be posted at Eyes on the Bay.