Impervious Surfaces

In 1989, Seattle Public Utilities (SPU) developed impervious surface ratings by land use categories. These ratings provide general information about the percentage of impervious surface associated with different kinds of development. The following table lists those percentages.

Based on the percentage of imperviousness and the land uses that occur in Thornton Creek Watershed, SPU determined that 50% of the watershed is covered by impervious surfaces. 46% of the area used by the Single Family category is impervious and 70% of Right-of-way is impervious. (Thornton Creek Watershed Characterization Report, Final DC Revision 1/31/00, page3-26). Building footprints in all land use categories account for 32% of the watershed’s imperviousness. (See Building Coverage Map)

The following table lists all land uses that occur adjacent to the tributaries:

Land Cover

The land cover map provides information on the various types of land cover found in the Thornton Creek watershed. Land cover is categorized into eight coverage types (based on a Landsat Satellite 1998 Land Cover classification developed by The Urban Ecology Lab at the University of Washington.)

The following table lists the types of coverage and the percentages in the watershed:

Vegetation along the riparian corridor helps to slow and filter stormwater as it flows toward the open tributaries. The following table provides information on the vegetative condition of the riparian corridor along the tributaries. The measurement assumes a riparian corridor of 35 meters from the centerline of the tributaries.