Freshwater Flow into Chesapeake Bay

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Explore resources here describing estimates of freshwater flow entering Chesapeake Bay.

The health of the Chesapeake Bay is greatly affected by freshwater flow from rivers draining its watershed. The amount of freshwater flow (also called streamflow) will:

• Change salinity levels in the Bay, which affect oysters, crabs, and finfish.

• Influence the amounts of nutrients, sediment, and contaminants delivered to tidal waters, which affects fisheries and submerged aquatic vegetation.

The USGS provides estimates of the monthly and annual amounts of freshwater flow into the Bay so scientists and managers can better understand effects on ecosystem conditions.

Monthly Mean Streamflow Over The Past 2 Years

Estimated monthly mean streamflow entering Chesapeake Bay.

Estimated monthly mean streamflow entering Chesapeake Bay over the past two years, line 1; period-of-record minimum flow, line 2; period-of-record 25th-percentile flow, line 3; period-of-record 75th-percentile flow, line 4; period-of-record maximum flow, line 5. Monthly flow is considered to be within a normal range if between the 25th and 75th percentiles.

Click HERE to view Chesapeake Bay Estimated Monthly and Annual Streamflow Interactive Graphs and Data.

Annual Mean Streamflow into the Chesapeake Bay

Estimated annual-mean streamflow entering the Chesapeake Bay

Estimated annual-mean streamflow entering the Chesapeake Bay. Normal streamflow values, falling between the 25th and 75th percentiles of period-of-record annual-mean streamflow values are shown as green bars. Blue bars represent above normal annual streamflow values. Red bars represent below normal annual streamflow values. Streamflows are summarized by water year, which documents the natural hydrologic cycle beginning October 1 and ending September 30.

For more information

More detailed results describing estimated freshwater flow entering the Chesapeake Bay may be found on the results page.

Methods used to estimate freshwater flow are based on streamflow data from the Susquehanna, Potomac, and James Rivers, and may be found on the related science page. The data and tools page documents detailed results from each of these rivers.

Need answers to questions about Chesapeake Bay Estimated Streamflow technical issues?  Contact Douglas L Moyer or Samuel H Austin.

For more information about USGS Chesapeake studies, contact Scott Phillips or Ken Hyer.

Estimated Monthly Streamflow to the Bay

The results on this page include: (1) monthly flow to the Bay, (2) amount of flow from different watersheds, and (3) annual flow to the Bay. Summary statistics for each month are also provided. 

Overview    Recent Flow Activity    Recent Flow Statistics Table    Summary of Methods    Website Programming History 

Most recent estimated monthly mean streamflow entering Chesapeake Bay for August 2021 was 46,900 cubic feet per second. This value, (Figure 1, Line 1), which is provisional and subject to revision, is considered to be above the normal range of August streamflow. Normal streamflow entering the Bay in August is between 19,800 cubic feet per second (Figure 1, Line 3) and 40,000 cubic feet per second (Figure 1, Line 4), the 25th and 75th percentiles, respectively, of all August streamflow values over an 84-year period of record.

Recent Monthly Mean Streamflow Entering Chesapeake Bay

The Amount of Freshwater from Different Watersheds

The majority of freshwater flow enters the Bay from three major watersheds: Susquehanna, Potomac, and James (Figure 2). These three watersheds, and the remaining areas, are represented by five streamflow cross sections across the Bay (A through E; Figure 2). The amount of freshwater flow from these areas is estimated each month (figure 3).

In August 2021, the largest proportion of streamflow entered at Section A, accounting for about 63 percent of total streamflow entering the Bay. This information, along with proportional flow in recent months, is shown in Figure 3.

Map of Major Chesapeake Bay Watersheds and Streamflow Stations

 Figure 2. Map of the drainage area of the Chesapeake Bay showing five cross sections across the Bay. Incremental streamflow entering the Bay between these cross sections is shown in Figure 3.



Chesapeake Bay graph-recent monthly mean streamflow for sections A-E

Figure 3. Estimated monthly mean streamflow entering Chesapeake Bay, shown proportionally by cross sections A-E. Water passing section A is largely from the Susquehanna River watershed; water entering between sections B and C is largely from the Potomac River watershed; and water entering between sections D and E is largely from the James River watershed.


The Annual Freshwater Flow into the Chesapeake Bay 

Estimated annual-mean streamflow entering the Chesapeake Bay

Figure 4:  Estimated annual-mean streamflow entering the Chesapeake Bay. Normal streamflow values, falling between the 25th and 75th percentiles of period-of-record annual-mean streamflow values are shown as green bars. Blue bars represent above normal streamflow values. Red bars represent below normal streamflow values. Click HERE to view interactive graphs.

The estimated annual-mean streamflow entering the Chesapeake Bay over the period of record has been calculated for each of the complete water years in the period of record, beginning in 1937 as shown in Figure 4. For the most recent complete water year (2020), this value was 77,665 cubic feet per second. The long-term annual-mean streamflow is 79,000 cubic feet per second, and "normal" annual-mean streamflow is between 67,250 cubic feet per second and 88,750 cubic feet per second — the 25th and 75th percentiles of all annual-mean values, respectively. Of the most recent ten complete water years, six were normal, three were above normal, and one was below normal.

Learn more about Chesapeake Bay streamflow:

A Table Listing Summary Statistics For The Past Two Years

Streamflow summary statistics for the past two years are listed below in tabular form. Streamflow summary statistics for the full period-of-record may be found HERE, under the Data and Tools tab. Streamflow codes are identified as follows: A, approved data; B, computed using only approved data; e, value involves manual editing by USGS personnel; L, computed using a summation of other values; M, computed using a model; N, computed using more than one model; P, provisional data; Q, computed using provisional data.

YearMonthDateInflow CodeEstimated Inflow (cfs)POR MeanPOR MinimumPOR 25th PercentilePOR 75th PercentilePOR Maximum