Freshwater Quality

  1. Basins
  2. Rivers and Streams
  3. Lakes

Data Tables



Basin Condition Rating

TRPC periodically performs a current condition assessment for Thurston County’s 70 river, stream, and lake basins based on their level of urbanization, canopy cover, and riparian vegetation. In 2016, 60% of basins were classified as either sensitive or intact, down from 63% in 2001. TRPC projects that only 59% of basins will be intact or sensitive in 2045.

Streams and Rivers

Through a system of monitoring sites, Thurston County tracks six measures of water quality: presence of 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 2020-2021 water year, 1 out of 34 monitoring sites was deemed to be of high concern for impairment based on the overall WQI score.


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 Capitol Lake (owned by the State of Washington) ended in 2011.

Of the ten lakes Thurston County monitored for the 2020-2021 water year, Summit Lake had the best water quality. All other lakes reported in that water year had poor or fair quality.


  • Thurston Regional Planning Council
  • Thurston County