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King County Water Quality Monitoring

The Water Resource Inventory Area (WRIA) 7 Routine Streams Monitoring Program was established during 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 South Fork of the Snoqualmie River at Snoqualmie Valley Trail Bridge, just north of the City of North Bend, Washington. King County is not currently conducting benthic macroinvertebrate or stream sediment monitoring in the WRIA 7 Routine Streams Monitoring Program.


Water Shed Image

The South Fork Snoqualmie originates near Snoqualmie Pass and flows more than 30 miles before joining the mainstem Snoqualmie near the City of North Bend, Washington. Total contributing area above the sampling location is greater than 52,000 acres. For most of its length, the South Fork follows Interstate 90. Most of the watershed is within the Mount Baker – Snoqualmie National Forest, and Olallie and Twin Falls Washington State Parks. The South Fork flows through the City of North Bend before passing through King County owned Tollgate Farm32, Mount Si Golf Course, and a small portion of the Three Forks Natural Area at the confluence with the mainstem (Kaje, 2009).

Total land use in the South Fork Snoqualmie River basin is dominated by forest (mostly evergreen) followed by scrub, developed, and other (barren land, grassland, and open water). Agriculture, as well as wetlands, represent a very small proportion of land use. 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% 9% 70% <15% <1% 6%


Resident cutthroat trout, rainbow trout, and mountain whitefish are found in the mainstem and numerous tributaries of the South Fork, with cutthroat ascending into headwater areas.

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 South Fork of the Snoqualmie 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, portions of the South Fork Snoqualmie River have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 16 °C. A short reach of the South Fork near Twin Falls State Park is listed on the Washington State Department of Ecology's (Ecology) 303(d) list for violations of pH standards (Category 5). There are two types of EPA-approved total maximum daily load (TMDL) plans in place and implemented: a Snoqualmie River Watershed Multiparameter TMDL to address DO and FC bacteria issues; and a Snoqualmie River Watershed Temperature TMDL (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 South Fork of the Snoqualmie 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 2023
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)15010.98.812.511.20.9
Temperature (°C)1508.
Turbidity (NTU)1381.570.2588.000.687.48
Conductivity (mSIEMS/cm)15063.821.1112.764.618.9
Total Suspended Solids (mg/L)1493.710.50215.001.0019.46
Ortho-Phosphorus (mg/L)1490.02050.00100.10700.01030.0229
Total Phosphorus (mg/L)1480.02930.00510.19900.01810.0288
Ammonia (mg/L)1490.04010.00220.30300.00900.0603
Nitrate (mg/L)1490.30340.09180.82100.28400.1348
Total Nitrogen (mg/L)1480.41330.14601.33000.38450.1653
Fecal Coliform(CFU/100ML)124671350014335


King County is not currently operating any stream, rain, or temperature gages on the South Fork of the Snoqualmie River. However the U.S. Geological Survey (USGS) does operate a stream gage on the South Fork above Alice Creek near the town of Garcia, Washington (12143400).

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 South Fork of the Snoqualmie River as part of the WRIA 7 Routine Streams Monitoring, King County (Roads) collected benthic data from several tributaries of the South Fork in 2010. To see this data, please visit the Puget Sound Stream Benthos webpage.