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King County Water Quality Monitoring

The Watershed Resource Inventory Area 7 (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 town of North Bend. 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. 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 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).


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

Water Quality

Water quality samples are analyzed monthly for temperature, dissolved oxygen, pH, conductivity, turbidity, total suspended solids, ortho-phosphorus, total phosphorus, ammonia, nitrate-nitrogen, total nitrogen, and fecal coliform 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 under the 2003 rules. A short reach of the South Fork near Twin Falls State Park is listed on the 2012 Washington State Department of Ecology's (Ecology) 303(d) list for violations of pH (Category 5), dissolved oxygen (Category 4a), and fecal coliform bacteria (Category 4a) standards.

Water Quality Index

A Water Quality Index (WQI) rating system was developed by the State Department of 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, visit the Water Quality Index page.

Table 1. Routine monitoring summary statistics for this station from 2011 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)9010.98.812.311.20.9
Temperature (°C)908.
Turbidity (NTU)791.
Conductivity (mSIEMS/cm)9061.227.096.859.717.9
Total Suspended Solids (mg/L)891.930.5023.701.103.02
Ortho-Phosphorus (mg/L)890.02060.00120.08890.01030.0222
Total Phosphorus (mg/L)880.02770.00510.11200.01750.0249
Ammonia (mg/L)890.05060.00270.27000.01850.0618
Nitrate (mg/L)890.31750.09710.68300.30100.1396
Total Nitrogen (mg/L)880.43940.17101.33000.40400.1690
Fecal Coliform(CFU/100ML)90851350014394


King County is not currently operating any stream, rain, or temperature gages on the South Fork of the Snoqualmie River. However the US Geological Survey does operate a stream gage on the South Fork above Alice Creek near Garcia, WA (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 web page.

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 Benthos webpage.