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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 Middle Fork of the Snoqualmie River at the 428th Ave Bridge just upstream of its confluence with the Middle Fork. King County is not currently conducting benthic macroinvertebrate or stream sediment monitoring in the WRIA 7 Routine Streams Monitoring Program.


Water Shed Image

The Middle Fork Snoqualmie is the largest subasin in the Snoqualmie watershed, draining nearly 110,000 acres. Like the other subbasins that form the eastern headwaters of the watershed, total land use in the Middle Fork Snoqualmie basin is mostly forest. Very little is developed and the basin is not used for agriculture. The remaining land consists of scrub, wetlands, and other (barren land, grassland, open water, and perennial ice/snow). See Table 1 below for more details on land use.

Table 1. Total land use in the basin

Developed Forest Scrub Wetlands Other
Total 1% 78% 10% <1% 10%

The upper two thirds of the Middle Fork are within the Mount Baker-Snoqualmie National Forest (MBSNF), much of it within the Alpine Lakes Wilderness. In the lower basin, apart from areas in close proximity to the City of North Bend, nearly all lands are within WDNR timber lands or in the Mount Si natural resource conservation area (NRCA). King County also owns roughly 644 acres along the Middle Fork in the Middle Fork Snoqualmie Natural Area. From its confluence with the North Fork, the first mile or so of the Middle Fork lies within the King County Three Forks Natural Area.


Cutthroat trout are distributed throughout the subbasin, including headwater tributaries, while rainbow trout are thought to occupy the mainstem up to approximately Rock Creek at river (RM) 28 (Washington Department of Fish and Wildlife resident fish distribution data). Mountain whitefish, a native salmonid, is also known to occupy the lower Middle Fork (Overman, 2008).

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 North Fork of the Snoqualmie River is now categorized as “Core Summer Salmonid Habitat” for aquatic life use and “Extraordinary Contact” for recreational use. As part of the updated water quality standards, portions of the Middle Fork Snoqualmie River have been assigned an additional “Supplemental Spawning and Incubation Protection” temperature criteria of 16 °C. A short reach of the Middle Fork near its mouth is not listed on the Washington State Department of Ecology's (Ecology) 303(d) list for any violations. There are several EPA-approved Snoqualmie River Watershed Temperature and Multiparameter total maximum daily load (TMDL) plans in place and implemented to address water temperature and FC bacteria (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.

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 Middle 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 2022
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)13911.58.413.611.81.3
Temperature (°C)1398.60.419.66.84.9
Turbidity (NTU)1279.020.52236.003.4923.76
Conductivity (mSIEMS/cm)13921.59.645.120.97.1
Total Suspended Solids (mg/L)13815.340.60427.003.9844.45
Ortho-Phosphorus (mg/L)1370.00210.00050.04930.00140.0046
Total Phosphorus (mg/L)1370.01870.00500.34300.00870.0375
Ammonia (mg/L)1380.00400.00200.01560.00330.0025
Nitrate (mg/L)1380.09520.01100.26700.08960.0623
Total Nitrogen (mg/L)1370.15210.05000.61500.13150.0821
Fecal Coliform(CFU/100ML)122221420848


King County is not currently operating any stream, rain, or temperature gages on the Middle Fork of the Snoqualmie River. However the U.S. Geological Survey (USGS) does operate a stream gage on the Middle Fork near the City of Tanner, Washington (12141300).