The US Geological Survey, in cooperation with the Maryland Department of
Natural Resources and the US Environmental Protection Agency’s
Chesapeake Bay Program, calculates the loads of nutrients and suspended
sediments contributed to the Chesapeake Bay from non-tidal areas using data collected at the
nine major Bay tributaries through the river input-monitoring (RIM) program.
In Maryland, only four major tributaries (Susquehanna, Potomac,
Patuxent, and Choptank) are included because, of the nine rivers monitored by the RIM program, most of the loads (92% of total
nitrogen and 69% of total phosphorus) are contributed to the entire Bay
through these four systems. The US Geological Survey calculates loads
with a statistical model using flow data and nutrient and suspended
sediment samples collected at these sites. Calculated loads to the Bay
are used to examine trends for effects of management actions, such as the
implementation of best management practices and upgrades to wastewater
treatment plants, and as input to the Bay watershed model.
In addition to trend assessment, calculated loads at the RIM program
stations are helpful in assessing how well Maryland and some
watersheds in several of the Bay States are doing in terms of
meeting the Chesapeake Bay watershed total maximum daily loads (TMDLs) for
nutrients and sediment. Nutrient and sediment TMDLs have been established for each
jurisdiction within the Bay watershed and each major tributary within that
jurisdiction. For example, the nitrogen allocation for the Susquehanna
River within the State of Maryland is 1.09 million pounds per year,
whereas the allocation for the Susquehanna River in Pennsylvania is 68.9
million pounds per year. Basin-wide TMDLs are 185.9 million pounds per year
for nitrogen, 12.5 million pounds per year for phosphorus, and 6.45
billion pounds per year for sediment. For a description of the
nutrient and sediment TMDLs see the following web site: (http://www.epa.gov/reg3wapd/tmdl/ChesapeakeBay/index.html
The Susquehanna River is the largest tributary to the Bay and consequently contributes the largest loads. Total nitrogen (TN) loads ranged from an annual average of 89,730 kg/day in 1999, a drought year, to 253,091 kg/day in 2004, a year characterized as record wet. The average TN load for 1985 through 2010 was 163,987 kg/day. Total phosphorus (TP) loads ranged from 2,218 kg/day in 1999 to 12,380 kg/day in 2004 with an average load of 5,744 kg/day. Suspended sediment loads ranged from 723,279 kg/day in 2001 to 8,801,376 kg/day in 1993 (a very wet year) with an average of 2,825,888 kg/day. There are no apparent trends or patterns in the loads for the Susquehanna River; loads are still largely a function of flow as evidenced by low loads during 2001 and high loads during 1993 and 2004. Loads from the Susquehanna River during lower flows are considerably lower than they might otherwise be as a result of the sediment trapping ability of Conowingo Dam and several other dams in Pennsylvania.
The Potomac River is the second largest tributary to the Bay. Total nitrogen loads ranged from 15,673 kg/day in 2002 (a drought year) to 144,140 kg/day in 1996 (a high flow year) and averaged 63,484 kg/day. Total phosphorus loads ranged from 761 kg/day in 2002 to 22,081 kg/day in 1996 with an average of 5,126 kg/day. Sediment loads on the Potomac River were highly variable and in some cases not well correlated with flow. The minimum sediment load of 230,262 kg/day occured in 2002 and the highest load of 33,483,811 kg/day was observed in 1996 (the average sediment load was 5,451,025 kg/day). There are no apparent trends or patterns in the loads for the Potomac River.
The Patuxent River is considerably smaller than the Susquehanna and the Potomac Rivers and this is reflected in the proportionally smaller loads. Total nitrogen loads ranged from an annual average of 844 kg/day in 2002 to 3,098 kg/day in 1989 (classified as a wet year) with an average of 1,986 kg/day. Total phosphorus loads ranged from 39 kg/day in 2002 to 275 kg/day in 1989 with an average of 149 kg/day. Total suspended sediment loads ranged from 9,420 kg/day in 2002 to 170,649 kg/day in 2003 with an average of 70,643 kg/day. The TN and TP load figures show that load and flow are more closely linked in the 1990s than during the 1980s. This indicates that the Patuxent has shifted from a point source dominated system to a non-point source dominated one.
The Choptank River is the smallest of Maryland’s river input tributaries and contributes the proportionally lowest loads. Total nitrogen loads ranged from 185 kg/day in 2002 to 1,295 kg/day in 2003. The average total nitrogen load is 616 kg/day. Total phosphorus loads ranged from 7 kg/day in 2002 to a maximum of 102 kg/day in 2003. Total phosphorus loads averaged 38 kg/day. Suspended sediment loads ranged from 601 kg/day in 2002 to 19,714 kg/day in 2010. The average sediment load is 7,039 kg/day. Unlike the loads in the Susquehanna and Potomac Rivers, the minimum and maximum loads for the Choptank are not well linked to the minimum and maximum flow periods possibly because the smaller drainage area results in more abrupt changes in stream flows.
*Data were provided by the US Geological Survey. Loading data for 2004-2007 are provisional due to the load modeling procedure (see USGS Water-Resources Investigations Report 00-4156 for additional information).
For more information, please contact Sherm Garrison at (410) 260-8624.
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