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Flooding in the Chesapeake Bay Watershed: Impacts on the Bay

by
Bill Whorton

Near record flow of the Susquehanna River was measured by the USGS on the morning of Friday, September 9th. River flow at Conowingo Dam, where the river enters the Chesapeake Bay, was 775,000 cubic feet per second (CFS)! 2011 will most likely be one of the highest annual flow years on record for the Susquehanna River, primarily as a result of both the September tropical storms and a wet spring across the watershed. In addition to the Susquehanna, high river flows were measured throughout other parts of the six-state Chesapeake Bay watershed. Your class can investigate real-time streamflow data at a site near your school by using the USGS WaterWatch website.

Likely Impacts on the Bay

Last month’s blog reported that the Chesapeake Bay received a short term water quality boost from Hurricane Irene due to the physical mixing of the Bay’s waters by extreme winds and waves that sent oxygen-rich surface waters into the deeper channels that are normally lacking oxygen at this time of year. It is true that the physical mixing that resulted from Hurricane Irene did increase the amount of dissolved oxygen near the bottom of the Bay; however the shear magnitude of the more recent flood waters, combined with the loads of nutrients and sediments, will likely have a negative effect on the Bay’s health. We will only understand the true impacts with the passage of time and through the combined monitoring and assessments by the Chesapeake Bay Program’s many partners.

Things to Watch:

  • Of potential concern to the next year’s crop of underwater bay grasses is the physical scouring of the Bay bottom (particularly in the Susquehanna Flats and the upper tidal Potomac River) resulting in the removal of vegetation living below the sediment surface—the ‘seeds’ for next spring’s plants.
  • The Bay’s oyster bars and other important hard bottom habitats will likely get a new layer of silt covering them in the coming days and weeks ahead, which will directly impact oyster and other bottom dwelling organisms.
  • Much of this sediment will stay around and, with the help of winds and tidal currents, find itself back up in the water column as early as this coming fall and well into the next year.
  • The flood of freshwater into a salty Bay can have impacts on the Bay’s critters like oysters which can’t just get up and move if the much lower salinity conditions last for an extended period of time.

Up on the Susquehanna River, the volume of flood waters will scour the bottom, causing sediment and nutrients previously ‘trapped’ behind the Conowingo Dam to be freed and sent down to the Bay. These released sediments will likely bring not only more nutrients to the Bay, but also long-buried chemical contaminants.

USGS will be taking samples for analysis of bacteria, pesticides and trace metals over the course of the flood event to help understand the chemical contaminant loads entering the Bay from such a major flood event. Several months from now we will be able to quantify these loads and the potential impact to a much higher degree.

Timing of the Floods Lessens the Opportunity for Further Impacts

When it comes to flood events and their impact on the health of local waterways and the Bay overall, it is timing that makes the big difference in terms of whether there is a short term (weeks to a month) or a long lasting (months to years) impact on the Bay ecosystem. Based on historical data, we expect and will be monitoring the following:

  • Bay grasses: We are at the end of the underwater Bay grasses peak growing season, so impacts will be fewer than if the flooding occurred in June or July.
  • Living organisms in the Bay: As this is not a major spawning period for Bay living resources, the long term impact on their populations will be minimal.
  • Nutrients & sediment to the Bay: Given that this flood event is happening as the summer season comes to a close, there is less opportunity for long lasting water quality impacts in terms of nutrient and sediment pollution. By the spring, a majority of the nutrients should have worked their way through the Bay system. Additionally, cooler temperatures, shorter days, reduced biological activity, and cloudy waters should prevent large algal blooms from growing in the excess nutrients.

Be sure to check in with Bay Backpack next week to learn how to teach about flooding in your classroom!