Trends in land and impervious cover provide insight into changing physical environment of Thurston County over time. Between 1992 and 2011, large-scale changes detectable from satellite imagery indicated that over 8,000 acres of land were converted to low, medium or high-density developed land covers.
While forests still cover over 40% of the county's area, forested land covers declined by over 42,000 acres between 1992 and 2011 due to logging, development, and other factors. Grass and agricultural lands have increased by more than 25,000 acres during the same period.
Wastersheds or basins that have an impervious land cover of more than 10% are generally assumed to have degraded water quality. Two watersheds in Thurston County were near or above this level in 2014: Henderson Inlet, with approximately 17.3% impervious surface coverage; and Budd/Deschutes with 9.4%.
Impervious surfaces include roads, driveways, rooftops, and other compact surfaces that do not allow rain to infiltrate into the soil and groundwater. Watersheds with large areas of impervious surfaces tend to have more runoff, which increases erosion and washes pollutants directly into streams, lakes, and Puget Sound.