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King County Water Quality Monitoring

The Water Resource Inventory Area (WRIA) 7 Routine Streams Monitoring Program was established in 2011 and consists of twelve sampling sites that are distributed throughout the Snoqualmie and Skykomish watersheds. Historically in King County, routine streams monitoring has been centered on WRIA 8 (Lake Washington/Cedar/Sammamish) and WRIA 9 (Green-Duwamish). However, in 2011 a Surface Water Management (SWM) fee increase allowed King County to expand its regular water quality monitoring efforts to include the King County portions of the WRIA 7 drainage. Objectives of this program are focused on quantifying long-term water quality trends to help inform the management of salmon recovery efforts, land use regulation, and to prepare for expected increases in climate variability.

King County collects monthly water quality samples on the Tolt River between its mouth and the Tolt Ave Bridge near the town of Carnation. King County is not currently conducting benthic macroinvertebrate or stream sediment monitoring in the WRIA 7 Routine Streams Monitoring Program.


Water Shed Image

The Tolt River sub-basin is one of the largest in the watershed. At 61,500 acres, it is larger than the South Fork Snoqualmie and roughly the same size as the North Fork Snoqualmie. The subbasin is significant and unique on a number of fronts. The South Fork Tolt reservoir is a core component of the regional water supply system. The geology of the basin - combined with historical and current forestry practices - have resulted in a history of landslides and erosion, with likely impacts on turbidity, particularly during high flows (Kaje, 2009).

Total land use in the Tolt River sub-basin is predominantly forest and over half of the forest is evergreen. The next largest land use is scrub. Developed, agriculture, and wetlands are very small portions of the sub-basin that amount to less of total land use combined than other (barren land, grassland, and open water). See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Agriculture Developed Forest Scrub Wetlands Other
Total <1% <4% 66% 21% 2% 8%%


The Tolt River basin is one of the most significant spawning areas for Snoqualmie Chinook salmon and for other salmonids. The Tolt River is also home to the only known naturally occurring spawning population of summer steelhead in the basin.

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen (DO), pH, conductivity, turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, total nitrogen, and fecal coliform (FC) bacteria. Results are compared to state water quality standards. Water quality standards are designed to protect public health and aquatic life. Comparing monitoring results to water quality standards allows an understanding of how safe the creek is for recreational contact as well as for aquatic life (see link at top of page to view current water data).

State water quality standards were revised in 2003. The Tolt River is now categorized as “Core Summer Salmonid Habitat” for aquatic life use and “Primary Contact” for recreational use. As part of the updated water quality standards, the Tolt River has been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 13 ºC to be applied from September 15th to June 15th. A short reach of the Tolt River near its mouth is listed on the Washington State Department of Ecology's (Ecology) 303(d) list as a "waters of concern" (Category 2) for pH, though no major violations of have been observed on the Tolt River. The river has an EPA-approved Snoqualmie River Watershed Temperature total maximum daily load (TMDL) plan in place and implemented (Category 4a).

See Table 2 below for routine monitoring summary statistics of water quality data collected to date. If stormwater data is available for this site, it will be shown as Table 3. Historical data reviews can be found in the annual reports produced by METRO/King County DNRP.

To view charts of current water quality data, please visit the Data Download webpage.

Water Quality Index

A Water Quality Index (WQI) rating system was developed by Ecology that evaluates several water quality parameters and gives a single rating of “high,” “moderate,” or “low” water quality concern. To see how Tolt River ratings compare with other stream sites, please visit the Water Quality Index webpage.

Table 1. Routine monitoring summary statistics for this station from 2011 to 2022
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)14011.79.613.911.71.0
Temperature (°C)1408.83.417.57.83.7
Turbidity (NTU)1287.850.35118.001.6718.43
Conductivity (mSIEMS/cm)14040.720.6104.040.210.0
Total Suspended Solids (mg/L)13914.910.50256.002.2042.68
Ortho-Phosphorus (mg/L)1380.00290.00060.07380.00180.0077
Total Phosphorus (mg/L)1380.01830.00500.19100.00810.0303
Ammonia (mg/L)1390.00440.00200.04950.00330.0051
Nitrate (mg/L)1390.24050.09970.43800.23200.0838
Total Nitrogen (mg/L)1380.31000.15700.62900.29950.0920
Fecal Coliform(CFU/100ML)122141180626


King County operates multiple stream gages on the Tolt River:

The U.S. Geological Survey (USGS) also operates several stream gages within the Tolt River basin. Station 12148500 is located just downstream of the confluence of the North and South Forks. Station 12147500 is located on the North Fork of the Tolt near the regulating basin. Station 12148300 is located on the South Fork near the regulating basin. Station 12148000 is just downsteam of the South Fork Tolt Reservoir and station 12147600 is located upstream of the reservoir.

Benthic Invertebrates

Benthic macroinvertebrates are small animals visible to the naked eye (macro) that lack a backbone (invertebrate) and live in or around the streambed (benthic). This group includes aquatic insects (such as mayflies and dragonflies), crustaceans, clams, snails, and worms. Benthic macroinvertebrates are of interest to scientists and water resource managers because they are an excellent indicator of the biological health of stream ecosystems and are a critical component of the food web in aquatic communities. Scientists quantify the composition and diversity of benthic macroinvertebrate populations in a stream to compare the biologic integrity of different streams. King County has utilized benthic macroinvertebrate sampling to assess biological health of numerous creeks across the county (see map).

For more information about benthic macroinvertebrates and King County’s Benthic Macroinvertebrate Program, please visit the King County Stream Bug Monitoring webpage.

While King County is not conducting benthic macroinvertebrate monitoring on the Tolt River as part of the WRIA 7 Routine Streams Monitoring, King County collected benthic data from two tributaries of the Tolt, Lynch and Stossel Creeks, in 2006. To see this data, please visit the Puget Sound Stream Benthos webpage.