A double rainbow appears over the Choptank River after a cloudburst passed over Tilghman Island, Md., on June 19, 2020. (Image by Carlin Stiehl/Chesapeake Bay Program)

You know the saying, “when it rains it pours.” Well, in the Chesapeake Bay watershed, this saying takes on a whole other meaning. Heavy rainfall can produce conditions that cause a number of changes to the Bay ecosystem by affecting salinity levels, water clarity and oxygen in the water. In turn, these factors can threaten (and in some cases help) Chesapeake Bay wildlife.

Salinity, water clarity and the Dead Zone

One of the most severe impacts of heavy rainfall results in the formation of dead zones. As rainwater travels to rivers and streams, it picks up nutrients which can fuel algal blooms. When blooms die and decompose, they remove oxygen from the water, which can suffocate underwater plants and animals, creating dead zones. The dead zone size in the Bay fluctuates from year to year, depending on weather and water temperatures. There is typically a correlation between the size of the mainstem Bay dead zone and excess rainfall and more runoff.

Rainfall also impacts the salinity levels of the Bay. In general, the Bay tends to have more fresh water in springtime when it receives runoff from snow melt upstream in the watershed, and greater rainfall than other seasons. The Bay tends to be saltier in the drier months. But during heavy rainstorms, river flows can rise delivering abundant freshwater into the Bay where salinity levels can change dramatically. In urban and suburban areas where there are a lot of impervious surfaces—such as roads, parking lots and walkways—the rain doesn’t have a chance to seep into the ground to replenish e groundwater aquifers or become absorbed by vegetation. Instead, the rain gushes into waterways.

Lastly, rainfall can create conditions that can reduce water clarity. During storms, excess sediment gets washed into waterways. In the case of severe precipitation events, rising water levels in the rivers and streams can erode the banks, producing even more sediment. All this excess sediment clouds the water and blocks sunlight from reaching underwater grasses, one of the most important habitats supporting the health of living resources in Chesapeake Bay.

Underwater grasses, oysters and finfish

All of these water quality impacts that result from rainfall can be harmful to the health of Bay’s critters and plants if rainfall frequency, duration and intensity increases over time.

Underwater grasses, otherwise known as submerged aquatic vegetation (SAV), are sensitive to nutrient pollution and respond quickly to changes in water quality. Abundant nutrients that arrive from high flow events after a rainstorm can produce algae blooms that are dense enough to reduce sunlight from getting to the SAV. Cloudy water caused by excess sediment also blocks sunlight from reaching the grasses, which can cause them to grow weaker and die. Lastly, changes in salinity levels contribute to declines in some types of SAV, like widgeon grass, that prefers regions where the water is moderately salty. However, some underwater grasses that enjoy lower or higher salinity levels in the water have seen population expansion and growth as recently as 2020.

Both wild and farmed oysters are also impacted by abundant runoff into the Bay from severe rainfall events. Oysters require salty waters to survive, grow and reproduce. Since they attach themselves to reefs early in their life, if conditions, like salinity, change when their reef suddenly receives abundant freshwater after a big storm, they simply can’t just pick up and move. Extra sediment that also arrives in the Bay can smother reefs and suffocate oysters. Abundant freshwater and excess sediment disrupt restoration successes and harvest opportunities. In 2019, abundant rainfall delivered the highest freshwater flows received by the Bay since record keeping began in 1937. The high freshwater flows had major negative impacts on oyster restoration efforts in portions of the Potomac River and in other tidal waters in Maryland. Abundant freshwater in the Bay caused massive die-offs among oyster aquaculture. However, an influx of freshwater isn’t always bad news for oysters. In some areas, the freshwater dilutes the salt water to levels that kill off some of the diseases that kill oysters, like MSX, but still allow the oysters to survive.

Unlike oysters, when salinity changes, finfish are able to move to areas better suited for them. This means salt-loving fish move south, and freshwater fish begin to appear in areas they aren’t normally seen. For example, in 2018, juvenile striped bass were spread out over a much larger area, which was attributed to a drop in salinity as a result of an abundance of freshwater flow to the Bay. However, these conditions can also give rise to invasive species. For example, blue catfish have been able to use the wide distribution of fresh and low salinity water to migrate through tidal waters in search of new habitats, further expanding its range around the Bay and its tributaries.

Climate Change and the Bay Program

As air circulation patterns change due to climate change, it is likely that precipitation rates will rise in some Bay regions and fall in others.

The unpredictability of weather means a greater need for climate resiliency. At the Chesapeake Bay Program, members of our Climate Resiliency Workgroup are working to increase the resiliency of the watershed through restoration projects and improved monitoring efforts.

The six watershed states and the District of Columbia, who are the signatories of the Chesapeake Bay Watershed Agreement, are also accounting for climate change impacts in their Phase III Watershed Implementation Plans (WIPs). In 2019, these jurisdictions included a written strategy in their Phase III WIPs, detailing how they currently manage land, water and living resources to account for impacts of climate change, and will account for expected changes in rainfall and river flows in their management efforts to reduce nutrient and sediment pollutant loads.

The Bay Program’s Water Quality Goal Implementation Team is also spearheading an update to its modeling and analysis tools used in the Chesapeake Bay Total Maximum Daily Load. The ability to project climate change effects through 2035 is integral to the next phase of the Chesapeake Bay Watershed Model.

Want to help reduce the impacts of severe rainfall on Bay water quality and fish, crab, and oyster health? Consider installing a rain barrel, planting a rain garden and reducing the amount of lawn fertilizer than can run off from your yard into local waterways.



Gary Judson

Great and informative article. Loved the use of term" Bay Criters". Would have liked to see a little more in regards to what local individuals could do to protect the health of the Bay.

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