Record Freshwater Flow in Water Year 2019 Affects Conditions in the Chesapeake Bay

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The U.S Geological Survey (USGS) reports that freshwater flow into the Chesapeake Bay during water year (WY) 2019 was the highest flow on record (fig. 1). The record freshwater flow washes more pollutants into the Chesapeake Bay and affects dissolved oxygen and habitat conditions for oysters, crabs, and finfish. The 2019 water year is the period from October 1, 2018, through September 30, 2019, which follows the natural hydrologic cycle.

The annual average freshwater flow into the Chesapeake Bay during WY 2019 was 130,750 cubic feet per second, which is the highest annual amount since 1937, the first year for which data are available. The previous record of 121,125 cubic feet per second was set in WY 1972 and was influenced by Hurricane Agnes. 

Figure 1. Annual mean streamflow into the Chesapeake Bay, water years 1937–2019.

Figure 1. Annual mean streamflow into the Chesapeake Bay, water years 1937–2019.

The record annual flow in WY 2019 was driven largely by the combined effects of above-normal monthly flows from October 2018 through February 2019 and the preceding above-normal monthly flows from May 2018 through September 2018 (fig. 2). Figure 2 shows monthly mean flows for WYs 2018 and 2019 (the solid blue line) relative to the observed range of flow conditions. Any monthly flow over the 75th percentile is considered above normal.

Rankings of monthly flows were extremely high during the first half of WY 2019. Monthly flows in October 2018, November 2018, December 2018, January 2019, February 2019, May 2019, and June 2019 were the 4th, 1st, 3rd, 8th, 5th, 10th, and 8th highest flows for their respective months, over the period of record beginning in 1937.

High flow conditions diminished overall from March 2019 through September 2019, and monthly flows dropped to within normal levels in August 2019 and September 2019 as a drought spread over the mid-Atlantic area. These changes, however, did not compensate for the exceptionally wet conditions that set up and produced the record-breaking WY 2019 flows.

Figure 2. Monthly mean streamflow into the Chesapeake Bay

Figure 2. Monthly mean streamflow into the Chesapeake Bay, water years 2018–2019, and minimum, maximum, 25th-percentile, and 75th-percentile flows for the period of record, water years 1937–2019.

Both WY 2018 and WY 2019 had above-normal conditions, but the timing of the high flows affected the Bay differently. Some of the primary effect on the Bay included:

Poor Dissolved Oxygen for Fisheries. High flows during the winter and spring of WY 2019 came during a critical time of year when the nutrients delivered to the Bay fuel algal blooms, which cause low dissolved oxygen in the summer. Low dissolved oxygen levels less than 2.0 mg/l (or hypoxia) are harmful to oysters, crabs and fish.  The high flows, and associated nutrient loads, during WY 2019 contributed to  low summer dissolved-oxygen levels in the Bay that were the 3rd worst recorded in Maryland waters, according to the Maryland Department of Natural Resources. During WY 2018, however, the high flows began later in the year, resulting in fewer low dissolved-oxygen levels during the previous summer.

Less Salt Harms Oysters: The high freshwater flows also made the Bay less salty, affecting oysters and other fisheries. Below-average salinities in the middle of the Chesapeake Bay persisted for 17 consecutive months, beginning in May 2018 (fig. 3). Record-low salinity levels were observed in February 2019 (fig. 3), with other months near record-low levels. Similar conditions prevailed on tidal tributaries such as the Potomac River, as evidenced by below-average salinities near the mouth of the river at Point Lookout, Maryland, from June 2018 to September 2019, with record-low salinities during January–March 2019 (Maryland Department of Natural Resources, 2019b). The salinity distribution in the Bay and its tidal tributaries rebounded to near-average conditions in September 2019.

Oyster deaths occurred in the upper Potomac River and upper Chesapeake Bay and were associated with shifts in suitable habitat caused by extended exposure to fresher (lower salinity) conditions. The low salinities also slowed some of the oyster restoration projects in tidal waters. However, reduced salinities also suppressed the prevalence of least one oyster disease (Dermo) (Wheeler, 2019).

Figure 3. Chesapeake Bay salinity levels at mid-bay station CB4.2C from January 2018 to September 2019.

Figure 3. Chesapeake Bay salinity levels at mid-bay station CB4.2C from January 2018 to September 2019. Data from Maryland Department of Natural Resources (2019a).

Spread of invasive species: The blue catfish, an invasive species in the Chesapeake, has spread over the last two summers due to the lower salinity levels. (Blankenship, 2019). Biologists and state fishery managers fear harm from the blue catfish, which can grow to 5 feet, to native species such as blue crabs, yellow perch and white catfish. Scientists are conducting studies to understand their potential impacts, with the USGS working with MD DNR on a new study in the Patuxent River.


Supporting information

The USGS estimates freshwater flow into the Chesapeake Bay each month, and more detailed information describing freshwater flows, including interactive graphs and data, may be found HERE and HERE.

The USGS also produces trends of nutrients and sediment in the watershed, and their delivery to the Bay, which can be found here

The Chesapeake Bay Program (CBP) is working with States and the District of Columbia to reduce nutrients and sediment being delivered to the Bay to improve water-quality conditions and habitat for fisheries. The CBP uses monitoring information from the Chesapeake Bay watershed and tidal areas to help evaluate how the Bay is responding to restoration efforts. More information on monitoring in the Bay and tidal waters, including dissolved oxygen and submerged aquatic vegetation, will be available in 2020, and can be found at


References Cited

Blankenship, K. 2019. Invasive blue catfish spread further into Virginia rivers. Chesapeake Quarterly, accessed October 24, 2019 at

Maryland Department of Natural Resources, 2019a, Fixed station monthly monitoring—Chesapeake Bay mainstem—MD mid bay (CB4.2C): Maryland Department of Natural Resources, Eyes on the Bay, accessed October 22, 2019, at

Maryland Department of Natural Resources, 2019b, Fixed station monthly monitoring—Lower Potomac River—Point Lookout (LE2.3): Maryland Department of Natural Resources, Eyes on the Bay, accessed October 22, 2019, at

Wheeler, T.B., 2019, Maryland oysters take a hit from a year of extreme rain: Bay Journal, accessed October 22, 2019, at


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