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Methodology: The Lower Nehalem Watershed Council utilized information from several sources to identify culverts in the watershed that are most likely to block migration of Coho. Our approach builds upon the GIS data layers provided by Portland State as part of their assessment of the watershed. Additional data layers were obtained from the Bureau of Land Management (BLM) and Coastal Landscape Analysis Modeling Study (CLAMS).
Data layers from the Portland State assessment:
Watershed Boundaries
Highways
Land Contours
Data layers from BLM:
24 k Stream
24 k Road
Data layer from the Coastal Landscape Analysis Modeling Study (CLAMS)
6th and 7th field
The Data Layers were displayed using ArcView GIS version 3.1. A new data layer was developed which consisted of the points at which there was a road stream crossing. Additional data was added to this layer as our analysis moved forward.
First we assigned a unique identifier to each road stream crossing. Within each 7th field we numbered the crossings starting from one. A total of 540 road stream crossings were found.
We trurned our attentions to road/stream crossings on low gradiant streams. These are the streams that are most improtant to our species of concern, Coho salmon.
We determined the approximate gradient of the stream above the road/stream crossing by counting the number of contour lines that the stream crossed in the first and second half miles above the crossing. Since a half mile is 5280/2=2640 feet and since each contour line represented a 40 foot rise in elevation each cross of a contour line represents 40/2640 = 1.5% rise in elevation. Thus crossing 4 lines in a half mile would mean a 4 X 1.5 = 6% gradient. One hundred seventy five (185) of the 540 road stream crossings were found to be below stream areas of 6% or less gradient.
Site visits were made to each of the crossings to determine if there is a fish passage problem. As of April 15, 2001 we have looked at 175 of these low gradient crossings and found 81 bridges, 60 culverts, 5 tidegates, 9 locations where the road no longer crosses the stream, and 11 crossings above a natural barrier that prevented salmon migration. We were unable to find five of the crossings and four were found to be tidally influenced and will require futher study to determine if they are fish passage barriers.
Twenty four(24) of the culverts appear to impede fish passage. We still have 10 locations left to visit.