The Effects of Florence and High River Flow During Summer 2018 on the Chesapeake Bay

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Prepared by the U.S. Geological Survey and Chesapeake Bay Program, Updated September 17, 2018.

Effects of Florence on the Chesapeake Watershed and Bay

  • Many rivers in the watershed are already well above normal in advance of the remnants from Hurricane Florence (see map below).  
  • The storm will move through the western portion of the watershed on Monday and Tuesday, with an additional 1-4 inches of rain.
  • The resulting high river flows will wash more sediment, nutrients, contaminants, and fresh water into the Bay.
  • Potential effects of Florence probably would have been short-lived, but they are compounding due to an unusual extended period of above normal river flow this year.
  • Since May 2018 (see graph) monthly total river flow to the bay has been unusually high, with a record high set for August.  The impacts to the Bay are discussed in the next section.
  • The Susquehanna River flows have been particularly above normal with three events over 200,000 cubic feet per second since late July. By comparison, average flows for the Susquehanna during the Summer are about 10,000 cfs.


Chart of the 2018 Summer river flow in to the Chesapeake Bay

Chart of the 2018 Summer monthly river flow into the Chesapeake Bay. The monthly flows have been above normal since May and set a record for August 2018.

Florence will Contribute to the Impacts of High Summer River Flows on the Bay Ecosystem

Even though the Chesapeake watershed missed the worst of Florence, the high river flows which have occurred since May, and the additional flows resulting from rainfall associated with Florence, will cause negative impacts on Bay conditions. 

There will likely be a decline in the amount of underwater grasses (submerged aquatic vegetation or SAV) in 2018. The acres of SAV in the Bay hit a record in 2017 (100, 000 acres) and has been increasing since 2011. However, during 2018, the SAV have been getting less light due to more turbid waters from the sediment washed in during storms. SAV provides important habitat for fish and crabs, food for waterfowl, and puts oxygen into the water. SAV acres and water clarity are key metrics to the Chesapeake Bay Water Quality Standards Attainment Indicator which also had its highest value since 1985 in the most recent assessment. Therefore, the next assessment period is likely to show a decline in the estimated water quality standards attainment due to 2018 weather events and their impacts on the bay ecosystem. 

Hypoxia conditions in the Bay are showing mixed results. There have been changes in the monthly hypoxia conditions (dissolved oxygen below 2mg/L) of Maryland waters according to MD Department of Natural Resources (MD DNR). Hypoxia was worse than average in June, was better than average in July, and was near average conditions for August. MD DNR stated that they attributed the poor June conditions to spring river flows which brought in more nutrients. For July, they attributed the reduction of hypoxia to several storm events, and water column mixing as a function of sustained winds of 20 knots before sampling which reduced stratification and mixed oxygen well into the deeper waters of the system.

The greater amount of fresh water into the Bay will affect survival of some oysters and distributions of fisheries. Oysters, fish, and jellyfish depend on certain levels of salty water to thrive in the Bay, and the freshwater from the rivers has lowered salinity levels.

  • Oysters and clams in the upper tributaries may be negatively impacted since they can’t migrate with the changes in salinity.
  • Past results (e.g., Hurricane Isabel) have shown that finfish communities respond to changes in salinity with shifts in their distributions.
  • There have been fewer jellyfish in the fresher waters of the mid to upper bay.    


 The Chesapeake Bay Program coordinates monitoring to document ecosystem conditions

  • Federal, state, and local agencies, and academic partners, coordinate their monitoring efforts through the Chesapeake Bay Program.
  • During the summer high flow conditions, NOAA shared satellite imagery tracking turbidity distributions in the bay and its tidal tributaries.
  • State’s, DC, and academic partners evaluate living resource impacts and storm effects on their habitat conditions in the watershed and Bay including impacts on SAV. 
  • During Florence there were additional efforts between the USGS working with FEMA, States, and localities to deploy real-time rapid deployment gauges and surge sensors in the lower Chesapeake Bay and at inland locations anticipated to have significant community impacts because of Hurricane Florence and the remnants of Hurricane Gordon.


For more information:

Contacts: Scott Phillips, USGS Chesapeake Bay Coordinator  (, 410 925-8098) and Peter Tango, USGS and Chesapeake Monitoring Coordinator (


Map showing real-time river flow on September 17, 2018 in to the Chesapeake Bay

Map of real-time river flows in the Chesapeake Bay watershed.
September 17, 2018
Sites with black symbols are record highs.
Sites with blue shades are well above normal.
Sites with green have normal flow levels.
Source: USGS Water Watch

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