This study provides information on channel and flood-plain processes and historical trends to guide effective restoration and monitoring strategies for the Sprague River Basin, a primary tributary (via the lower Williamson River) of Upper Klamath Lake, Oregon. The study area covered the lower, alluvial segments of the Sprague River system, including the lower parts of the Sycan River, North Fork Sprague River, South Fork Sprague River, and the entire main-stem Sprague River between the confluence of the North Fork Sprague and the South Fork Sprague Rivers and its confluence with the Williamson River at Chiloquin, Oregon. The study included mapping and stratigraphic analysis of flood-plain deposits and flanking features; evaluation of historical records, maps and photographs; mapping and analysis of flood-plain and channel characteristics (including morphologic and vegetation conditions); and a 2006 survey of depositional features left by high flows during the winter and spring of 2005–06.

Analyses focused on the channel and flood plain within an area defined as the “geomorphic flood plain,” an area encompassing active fluvial and riparian processes. The geomorphic flood plain was subdivided into 13 valley segments of distinct fluvial environments on the basis of valley form and major tributary junctions: nine segments span the 136.1 kilometers of main-stem Sprague River, two segments for the lower Sycan River, and one segment for each part of the South Fork Sprague and North Fork Sprague Rivers within the study area. Segment characteristics range from steep and narrow canyons to low-gradient reaches with expansive flood plains. The wide flood-plain valley segments are broadly similar; most contain a sinuous, low-gradient channel that migrates slowly across the valley bottom. The narrow valley segments include the steep, boulder-and-cobble-bed reaches at downstream and upstream ends of the study area as well as other confined valley segments that have similar gradients and substrates as adjacent unconfined valley segments, but much lower sinuosities. Although the geologic setting of the expansive South Fork valley segment resulted in historical conditions of sinuous and poorly defined channels and wet meadows, flanking levees now narrowly confine the channelized South Fork Sprague River for much of its length.

Stratigraphic analyses show that before the Mazama eruption of 7,700 calendar years before present, wetlands and low flood plains flanked the main rivers of the study area. The eruption, however, covered much of the northern basin with sand- and granule-size pumice clasts, transforming channels by increasing bed-material transport and promoting bar formation and channel migration, particularly for the Sycan and North Fork Sprague Rivers, and for the Sprague River downstream of the Sycan River confluence. The South Fork Sprague River, which had much less Mazama pumice deposited in its watershed, remained a wet-meadow fluvial system until historical channelization and diking.

The analysis of historical maps and aerial photographs covering the geomorphic flood plain show changes in sinuosity, migration rates, and vegetation conditions since the 1800s. Most quantitative information is for the period between 1940 and 2000. The decrease in sinuosity since 1940 for nearly all the unconfined reaches resulted partly from decreased migration rates, but mostly from several cutoffs and avulsions formed between 1940 and 1975. The river shortening and steepening possibly resulted from (1) flood-plain confinement by levees, dikes, roads, and railroads leading to deeper and faster overbank flow, thereby promoting erosion of new flood-plain channels; and (2) flood-plain disturbances such as trails, ditches, and vegetation manipulation or eradication that locally concentrated overbank flow and decreased surface resistance to channel erosion.

The most evident vegetation change has been the loss of short woody vegetation adjacent to the river channels: only one-half the near-channel area covered by short woody vegetation in 1940 was similarly covered in 2000. Woody vegetation removal in the 1950s and 1960s and continuing grazing and trampling by livestock probably are the main reasons for the decrease in short woody vegetation from the dense riparian corridors of willows (Salix sp.) and other riparian shrubs noted in the early 20th century.

The alluvial corridor of the South Fork Sprague River, compared to other Sprague River Basin rivers, has been the most substantially transformed since first historical observations. The present channel is incised, straightened, and separated from the rarely inundated flood plain by levees.

Despite these effects of human disturbances, many of the fundamental physical processes forming the Sprague River fluvial systems over the last several thousand years still function. In particular, flows are unregulated, sediment transport processes are active, and overbank flooding allows for floodplain deposition and erosion. Therefore, restoration of many of the native physical conditions and processes is possible without substantial physical manipulation of current conditions for much of the Sprague River study area. An exception is the South Fork Sprague River, where historical trends are not likely to reverse until it attains a more natural channel and flood-plain geometry and the channel aggrades to the extent that overbank flow becomes common.