Freshwater Flow into Chesapeake 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. 

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Most recent estimated monthly mean streamflow entering Chesapeake Bay for September 2020 was 32,000 cubic feet per second. This value, (Figure 1, Line 1), which is provisional and subject to revision, is considered to be within the normal range of September streamflow. Normal streamflow entering the Bay in September is between 17,000 cubic feet per second (Figure 1, Line 3) and 36,000 cubic feet per second (Figure 1, Line 4), the 25th and 75th percentiles, respectively, of all September streamflow values over an 84-year period of record.

Recent Monthly Mean Streamflow Entering Chesapeake Bay

Figure 1: Estimated monthly mean streamflow entering Chesapeake Bay. Click HERE to view interactive graphs.

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 September 2020, the largest proportion of streamflow entered at Section D-E, accounting for about 32 percent of total streamflow entering the Bay. This information, along with proportional flow in recent months, is shown in Figure 3.

 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.

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 

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:

• View Chesapeake Bay recent monthly streamflow compared to historic conditions
• View Chesapeake Bay period-of-record summary statistics
• View and download Chesapeake Bay data for historic monthly and annual streamflow
• Read about Chesapeake Bay streamflow estimation methods
• Read about the 2019 water year record setting annual flow

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.