DOWNSTREAM CHANNEL CHANGES AFTER A SMALL DAM REMOVAL: USING AERIAL PHOTOS AND MEASUREMENT ERROR FOR CONTEXT;CALAPOOIA RIVER, OREGON

Scope/Content: Abstract: Dam removal is often implemented without adequate baseline monitoring to distinguish background variability from channel changes due to the removal. This study evaluated aerial photos as substitutes for multiple-year pre-removal field data to assess downstream channel changes associated with a small dam removal. The Brownsville Dam, a 2.1m tall concrete dam on the Calapooia River, Oregon, was removed in 2007.We mapped bars and the low flow channel downstream from the dam and in an upstream control reach using aerial photos (1994-2008) and in the field prior to (2007) and following (2008) removal. The locations and magnitudes of changes in bar area and wetted width, relative to errors, indicate that downstream channel changes were a result of the removal. The maximum changes (3520 1460m2 for bar area, 32 8m for wetted width) observed prior to dam removal with aerial photos were far downstream. In contrast, the maximum changes after removal were immediately below the dam (200 90m2 for bar area,11 3mfor wetted width), and small in the upstream control (150 130m2 for bar area, 9 4m for wetted width). The dominant errors were photo specific: exposure error for spring to summer comparisons, position error for photos not processed for this study and identification error for small scale photos not scanned from film. We found aerial photos to be an acceptable but coarse substitute for multi-

Scope/Content: Dam type: concrete

Scope/Content: Height: 5 feet. Length: 14 feet.

Scope/Content: Date constructed: 1880, 1967. Date removed: 2007.