Spatial and temporal characterizations of Dissolved Oxygen levels in the Mill Creek Basin, (King County, WA)

Virginia Travers
Forestry

The Mill Creek basin is approximately fifteen square miles. It is located in Southwest King County and runs west of and adjacent to the Green River. According to the Green-Duwamish Watershed Non-point Action Plan, the Mill Creek basin has some of the most polluted streams draining into the Green-Duwamish river system. As a result, King County's Department of Natural Resources and the Center for Water and Watershed Studies are monitoring dissolved oxygen (DO) levels to determine the duration and timing of DO levels that are below rearing and incubation standards of salmonid species. The two streams of primary interest are Mill Creek and Mullen Slough; the Municipality of Metropolitan Seattle has consistently given Mill Creek a poor water quality rating. King County is interested in characterizing the spatial and temporal variations of DO in both surface and subsurface compartments within the basin.

An estimated 2.7 square miles of the Mill Creek basin are wetlands. During the past century, large portions of the valley have been converted to agricultural land use areas. Historically after these conversions, seasonal low levels of DO have been measured and recorded in Mill Creek and Mullen Slough. Sections of these creeks are heavily vegetated (predominately reed canary grass) along the banks and in the main channel. A section of Mullen Slough, where low levels of DO have been measured, intersects fields used for agricultural and dairy farming. The purpose of the study is to evaluate and monitor the DO levels in suface and subsurface waters of Mill Creek and Mullen Slough.

During August and September (2002), the waters flowing from Peasley Canyon entered the valley oxygenated at approximately 80% saturation or higher and close to 9mg/l then within a quarter of a mile, the DO levels dropped significantly.

Hydrolabs, multi-parameter monitoring instruments, have been deployed at several sites in the basin and are continuously collecting data at 15-minute intervals. Expansion of the study will include assessing groundwater and/or hyporheic flows and measuring DO concentrations by installing piezometers in clusters at staggered depths. During the summer (2003), Biological Oxygen Demand studies will commence to determine how much the instream growth (both algal and vegetative) contributes to the low DO.