GEOLOGIC AND ANTHROPOGENIC HISTORY OF SPOKANE REVEALED BY SUBSURFACE PROJECTIONS FOR STORMWATER DISPOSAL

Increasing population and development in urban areas pose challenges for stormwater disposal. Water Well Reports were interpreted and projected to find potential stormwater infiltration zones in the City of Spokane, such as paleochannels. The City of Spokane is generally shallow Miocene Columbia River Basalt that has been incised and covered with Quaternary megaflood deposits, however, in the city there can also be extensive anthropogenic fill. Projecting the lowest contact of encountered sand and gravel units described in well logs provided a map of the minimum depth of potentially permeable material, fill or native. Projections were also made of the upper contact of basalt as this would be the expected limiting layer for infiltration, however, the large number of shallow wells did not encounter basalt, especially to the west side of Spokane. Subsurface projections of the potentially permeable deposits yielded a series of northward draining buried channels that seem to correlate with channels presented in a late-1800’s artist rendition of Spokane. One notable paleochannel corresponds with Cowley Creek, which is now a heavily controlled stream that flows north to north west through the City of Spokane largely confined to buried pipes. Identifying paleochannels in the City of Spokane may allow for sustainable stormwater drainage.