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King County Water Quality Monitoring

King County monitors the ecological health of Longfellow Creek in a variety of ways including collecting and analyzing water and sediment samples. Water quality samples have been collected monthly from Station C370 located at the footbridge on SW Yancy Street, near 28th Ave SW. Sampling began in 1979 and continued until 2008 when regular monitoring was discontinued due to budget cuts. However, sampling at station C370 resumed in February 2013. In 1992 another water quality station (J370) was added further upstream at Brandon Street but monitoring at this site ended in 2008 and has not been reinstated. Sediment samples have been collected from Longfellow Creek as part of the Streams Sediment Monitoring Program starting in 1987.

From time to time special studies have been conducted at various streams in King County. Click here for information about Special Studies of Longfellow Creek.


Water Shed Image The Longfellow Creek subbasin is comprised of roughly 2,685 acres in West Seattle within the Water Resource Inventory Area (WRIA 9). The creek flows for approximately 4.2 miles through heavily urbanized areas into the West Duwamish Waterway (Kerwin and Nelson 2000). Currently, the upper 4,900 feet of the creek has been diverted into underground pipes and roughly one third of the total creek flow drains through pipes beneath shopping centers, houses and roads.

The headwaters of Longfellow Creek were historically located in a natural wetland and peat bog in what is now Roxhill Park. (Kerwin and Nelson 2000). The Roxhill bog was recently restored with the completion of the Longfellow Creek Legacy Trail in West Seattle.


Historically, Longfellow Creek contained populations of coho salmon, cutthroat trout and steelhead trout (Kerwin and Nelson 2000). In 1999, Seattle Public Utilities conducted spawning surveys on Longfellow creek and noted the presence of sixty adult coho salmon. In addition, juvenile rainbow trout and coho salmon were captured during electrofishing surveys in 1999 by Washington Trout. Numerous groups have released coho fry into the creek over the years.

Longfellow Creek has two small-unnamed tributaries. These tributaries are believed to be too small for anadromous fish use, but may be important refuge for over wintering salmonids during periods of high flow.

Key findings in the Habitat-limiting Factors and Reconnaissance Report (Kerwin and Nelson 2000) identified extensive channelization, severely altered hydrologic regime, lack of instream structures, habitat barriers, quality and quantity of gravels, and poor water quality as some of the factors affecting anadromous fish in Longfellow Creek.

Beginning 2001, NOAA's Northwest Fisheries Science Center has been studying coho pre-spawn mortality in urban streams of the Puget Sound Basin. Longfellow Creek is part of this study. Fish suffering from pre-spawn mortality show behavioral symptoms that are indicative of an underlying neurological or respiratory disorder. Fish with the disorder exhibit loss of orientation, followed by loss of equilibrium, and eventual death.

Volunteers in the King County Salmon Watcher Program have been making observations in Longfellow Creek beginning 1999 and have consistently sighted coho and chum salmon. Chinook salmon and cutthroat trout have also been spotted on occasion.

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 bacteria (FC). 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. Longfellow Creek is now categorized as “Salmonid Spawning, Rearing and Migration Habitat” for aquatic life use. The creek is designated as “Extraordinary Contact” for recreational use. Both sampling sites are listed on the 2012 Washington Department of Ecology’s (Ecology) 303(d) list for violation of DO and FC standards (Category 5). See Table 1 for a summary of water quality violations in the creek during the most recent water year.

A 2000 water quality data review summarized characterized Longfellow creek as having “fair” water quality for aquatic life (Kerwin and Nelson 2000). Under base flow conditions water quality criteria were generally met. However, criteria for copper and temperature were occasionally exceeded, and pH levels were below criteria under storm flow conditions.

To view charts of current data for Longfellow Creek, visit the links above.

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 Longfellow Creek ratings compare with other stream sites, visit the water quality index page.

Long-term Trends

A 25-year (1979 – 2004) trend analysis was conducted with baseflow water quality data from station C370 showed some significant changes in the water quality since 1979. Improvements in water quality over this time period are indicated by a significant decrease in temperature and an increase in dissolved oxygen concentrations. Water in Longfellow Creek is becoming more acidic as indicated by the significant decrease in pH. However, the pH remains within acceptable range relative to the state standards. Water quality declined with regards to nutrient enrichment as there was a significant increase in total phosphorus and total nitrogen.

Table 1. Routine monitoring summary statistics for this station from 1979 to 2017
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)16910.
Temperature (°C)26111.11.920.210.83.9
Turbidity (NTU)2787.330.60122.003.9012.35
Conductivity (mSIEMS/cm)154277.699.9342.0301.056.9
Total Suspended Solids (mg/L)24811.030.50350.003.6032.69
Ortho-Phosphorus (mg/L)2770.04450.00510.15000.04170.0206
Total Phosphorus (mg/L)2780.08810.01601.60000.07410.1112
Ammonia (mg/L)2770.03910.00310.56400.03040.0517
Nitrate (mg/L)2760.96640.37302.61000.93500.2925
Total Nitrogen (mg/L)2781.26960.17403.25001.20500.3830
Fecal Coliform(CFU/100ML)2728309190002102286

Table 2. Storm water monitoring summary statistics for this station from 1979 to 2017
ParameterNumber of SamplesMeanMinimumMaxmiumMedianStandard Deviation
Dissolved Oxygen (mg/L)0     
Temperature (°C)0     
Turbidity (NTU)574.7246.40105.0070.4021.29
Conductivity (mSIEMS/cm)0     
Total Suspended Solids (mg/L)0     
Ortho-Phosphorus (mg/L)40.05180.02630.07240.05420.0205
Total Phosphorus (mg/L)40.20430.15600.29400.18350.0621
Ammonia (mg/L)60.04190.02040.10200.03080.0315
Nitrate (mg/L)40.67380.54700.80400.67200.1425
Total Nitrogen (mg/L)0     
Fecal Coliform(CFU/100ML)0     


King County is not currently operating any stream, rain, or temperature gages in Longfellow Creek.

Stream Sediment

Sediment samples were collected from one location along Longfellow Creek in the summer of 2004 (see Map). Concentrations of detected chemicals in all samples were below all sediment quality guidelines, suggesting little potential for adverse effects in sediment-dwelling organisms.

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).

King County has not conducted regular benthic macroinvertebrate sampling on Longfellow Creek. However, the City of Seattle has occasionally monitored benthos populations in Longfellow Creek. To see this data for Longfellow Creek, please visit the Puget Sound Benthos webpage.

Special Studies

Coho Pre-spawn Mortality in Urban Streams

Research scientists in NOAA's Ecotoxicology and Environmental Fish Health Program are actively investigating the problem of coho pre-spawn mortality in urban streams of the Puget Sound Basin.