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USGS Chesapeake Bay Activities



Feature—Measuring Nutrient and Sediment Loads to Chesapeake Bay

Prepared by the U.S. Geological Survey, May 9, 2011.

The U.S. Geological Survey (USGS) monitors nutrient and suspended sediment loads delivered to the Chesapeake Bay though a long-term monitoring partnership with the States of Maryland and Virginia known as the Chesapeake Bay River Input Monitoring (RIM) Program. Through this partnership, nutrient loads are determined for nine rivers based on (1) continuous streamflow monitoring, (2) extensive water-quality sampling, and (3) advanced statistical analysis. Together, the nine stations reflect loads delivered to the Chesapeake Bay from 78 percent of its 64,000-square-mile watershed.

Chesapeake River Input Monitoring stations

Figure 1. Locations of River Input Monitoring stations in the Chesapeake Bay watershed.

Table 1. River Input Monitoring (RIM) stations and contributing drainage area

Station Name Upstream land surface area (mi2)

Susquehanna River near Conowingo, Md.

27,100

Potomac River at Chain Bridge, Washington D.C.

11,600

James River at Cartersville, Va.

6,252

Rappahannock River near Fredericksburg, Va.

1,596

Appomattox River at Matoaca, Va.

1,342

Pamunkey River near Hanover, Va.

1,078

Mattaponi near Beulahville, Va.

603

Patuxent River at Bowie, Md.

348

Choptank River near Greensboro, Md.

113

Highlights of Nutrient and Suspended Sediment Loads and Streamflow Measures from 2010

The USGS provides preliminary estimates of loads from the RIM stations to support the Chesapeake Bay Program's (CBP) Bay Barometer, an annual report that describes progress in restoration activities and conditions within the Bay and its watershed. The following findings describe the conditions observed in 2010 relative to recent and long-term conditions.

RIM Measures Most of the Nutrients Entering the Chesapeake Bay

River monitoring provides reliable measurements for 58 percent of the nitrogen and 60 percent of the phosphorus reaching Chesapeake Bay (table 2). Additional sources of information are required for land areas that are not included within monitored watersheds in order to provide a comprehensive assessment of nutrients reaching tidal waters.

The CBP uses a number of tools to determine loads from wastewater discharges, nonpoint-source loads, and direct inputs from atmospheric deposition. Wastewater discharges are compiled from reported discharges from each wastewater facility in the watershed and are summarized for those located downstream of RIM monitoring sites. Nonpoint-source loads are determined based on the Chesapeake Bay Watershed model for those areas downstream of monitoring stations. These loads include nutrients delivered from all agricultural, urban, suburban, and forest lands located downstream from monitoring stations. A significant amount of nitrogen is deposited directly to the bay waters through the atmosphere; these loads are determined by the National Atmospheric Deposition Program of the U.S. Geological Survey. These loadings are summarized for areas downstream from RIM monitoring stations and are presented along with measured loads as an indicator of total loads to Chesapeake Bay in the Bay Barometer.

The load indicator and supporting information can be viewed at: http://www.chesapeakebay.net/status_pollutants.aspx?menuitem=19795.

Table 2. Mean annual nitrogen and phosphorus loads to Chesapeake Bay, 1990–2010

Source of Load Nitrogen Phosphorus
Load in pounds per year Percent of total Load in pounds per year Percent of total

River input measured load

206,000,000 58% 12,100,000 60%

Wastewater downstream of monitoring

46,200,000 13% 2,180,000 11%

Nonpoint sources downstream of monitoring

82,100,000 23% 6,000,000 30%

Atmospheric deposition to tidal waters

20,900,000 6% -- --

 Total load to Chesapeake Bay

355,200,000   20,280,000  

Susquehanna, Potomac, and James Rivers Provide the Greatest Nutrient and Sediment Loads to Chesapeake Bay

As the three largest tributaries to Chesapeake Bay, the Susquehanna, Potomac, and James Rivers deliver the largest loads to Chesapeake Bay on an annual basis (fig. 2). Nitrogen, phosphorus, and sediment are the three greatest sources of impairment to the Chesapeake Bay and its tidal tributaries. As the largest sources of loadings, conditions in these three rivers and their watersheds are among the greatest factors influencing water quality within the bay. The USGS provides measures of nutrient loads to track the inputs of these constituents to Chesapeake Bay over time. These data support ongoing efforts to manage restoration of the Bay and its watershed, as well as to support scientific efforts to understand the response of the Bay Ecosystem to variations in nutrient and sediment inputs.

Chart showing mean annual nitrogen, phosphorus, and suspended sediment loads measured at River Input Monitoring stations, 1990-2010

Figure 2. Mean annual (A) nitrogen, (B) phosphorus, and (C) suspended sediment loads measured at River Input Monitoring stations, 1990–2010.

Long-Term Tracking of Streamflow to the Chesapeake Bay Supports Understanding of Natural Pressures on the Bay Ecosystem

The USGS measures streamflow to Chesapeake Bay through its network of real-time continuous streamflow gages and an automated Web site that delivers summarized data that places recent conditions in context with long-term measurements. Long-term annual streamflow estimates (fig. 3) show significant differences from year to year with periods of flood, drought, and normal conditions. Conditions in the estuary, which include areas of fresh, brackish, and saline water, are strongly affected by the amount and timing of freshwater that is received.

For more information about recent and historical streamflow to Chesapeake Bay, visit the USGS Chesapeake Bay inflow site at http://md.water.usgs.gov/waterdata/chesinflow/.

The USGS also provides real-time access to its network of streamflow gages at http://waterdata.usgs.gov.

Annual-mean streamflow entering the Chesapeake Bay

Figure 3. Long-term, annual-mean streamflow to Chesapeake Bay, 1938–2010.

River Loads Vary Dramatically Year to Year Based on Streamflow

Climatic variations drive the amount of nitrogen, phosphorus, and sediment delivered to Chesapeake Bay. Precipitation and its associated runoff and streamflow carry nutrients and sediment from the land surface to streams and rivers; the nutrients and sediment are eventually delivered to tidal waters. With greater amounts of precipitation, more nutrients and sediment reach the bay. The graphs below show annual loads as measured at the nine RIM monitoring sites from 1990 to 2010, along with streamflow for the same period. Nitrogen, phosphorus, and sediment loads show significant variations from year to year, and these variations are mostly related to rainfall, runoff, and streamflow conditions during the year. Over long time scales, however, the amount of nutrients delivered to the bay are expected to decline as management and restoration activities are put in place to lower the amount of nutrients applied to lands, retain nutrients and sediment on the land surface, decrease the peak runoff, and decrease wastewater inputs to streams.

USGS studies of long-term trends in nutrients and sediment in these rivers use statistical techniques to normalize for variations in streamflow in order to assess changes in concentrations and loads that are related to management and restoration activities. Figure 4 shows the cumulative nitrogen, phosphorus, and sediment loads delivered to the Bay from the nine River Input Monitoring basins. The periods of low load coincide with periods of low flow and similarly the loads are higher during years with greater streamflow.

Chart of annual nitrogen, phosphorus and suspended sediment loads

Figure 4. Annual (A) nitrogen, (B) phosphorus, and (C) suspended sediment loads measured at River Input Monitoring stations, 1990-2010.


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