In the mid-1980's, the USGS Chesapeake Bay River Input Monitoring (RIM) Program was established to quantify loads and long-term trends in concentrations of nutrients and suspended material entering the tidal part of the Chesapeake Bay Basin from its nine major tributaries. These nine rivers account for approximately 93% of the streamflow entering Chesapeake Bay from the non-tidal part of its watershed. Results of the RIM program are being used by resource managers, policy makers, and concerned citizens to help evaluate the effectiveness of strategies aimed at reducing nutrients and sediment entering Chesapeake Bay from its tributaries.
Water samples are collected upstream of the transition area between the tidal and non-tidal regions of the nine rivers (Figure 1). This transition zone historically has been referred to as the "Fall Line" for the many sets of falls and rapids that are found at this point on the rivers. Below the Fall Line in the tidal areas of these rivers, tides can transport water, nutrients, and suspended material from downstream, making it difficult to determine the cause of any observed changes. Because water-quality samples are collected above the influence of tides, any observed changes in nutrients or suspended material can be attributed to upstream causes.
Figure 1. Location of the nine River Input Monitoring sites. The general
location of each site
is shown as a solid red circle. The upstream drainage area
for each site is shaded. Detailed
information on the site and general information on
the upstream drainage area can be
obtained by clicking a region on the map or a site
name from the legend. For a detailed
version of this map click
here.
The concentration of nutrients and suspended material in a river is constantly changing. Factors such as river discharge, season, soil conditions, and land use changes affect the concentration measured on any given day. To account for these variations, one or two samples representing base-flow conditions are collected each month at every site and as many as 20 additional samples are collected at each site throughout the year during high-flow conditions.
Using the water quality data and daily records of streamflow, RIM Program scientists estimate the load of nutrients and suspended material carried to Chesapeake Bay by the nine rivers each year. Estimates of the load are of interest because they enable comparisons to be made between rivers. As an example, the Susquehanna River contributes more nitrogen to Chesapeake Bay than the Rappahannock River. Loads are also used to compute yield, or the load of nutrients and suspended material per unit area of basin. Yield computations allow one to make comparisons between basins of different sizes. For example, on average, an acre of land in the Rappahannock River Basin contributes more suspended material to the river than an acre of land in the Susquehanna River Basin. Loads and yields are the starting points from which many nutrient reduction plans are evaluated.
Sources of nutrients and suspended material broadly can be identified as coming from point or nonpoint sources. Point sources contribute nutrients and suspended material to a river at more-or-less a single point. Major point-source contributors in the Chesapeake Bay watershed include municipal wastewater treatment plants and industries. Nonpoint sources contribute nutrients and suspended material to a river over a broad area. Nonpoint nutrients may enter the river through ground water or storm-water runoff. Major nonpoint sources of nutrients and suspended material include runoff from agricultural fields, pastures, and urban areas. In general, river basins with more point sources and greater areas of urbanization and agriculture have higher yield of nutrients. A comparison of land use and point-source discharges between basins can be found in Table 1.
Table 1. Comparison of land use and wastewater discharge upstream from the nine
River
Input Monitoring sites. Click on the RIM site name for detailed information on
the site and
general information on the upstream drainage area.
| Station Name | Upstream land surface area (mi2) | Land Use (percent) | Major upstream wastewater discharges (Mgal/day) | ||||
| Urban | Agricultural | Forested | Other | ||||
| Susquehanna River near Conowingo, Md. | 27,100 | 2 | 29 | 67 | 2 | 437 | |
| Potomac River at Chain Bridge, Washington D.C. | 11,600 | 3 | 35 | 61 | 1 | 126 | |
| James River at Cartersville, Va. | 6,260 | 1 | 16 | 80 | 3 | 89.4 | |
| Rappahannock River near Fredericksburg, Va. | 1,600 | 1 | 36 | 61 | 2 | 4.7 | |
| Appomattox River at Matoaca, Va. | 1,340 | 1 | 20 | 72 | 7 | 1.1 | |
| Pamunkey River near Hanover, Va. | 1,081 | 1 | 24 | 68 | 7 | 5.0 | |
| Mattaponi near Beulahville, Va. | 601 | 1 | 19 | 69 | 11 | 0.1 | |
| Patuxent River at Bowie, Md. | 348 | 13 | 41 | 38 | 8 | 30 | |
| Choptank River near Greensboro, Md. | 113 | 1 | 50 | 29 | 20 | 0.0 | |
| [Mgal/day, millions gallons per day; land-use data from Vogelmann and others, 1998.] | |||||||
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U.S. Department of Interior,
U.S. Geological Survey
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Last modified: 12/07/04 10:35:36 AM