SPRAGUE RIVER, OREGON; GEOLOGIC FRAMEWORK STUDIES FOR ESTABLISHING RESTORATION PRIORITIES

The Sprague River in south-central Oregon has been a target for aggressive restoration by state and federal agencies because (1) of its role as principle water source for upper Klamath Lake; (2) the river provides spawning and rearing habitat for listed fish species; and (3) several local landowners are motivated to improve river and riparian conditions. Ongoing and planned investment in the river corridor has prompted a multi-year, multi-approach analysis of river processes and conditions so to guide cost-effective, efficient, and sustainable management. Our effort has focused on documenting pre-settlement, historic, and current conditions to identify key channel and floodplain processes, and how these processes have been affected by geologic, climatologic, and land-use conditions. Our approach combines (1) surficial geologic (and soils) mapping and stratigraphy, (2) GIS analysis of historic channel and floodplain morphology, channel migration, and riparian vegetation conditions (on the basis of historic photos, maps, and LiDAR topography), and (3) quantitative analysis of sedimentation patterns resulting from high flows of early 2006. Preliminary results of overall geologic framework studies indicate long-term structural control of valley segments and spring inflows, with low-gradient and high-sinuosity alluvial reaches occupying basins between canyon segments formed in uplifted fault blocks and volcanic constructional features. Large springs are mainly where faults intercept the valley bottom. Within the alluvial reaches, which are the focus of most restoration efforts, the channel has shifted by avulsion and lateral migration for at least the last 3500 years, including several historic meander cut-offs. Late Holocene channel dynamics have also been profoundly affected by the ca 6700 yr BP Mount Mazama eruption, which introduced a tremendous volume of sand-size pumice into an otherwise sand-poor drainage basin, in part by a large sediment laden flood down the tributary Sycan River.