SOILS IN THE WATERSHED

The surface geology within the Thornton Creek Watershed is dominated by the presence of vashon till, which covers 73.6% of the total watershed area. (See soilarea and surface geology types) This is significant because vashon till generally has less established vegetation and higher runoff rates compared to other types of surface geology. This type of underlying geology presents challenges to storm water strategies focused on the infiltration and recharge of water on-site, as percolation rates will be lower.

Seventy-three (73%) of residential family homes in Thornton Creek Watershed are located on vashon till. In order to reduce the run-off contribution of single family homes in the watershed located within this till area (See %landuse), it may be appropriate to consider retention/detention strategies. Infiltration strategies may be more appropriate within a surface geology made up of Esperance Sand (see surface geology map). This type, which has higher rates of percolation covers 14% of the Thornton Creek watershed (see soilarea).