Rivers

Rivers and streams are a pathway that delivers upstream watershed sources of nitrogen to Puget Sound. These river loads contribute the second-largest local input of inorganic nitrogen to Puget Sound (after wastewater loading). There are about 13 major rivers that discharge to Puget Sound, in addition to a number of smaller rivers, creeks, and streams. Rivers transport and transform nitrogen loads from all upstream sources within the watersheds that they drain. Therefore, human activities in the watershed, land use/land cover, and in-stream processes all affect how much nitrogen is in our rivers and streams and how much of that eventually enters Puget Sound.

River nitrogen loading varies by season

This map shows the difference between annual (light blue) and summer (dark blue) dissolved inorganic nitrogen (DIN) loads from rivers entering Puget Sound and the Straits. Values are annual averages over the years 1999 through 2008.

The total DIN load from rivers and streams entering Puget Sound is about 22,600 kg/day, though the load varies seasonally from approximately 10,000 to 50,000 kg/day. This is in contrast to wastewater treatment plants, which do not show such a large seasonal variation. River loads are significantly lower in the late summer months (July – September average) when streamflows are at their lowest, and higher loads occur during months when we get most of our rainfall (November – January). These estimates are based on a statistical technique that uses monthly monitoring data for concentrations and daily flows and may underestimate loads during storm events.

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.

Top 5 rivers in terms of dissolved inorganic nitrogen load

(estimates are annual averages from 1999-2008)

  1. Snohomish River – 5,945 kg/day
  2. Skagit River – 4,225 kg/day
  3. Stillaguamish River – 2,440 kg/day
  4. Puyallup River – 2,105 kg/day
  5. Green River – 1,635 kg/day

When Canadian rivers are included, the Fraser River is the largest freshwater inflow to the Salish Sea (103,200 cubic feet per second) and has the largest river nitrogen load (33,140 kg/day). The influence of the Fraser River within Puget Sound waters has been studied and is believed to be most significant during fall and winter (Banas et al., 2015).

Relative loads & spatial variation

This map shows relative loads of dissolved inorganic nitrogen (DIN) from different Puget Sound watersheds. The relative load is a way to normalize the load from watersheds by taking into account differences in watersheds size (area) so that differences in load are not just a result of differences in watershed size.

The Nooksack, Samish, and Stillaguamish Rivers, along with several watersheds located in South Puget Sound, have the highest relative DIN loads compared to others watersheds in Puget Sound. The watersheds in the Olympic Peninsula, which have the least amount of development, have the lowest relative DIN loads.

River nitrogen load contributions vary across the different regions of Puget Sound with the largest loads from rivers entering Whidbey Basin (56%), and South Sound (18%).

Upstream watershed sources

This image shows the results of the USGS SPARROW model for the Pacific Northwest. SPARROW (Spatially Related Regressions on Watershed attributes) is a watershed model that is used to estimate nutrient conditions (total nitrogen and total phosphorus) and identify important nutrient sources in catchments of the Pacific Northwest region for 2002. 

Though the model is not optimized for the Puget Sound region, it includes the Puget Sound region, tracks various land-based sources of nitrogen within smaller watersheds and subbasins, and routes these sources to downstream water bodies.

The SPARROW model output can be viewed in more detail to access maps of nutrient contributions from specific watersheds in the Pacific Northwest.