Trends in land and impervious cover provide insight into the changing environment of Thurston County over time. Between 1992 and 2016, large-scale changes detectable from satellite imagery indicated that over 9,000 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 more than 41,000 acres between 1992 and 2016 due to logging, development, and other factors. Grassland and scrub/shrub land covers increased by more than 28,000 acres and cultivated and pasture/hay land covers increased by just over 300 acres during the same period.
In 2016, approximately 4.6% of Thurston County was covered by impervious surfaces.
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.