- The Columbia River system in the Pacific Northwest is currently comprised of a series of
reservoirs, resulting from regulation of river flow due to a series of hydroelectric dams.
A return to pre-dam conditions might improve salmon (Oncorhynchus sp.) survival rates
and increase migration efficiency. This study modeled both historic and current river
conditions for a part of a reservoir on the Columbia River in order to characterize the
impact of a river reservoir drawdown to historic (pre-dam) water levels. The study area is
a 48-kilometer stretch of the John Day reservoir in the middle Columbia region that is
currently being considered for water level drawdown. Within this study, historic pre-dam
aerial and oblique photographs and hard-copy bathymetric survey maps were
incorporated into a GIS (Geographic Information System) to characterize the wetted
perimeter of the river, substrate distribution, water depth, and overall channel
morphology of the original channel. The current river conditions were also characterized
using digital orthophotos, hydrographic survey data, and fisheries sampling information.
The spatial distribution of habitat in the historic and current river channels were
compared using existing knowledge of salmonid behavior and habitat requirements in
large river systems for adult spawning activities. The results of this study can be used to
indicate similarities and differences between historic and present conditions, which may
help designate how well a "return to the river" drawdown concept could be implemented
to achieve the desired increase in preferred salmon habitats.
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