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USGS Fact Sheet FS-009-98
March 1998

Nutrient Transport in the Major Rivers and Streams of the Puget Sound Basin, Washington

by E.L. Inkpen and S.S. Embrey




Why are nutrients important in the Puget Sound Basin?

The nutrients phosphorus and nitrogen are essential for the life of plants and animals. However, an overabundance of nutrients in surface water can cause overgrowth of aquatic plants such as algae. This overgrowth can lead to a variety of water-quality problems, including daily, wide swings in concentrations of dissolved oxygen. Fish and other aquatic life can be adversely affected by these swings if the concentrations become too low.

High nutrient concentrations and associated water-quality degradation are of concern in some Puget Sound Basin rivers and lakes near intensive agriculture and population centers (Butkus and Lynch, 1996). A recent study by the Washington State Department of Ecology (Newton and others, 1997) also identified several marine embayments of Puget Sound as sensitive to developing low dissolved oxygen levels as a result of high nutrient inputs. This fact sheet evaluates sources and transport of nutrients in the major rivers to help determine factors that affect the quantity of nutrients reaching Puget Sound and its adjacent waters.

The Puget Sound Basin National Water-Quality Assessment team compiled historical data on nutrient concentrations and streamflows for 22 rivers and streams in the Puget Sound Basin. The data were used to estimate loads and yields of inorganic nitrogen (nitrate, nitrite, and ammonia), organic nitrogen, and total phosphorus for the period 1980-1993 (Embrey and Inkpen, in press). Total nitrogen (inorganic plus organic nitrogen) and total phosphorus inputs to basins from agricultural, urban, and atmospheric sources were also estimated.


What factors affect nutrient loads and yields?

Each year, the major rivers and streams of the Puget Sound Basin transport loads of approximately 11,000 tons of inorganic nitrogen, and 2,100 tons of total phosphorus to Puget Sound and its adjacent waters (fig 1). About 9,900 tons of organic nitrogen are transported by 8 of the largest Puget Sound Basin rivers. Data were not available to compute organic nitrogen loads for all rivers and streams.

[map of nutrient loads, Puget Sound Basin]Figure 1 (GIF, 61,554 bytes)
Annual nutrient loads transported by Puget Sound Basin rivers and streams.


As expected, the largest nutrient loads are carried by the rivers having the largest basins and consequently the largest streamflows. The Skagit and Snohomish River Basins together comprise 47 percent of the drainage area of the Puget Sound Basin, and transport about 49 percent of the inorganic nitrogen and 45 percent of the phosphorus loads carried by rivers to Puget Sound and its adjacent waters.

Inorganic nitrogen yields in tons per square mile vary with concentrations of inorganic nitrogen in water (fig. 2). The Samish River and Issaquah Creek have the highest concentrations and yields, whereas the Dungeness and Elwha Rivers have some of the lowest. Nutrient yields also vary with land use . Yields from basins in the eastern part of the Puget Sound Basin that are dominantly influenced by urban or agricultural land use, or a mix of both, are on average three times larger than yields from the forested basins in the Olympic Mountains (figs. 3 and 4). (Dominant Land Use: Although most basins in the Puget Sound Basin have large forested areas, the types of land use in the lower parts of the basin where water-quality samples are taken are considered the dominant land use.)

[graph of N yield and inorganic N]Figure 2 (GIF, 8,577 bytes)
Nitrogen yield varies with concentrations of inorganic nitrogen in water.


[map of nutrient yields, Puget Sound Basin]Figure 3 (GIF, 55,665 bytes)
Annual nutrient yields from river basins in the Puget Sound Basin.


[map of rivers and land use, Puget Sound Basin]Figure 4 (GIF, 102,347 bytes)
Rivers and land uses of the Puget Sound Basin.


Comparing loads and yields


Loads carried by the Skagit and Samish Rivers are indicative of their basin sizes. Yields are indicative of these basins' land use. The Skagit River yield is more influenced by the large percentage of its basin that is forest, and the Samish River Basin yield is more influenced by the agricultural areas (fig. 4).

------------------------------------------------------------------- Inorganic Inorganic nitrogen nitrogen load yield Dominant Basin (area) (tons/yr) [(tons/mi2)/yr] land use =================================================================== Skagit R. (3,090 mi2) 2,800 0.9 forest Samish R. (87.8 mi2) 250 2.8 agriculture ====================================================================


What are major sources of nutrients?

The top three sources of nutrients and their annual inputs, in tons, to the Puget Sound Basin are:

				Nitrogen	Phosphorus

	Animal manures	          15,000           3,000

	 fertilizers	           9,300           1,100

	 deposition                7,800             900


[table 1: nutrient inputs, yields; land use]  Table 1 (HTML/GIF, 22,216 bytes)
Annual nutrient inputs, yields, and land use of rivers
draining to Puget Sound and adjacent marine waters


Table 1 shows the inputs from animal manures, agricultural fertilizers, and atmospheric deposition to each basin. These sources are considered non-point sources of nutrients because they are distributed over the basin landscape, and the nutrients are transported to rivers in surface runoff and ground-water discharge. Some portion of the nutrients are retained on soil particles or taken up by plants and so are prevented from entering surface waters and becoming part of the river load. For this reason, the non-point sources are considered as potential sources of nutrients to the rivers. Other non-point sources that might be important in urban areas but for which data were not available include domestic fertilizer applications and septic systems.

In urban areas, wastewater treatment plants (WWTPs) are typically an important source of nutrients. WWTPs are known to be point sources of nutrients because the nutrients are discharged directly to a river and become part of the transported load. In the Puget Sound Basin, however, most WWTPs discharge directly to Puget Sound or its adjacent waters and so are generally not a major source of nutrients to the rivers. Two exceptions are the Puyallup River, where WWTPs account for 22 percent of the transported nitrogen load, and the Snohomish River, where WWTPs account for 15 percent of the transported phosphorus load (table 1). Other point sources that might be important but for which data were not available include nutrients from commercial and industrial discharges.

How does land use affect nutrient inputs?

In the Puget Sound Basin, land use is a major factor that influences what sources will contribute nutrients to a river's basin, and, in part, the quantity of inputs. The Samish and Nooksack Rivers drain basins dominantly influenced by agriculture. These two basins receive the largest nutrient inputs (up to 10 (tons/mi2)/yr of nitrogen and up to 1.5 (tons/mi2)/yr phosphorus), 90 percent of which comes from animal manures and agricultural fertilizers (table 1). In contrast, the Green River Basin, which has more urban areas and less agricultural areas, receives from manures and atmospheric deposition less than half the nutrient input to the Samish and Nooksack Basins. Forested basins receive the smallest inputs of nutrients, and atmospheric deposition is the only major nutrient source identified (table 1). Other nutrient sources to forested watersheds could include recreation, logging, and other forest practices, but the lack of data on human activity and natural processes in those areas prevented a more comprehensive estimate.

Yields of inorganic nitrogen from river basins are related to total nitrogen inputs from the major non-point sources (fig. 5). The Samish and Nooksack Rivers in the eastern part of Puget Sound Basin, where there are greater areas of urban and agricultural land use, have some of the highest yields, and these basins also receive the largest nutrient inputs (table 1). In contrast, the forested watersheds, such as the Elwha and the Duckabush in the Olympic Mountains, yield considerably smaller amounts of nutrients and have much smaller nutrient inputs. However, phosphorus yields are not related to inputs of phosphorus, most likely because of its tendency to adsorb to soil particles.

[graph, N yield and N input]Figure 5 (GIF, 7,633 bytes)
Estimated nitrogen yields vary with nitrogen inputs.


Nutrient inputs to a basin are generally greater than the yield from that basin. This is to be expected because some nutrients will be removed by natural processes before they reach a river. In the Hamma Hamma, Skokomish, Dosewallips, and Duckabush River Basins, however, yields are greater than inputs. Because so little information regarding nutrient inputs and sample concentration data were available for these basins, estimates of inputs and yields for these basins were made with less confidence than for other basins.

Future plans and nutrients in the Puget Sound Basin

The Puget Sound Basin National Water-Quality Assessment team plans to refine estimates of nutrient loads for the Nooksack River, Skokomish River, and Green River Basins using data collected monthly and also during storms and floods. To estimate total nitrogen loads, organic nitrogen will also be included. New data on nutrient concentrations and loads will help to further the understanding of the relation between land use and water quality.

References Cited:

Butkus, Steve, and Lynch, Donna, 1996, 1996 Washington State water quality assessment, section 305(b) report: Olympia, Wash., Washington State Department of Ecology, Water Division, Water Quality Program, No. WQ-96-04 (Revised), August 1996, 98 p.

Newton, J.A., Albertson, S.L., and Thompson, A.L., 1997, Washington State marine water quality in 1994 and 1995: Olympia, Wash., Washington State Department of Ecology, Environmental Investigations and Laboratory Services Program, No. 97-316, 71 p.

This fact sheet is based on the report :

Embrey, S.S., and Inkpen, E.L., 1998, Nutrient transport in rivers of the Puget Sound Basin, Washington 1980-1993: U.S. Geological Survey Water-Resources Investigation Report 97-4270, 30 p.

National Water-Quality Assessment (NAWQA) Program

The Puget Sound Basin is part of a national program consisting of more than 50 NAWQA study areas nation-wide. Teams in each study area will assess the water quality of surface- and ground-water resources, how the water quality changes over time, and determine what natural and human factors affect the water quality.

For more information contact:

Project Chief
Puget Sound Basin NAWQA
1201 Pacific Ave., Suite 600
Tacoma, WA 98402
(253) 428-3600
email: gs-w_nawqa_pugt_wa@usgs.gov

World Wide Web: http://wa.water.usgs.gov/projects/pugt/index.htm

USGS Fact Sheet 009-98
March 1998

Water Resources of Washington State

Water Resources of the United States

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