Upstream Sources of Nitrogen via Rivers and Streams

Ecology has studied nitrogen from upstream sources to Puget Sound by testing water at the mouths of rivers.

Upstream sources include agriculture, urban stormwater runoff, atmospheric deposition to land surfaces, permitted-source effluents such as wastewater treatment plants with outfalls in rivers, septic systems, and natural sources, such as vegetation and wildlife, as well as groundwater.

Nitrogen entering rivers from upstream sources also undergo various instream transformations before entering Puget Sound. We don’t yet know how much of the nitrogen load at the mouth of a particular river is from a particular upstream source.

Sources to rivers

Ecology’s estimates of nitrogen loading from rivers and streams presented in this section were developed to support ongoing studies and modeling efforts which required loading estimates at the mouth of rivers and streams entering Puget Sound. The estimates presented in this section therefore include nitrogen from all upstream watershed sources.

Some of the nitrogen delivered to Puget Sound via rivers is in the form of baseflow, which is the portion of river flow from groundwater. Additional groundwater enters Puget Sound directly via submarine groundwater discharge. Best available information from USGS suggests that the flow volume of submarine groundwater discharge is small, especially relative to total river flows.

In addition to groundwater, stormwater also contributes flow and nitrogen to rivers. Stormwater runoff includes water that runs off impervious surfaces and into the nearest waterway or storm drain during rain events.

Concentrations, Loads, and Yields explains some of the ways that we describe the amount of nitrogen from various sources.

River streamflow estimates

Ecology’s Puget Sound Dissolved Oxygen Modeling study estimated flows and loads from all rivers discharging to Puget Sound. U.S. watersheds contributed an average annual flow of 55,600 cubic feet per second (cfs). This is comparable to the USGS estimate of 50,000 cfs. Ecology’s Puget Sound Toxics Assessment project estimated the total flow from U.S. watersheds as 56,300 cfs, based on 30-year average rainfall, evapotranspiration, and runoff patterns.

Largest Rivers in Terms of Flow

River Annual Streamflow Draining Area
Skagit River  16,300 cfs 3,200 mi2
Snohomish River  10,100 cfs 1,870 mi2
Stillaguamish River 4,600 cfs 740 mi2
Puyallup River 3,400 cfs l,050 mi2
Nisqually river  2,100 cfs 720 mi2

Canadian watersheds contribute an additional 166,100 cfs to the Straits, over 60% of which is flow from the Fraser River.

Nitrogen river concentrations

Generally, lower nitrogen concentrations are found in the rivers that drain the Olympic Peninsula. These watersheds remain largely forested and have significantly smaller populations and fewer human sources of nitrogen. Nitrogen concentrations are higher in rivers in South and Central Puget Sound watersheds, as well as those rivers that discharge north of Whidbey Basin. These watersheds are generally more densely populated, more urbanized, and/or have more agricultural activities.

Nitrogen concentrations are higher in rivers in South and Central Puget Sound watersheds, as well as those rivers that discharge north of Whidbey Basin. These watersheds are generally more densely populated, more urbanized, and/or have more agricultural activities.

Concentrations of nitrogen in rivers that drain into Puget Sound (Source: Mohamedali et al., 2011a)

Nitrogen river loads

The total annual DIN load to Puget Sound from rivers and streams is 22,600 kg/d. Rivers that contribute the largest flows to Puget Sound are generally the ones that also contribute the largest nitrogen loads, even though there are smaller streams with higher concentrations.

Nitrogen loads from Puget Sound rivers (Source: Mohamedali et al., 2011a)

The five rivers with the largest nitrogen loads to Puget Sound are:

Rivers with Largest DIN Loads

River N kg/d
Snohomish:  5,940 kg/d 
Skagit:  3,860 kg/d 
Stillaguamish:  2,440 kg/d 
Puyallup:  2,100 kg/d 
Green:  1,640 kg/d 

The Nooksack River, which discharges to Bellingham Bay, has a nitrogen load of 4,180 kg/d, which is the largest nitrogen load of all U.S. rivers north of Puget Sound.

When Canadian rivers are included, the Fraser River is, by far, the largest freshwater inflow to the region (103,200 cfs, with a watershed area of 90,880 mi2) and has the largest river nitrogen load (33,140 kg/d). However, since the Fraser River discharges to the Strait of Georgia, nitrogen contributions from this region likely have a lesser influence on Puget Sound.

Relative contribution to Puget Sound Basins

River load contributions vary across the different regions of Puget Sound with the largest loads from rivers entering Whidbey Basin (56%), and South Sound (18%).  Relative DIN load entering the seven main basins of Puget Sound are illustrated below.

Annual U.S. river DIN loads into different regions of Puget Sound (Source: Mohamedali et al., 2011a).

Load variation by season

The total nitrogen load from rivers to Puget Sound varies seasonally from approximately 10,000 kg/d to 50,000 kg/d. This is in contrast to wastewater treatment plants, which do not show such a large seasonal variation. River loads are significantly lower in late-summer months (July – September) when streamflows are at their lowest, and higher loads occur during months with higher rainfall (November – January).

Nitrogen river yield

Another way to compare nitrogen loading between rivers and streams is to normalize the load by watershed area (i.e. calculate the yield) and then calculate relative loads by multiplying the individual river yields (load/area) by the total yield of the whole region.

This relative load allows comparisons across watersheds of different sizes. The Nooksack, Samish and Stillaguamish Rivers, along with several watersheds located in South Puget Sound, have the highest loads compared to their watershed size.

Comparison of nitrogen yields from Puget Sound Rivers (Source: Mohamedali et al., 2011a).