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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 the City of Enumclaw, Washington.

Boise Creek is part of the Puyallup River Basin in Water Resource Inventory Area (WRIA) 10. WRIA 10 drains approximately 1,065 square miles, which includes the Puyallup River and 2 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 on Boise Creek . Click here for information about Special Studies.


The Boise Creek watershed drains approximately 15.4 square miles. Its headwaters originate from the Cascade Mountains east of Enumclaw and continue down to meet up with the Carbon River around river mile (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. Land use in the basin is a mix of agriculture, developed, and forest. Forestland is mostly evergreen forest, agriculture is almost entirely pasture/hay, and development is a mix between low intensity and open space. At 13%, there is more scrub than agriculture. Other types of land use consist of 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 11% 14% 53% 13% 3% 6%


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 (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.

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 State Department of Ecology’s (Ecology) 303(d) list for violation of water temperature and pH standards (Category 5). water quality results 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 are indicators that delivery of nutrients, toxins, and pathogens is increasing (King County, 2004). Alteration to pH is generally caused by chemicals such as de-icers (alkalinity) and fertilizers (alkalinity), agricultural production (acidity), or by natural causes from organic matter (acidity) and minerals (acidity or alkalinity) associated with agricultural production.

The creek has an EPA-approved Puyallup River Bacteria total maximum daily load (TMDL) plan in place and implemented (Category 4a). In the Puyallup River watershed FC TMDL area, Boise Creek is the largest FC loading source of any tributary requiring the largest FC 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). FC load reduction targets are substantially higher during low flow conditions and have been defined for more locations than during winter months. However, 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 to bacterial and viral pathogens via contact with water from Boise Creek could be a significant public health concern (Timm, 2013).

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.

Table 1. Routine monitoring summary statistics for this station from 2015 to 2019
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)5911.19.313.411.11.0
Temperature (°C)5910.02.318.69.04.0
Turbidity (NTU)594.691.0556.202.298.44
Conductivity (mSIEMS/cm)5975.046.2105.073.912.0
Total Suspended Solids (mg/L)596.630.8084.802.4013.31
Ortho-Phosphorus (mg/L)590.02010.00660.20000.01330.0274
Total Phosphorus (mg/L)590.04460.01570.28000.03020.0466
Ammonia (mg/L)590.05480.00421.51000.00870.2153
Nitrate (mg/L)590.53220.11801.40000.47300.3054
Total Nitrogen (mg/L)590.79000.25703.00000.70500.4956
Fecal Coliform(CFU/100ML)591927820100196


King County operates two streamflow gages in Boise Creek: Boise at 276th Ave SE(69C) and Boise at 268th Ave SE (69B).

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

Special Studies

Lower Boise Creek Channel Restoration Project

The Lower Boise Creek Channel Restoration Project is located on a 15 acre parcel at the confluence of Boise Creek and the White River near Enumclaw. In the 1930’s, Lower Boise Creek was routed into a ditch beside the railroad making the creek straight, steep and rocky. Spawning gravels, slow water, and cover greatly declined. Restoration began in 2009 and was completed in 2010. As a result of the project, more salmon and trout used the site and their numbers were higher than in a comparable reach upstream.

Boise Creek Bacterial Source Tracking Study: 2012 Summary of Findings

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.

Puyallup River Watershed Fecal Coliform Total Maximum Daily Load: Water Quality Improvement Report and Implementation Plan

During 2006 - 2007, Ecology conducted a 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.

Boise Creek Rapid Rural Reconnaissance Report

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, problems related to surface water in these rural areas, and high priority capital improvement needs and opportunities.