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King County Water Quality Monitoring

King County monitors the ecological health of the Boise Creek basin in a variety of ways including collecting and analyzing water quality samples. Water quality samples have been collected monthly since 2015 from one station in Boise Creek. Station BSE_1MUDMTNRD is located near the downstream side of SE Mud Mountain Road in Enumclaw, WA.

Boise Creek is part of the Puyallup River Basin in Watershed Resource Inventory Area 10 (WRIA 10). WRIA 10 drains approximately 1,065 square miles which includes the Puyallup River and two primary tributaries, the White and Carbon Rivers. Together, these rivers comprise the Puyallup drainage that empties into Puget Sound at Commencement Bay near the city of Tacoma, Washington.

From time to time additional studies have been conducted in the Boise Creek basin. Click here for information about Special Studies of Boise Creek.


The Boise Creek watershed drains approximately 15.4 square miles, with the headwaters originating from the Cascade Mountains east of the City of Enumclaw, Washington and continuing down to meet up with the Carbon River around RM 23.3. The Boise Creek main channel runs 12.2 miles and is designated a “shoreline of the state” from approximately RM 11 down to the confluence with the White River. The upper basin is primarily forestland (~65%) and lies within the Forest Production District. The lower extents of the basin drain the City of Enumclaw as well as a mix of rural residential, agriculture, and commercial lands. Around 3% of the land area is comprised of farms enrolled in the farmland preservation programs of King County which means that they will maintain agricultural character into the foreseeable future (King County, 2012).


The Boise Creek basin is known to have the following salmonids: Chinook, coho, and steelhead (Kerwin 1999). The distribution of anadromous species is limited by a natural falls at RM 4.4, with resident rainbow and cutthroat trout inhabiting the reaches upstream.

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.

State water quality standards were revised in 2003. Boise Creek is now categorized as "Core Summer Habitat" for aquatic life use and "Primary Contact" for recreational use. Boise Creek is on the Washington Department of Ecology’s (Ecology) 303(d) list, Category 5, for violation of water temperature and pH standards.

Boise Creek has impaired water quality requiring remedial action, resulting primarily from increased nutrient loading, which can fuel algal growth and, in turn, increase temperature and biological oxygen demand within the system (King County, 2004). Alteration to pH within Boise Creek, which is generally caused by chemicals such as de-icers (alkalinity), fertilizers (alkalinity), agricultural production (acidity), or by natural causes from organic matter (acidity) and minerals (acidity or alkalinity) associated with agricultural production, are indicators that delivery of nutrients, toxins, and pathogens is increasing (King County 2004).

In the Puyallup River watershed fecal coliform total maximum daily load (TMDL) area, Boise Creek in the largest fecal coliform loading source of any tributary requiring the largest fecal coliform reduction of any dry season source (Mathieu and James, 2011). Boise Creek has subsequently been identified as the number one priority for improvement within the Puyallup TMDL study area. High bacterial concentrations occur in Boise Creek during summer low flow and winter high discharge conditions (Mathieu and James, 2011, Timm. 2013). Fecal coliform (FC) load reduction targets are substantially higher during low flow conditions and have been defined for more locations than during winter months. But, during both seasons, FC reduction targets are high. Depending on location in the watershed, reduction targets range from 57% to 92% in summer, to 61% to 67% in winter in order to achieve Washington State water quality guidelines of 100 colony forming units (cfu) per 100mL (Mathieu and James, 2011). The high summer FC loadings suggest non-storm driven sources such as leaking sewer pipes, septic systems, or cross-connections that could be implicated in bacterial contamination. High numbers during the wet season likely suggest runoff from the landscape as a contributor as well. As some potential sources are human in origin, increased exposure via contact with water from Boise Creek to bacterial and viral pathogens could be a significant public health concern (Timm. 2013).

Table 1. Routine monitoring summary statistics for this station from 2015 to 2018
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)3811.19.313.411.31.0
Temperature (°C)389.82.318.68.83.9
Turbidity (NTU)385.081.0556.202.2010.12
Conductivity (mSIEMS/cm)3874.246.2105.071.912.4
Total Suspended Solids (mg/L)387.241.5084.802.4015.83
Ortho-Phosphorus (mg/L)380.02200.00660.20000.01510.0310
Total Phosphorus (mg/L)380.04470.01630.28000.03020.0486
Ammonia (mg/L)380.06110.00431.51000.00910.2442
Nitrate (mg/L)380.57830.11801.40000.56000.3130
Total Nitrogen (mg/L)380.82970.25703.00000.68450.5373
Fecal Coliform(CFU/100ML)38187782096209


King County maintains two streamflow gauges in Boise Creek (69b) and 69c).

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. To see the benthic macroinvertebrate data for Boise Creek, please visit the Puget Sound Benthos webpage.

Special Studies

Boise Creek Bacterial Source Tracking Study: 2012 Summary of Findings (Timm, 2013)

This study was conducted to understand potential sources of microbial contamination in Boise Creek in an effort to help address the impairments identified in the Puyallup River Fecal Coliform TMDL. Specifically, this microbial source tracking study focused on water quality impairments in Boise Creek related to a history of bacterial contamination. For more information, download a copy of the report.

Puyallup River Watershed Fecal Coliform Total Maximum Daily Load Water Quality Improvement Report and Implementation Plan (Mathieu and James, 2011)

During 2006-07, Washington State Department of Ecology conducted this study to help evaluate the TMDL needs for the Puyallup River Watershed. The study focused on collecting bacteria and streamflow data from 55 sites throughout the study area twice per month. These data were analyzed to determine how much the current bacteria levels needed to be reduced to meet the water quality standards. For more information, download a copy of the report.

Boise Creek Rapid Rural Reconnaissance Report (King County, 2004)

In order to gain a better understanding of drainage systems, streams, and habitat issues in the Boise Creek, the King County Council authorized, in 2001, the accelerated development of drainage plans for this area. The Boise Creek Rapid Rural Reconnaissance Report provides a general overview of the existing stream and basin conditions and problems related to surface water in these rural areas and identify high priority capital improvement needs and opportunities. For more information, download a copy of the report.