Basin Condition Rating TRPC periodically performs a current conditions assessment for Thurston County's 70 river, stream, and lake basins. Each basin's level of urbanization, canopy cover and riparian vegetation, overall water quality, and aquatic life are used to determine the basin's condition. In 2011, 93% of basins were classified as either sensitive or impacted. Between 2006 and 2011, the number of intact basins decreased from four to two while the number of very degraded basins decreased from one to zero.
Streams and Rivers Through a system of monitoring sites, Thurston County tracks seven measures of water quality: presence of fecal coliform bacteria, dissolved oxygen, water pH, total phosphorus, temperature, presence of nitrites and nitrates, and turbidity. Using a Water Quality Index (WQI), these measures are used to indicate the level of concern for water quality impairment. The higher the WQI, the better the water quality.
For the 2017-2018 water year, only two monitoring sites were deemed to be of high concern for impairment based on the overall WQI score.
Lakes Thurston County began monitoring lake water quality in 1988. Sampling sites for lakes are typically located in the deepest part of the lake, and lakes sampled may change from year to year based on a number of factors. Monitoring on Deep Lake (in Millersylvania State Park) and Capitol Lake (owned by the State of Washington) ceased in 2011.
Of the eight lakes Thurston County currently monitors, Summit Lake has the best water quality while Black Lake has the worst.