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 October 2019 was 33,000 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 October streamflow. Normal streamflow entering the Bay in October is between 22,100 cubic feet per second (Figure 1, Line 3) and 55,700 cubic feet per second (Figure 1, Line 4), the 25th and 75th percentiles, respectively, of all October streamflow values over an 83-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 October, 2019, the largest proportion of streamflow entered at Section A, accounting for about 55 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 (2019), this value was 130,750 cubic feet per second, the highest annual flow on record. Detailed discussion of this record breaking annual flow during the 2019 water year may be found HERE. The long-term annual-mean streamflow is 79,600 cubic feet per second, and "normal" streamflow is between 67,500 cubic feet per second and 89,800 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
2019102019-10PN330004440091402210055700177000
201992019-09PN210003830084801690038575207000
201982019-08PN320003450098101927540000132000
201972019-07PN5400039600111002495044550122000
201962019-06PN9500065200179004125075200342000
201952019-05PN1470001000003290062300128000221000
201942019-04PN1540001420005620095700179000389000
201932019-03AN16000014500051800104000182000301000
201922019-02AN1820001020003610069400134000231000
201912019-01AN166000910001750057200118000249000
2018122018-12AN208000842001730050800107000222000
2018112018-11QeN21500060400146003110078900215000
2018102018-10QeN1320004440091402210055700177000
201892018-09QN2070003830084801690038575207000
201882018-08QN1320003450098101927540000132000
201872018-07QN10000039600111002495044550122000
201862018-06QN11400065200179004125075200342000
201852018-05QN1610001000003290062100128000221000
201842018-04QN1500001420005620095625178250389000
201832018-03QeN10400014500051800103250178250301000
201822018-02QN1580001020003610069225132500231000
201812018-01QN70000910001750056775115000249000
2017122017-12QN28500842001730050275106250222000
2017112017-11QN6010060400146003110078900215000
2017102017-10QN245004440091402210055700177000
201792017-09BN211003830084801690038575207000