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

Highlights

• Each year, approximately 11,000 tons of inorganic nitrogen and 2,100 tons of total phosphorus are transported by rivers and streams to Puget Sound and its adjacent waters
• Rivers that have the largest streamflows carry the largest nutrient loads
• Nutrient yields are largest from basins with higher percentages of urban and agricultural areas and that receive the highest inputs of nutrients

 

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.

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.)

Comparing loads and yields

        -------------------------------------------------------------------
        		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

	Agricultural
	 fertilizers	           9,300           1,100

	Atmospheric
	 deposition                7,800             900

 

  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.