Many watershed processes are influenced by bedrock lithology. The geologic formations that underlie each watershed determine how groundwater moves; how stream channels form; how soils form, weather, and erode; and many other watershed characteristics that directly and indirectly influence salmonid habitat. Bedrock lithology in the study area is mapped in the shapefile geo62500_m6.shp. We obtained the original layer (geo62500.shp) from the MCWC GIS, intersected it with 6th fields, and analyzed for igneous versus sedimentary formations. We divided the 39 separate formations in the original layer into three main groups: 1) igneous formations; 2) sedimentary formations; and 3) quaternary alluvial and colluvial deposits.

Summary of results Sedimentary formations occupy the largest area (252,107 ha or about 67%). About a quarter of the study area is occupied by igneous formations, with the remaining 8% consisting of quaternary formations. The Alsea Basin has the largest absolute area of sedimentary formations (98,704 ha or about 381 sq mi). The Siletz Basin has the highest absolute area of igneous formations (36,786 ha or about 142 sq MI). The Ocean Tributaries basins have the highest area of Quaternary formations (11,139 ha or about 43 sq MI).

Information on lithology helps interpret the results of other analyses. For example, knowing the underlying lithology can help in interpreting stream substrate data. Gravels, cobbles and boulders formed from igneous rock tend to be quite durable, compared to those formed from sedimentary formations, which may break down within periods of tens to hundreds of years (Siuslaw National Forest, 1997 #32).

Spreadsheet Data: geo62500_m6.xls

Shapefile: geo62500_m6.ZIP

Metadata: MD_geo62500_m6.doc

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Please address questions or comments to:

Dr. Ralph Garono
Principal Investigator
Earth Design Consultants, Inc.
rgarono@earthdesign.com

Laura Brophy
Principal Investigator
Green Point Consulting
brophyl@peak.org | homepage