UTILIZING GEOMORPHIC INFORMATION AS A GUIDE FOR REHABILITATING SALMONID HABITAT ALONG THE MIDDLE FORK JOHN DAY RIVER, OREGON

Anthropogenic activities over the last 150 years including vegetation clearing, logging, dredge mining, and the construction of a railroad have profoundly impacted large portions of the available salmonid habitat along the Middle Fork John Day River. Various types of geomorphic information were integrated into the current study to provide a better understanding of the physical setting and processes affecting a river system that has been highly altered historically and to provide a valuable tool in guiding salmonid habitat rehabilitation efforts. Several aspects of the geomorphology are important to physical processes that influence this river system and are largely responsible for the formation of specific salmonid habitat. The bedrock along the river is comprised of mildly folded volcanic rocks. Structural and lithological characteristics of these rocks are directly responsible for the alternating valley and canyon morphology and for the position of the river regionally as it flows along the axis of a syncline. The valley reaches are relatively wide, contain a thick sequence of alluvium, and have low gradients. Canyon reaches are narrow, alluvium-deficient, with steep slopes. Landslides are common in the basin, exert control on the channel position and form, create local grade control, and represent a source of sediment input. Alluvial fans along the margins of the valley have formed at the mouths of the tributaries to the river. These fans create local constrictions that also control the position of the river, aspects of the channel geometry, local grade control, and contribute stream flow and sediment to the main channel. The geologic structure also influences the groundwater system and numerous springs and seeps along the valley margins play an important role in the base flow and thermal characteristics of the river. Stratigraphic studies of the alluvium were undertaken to better understand the history of the river and place current physical processes into their long-term context. This is important to understanding factors that have influenced the presence of salmonid species and the development of their habitat in this basin. Results indicate an overall trend of incision over the last 7,600 years followed by a period of lateral channel migration. Preserved meander scars and abandoned channels indicate that the areal extent of the floodplain was much greater prehistorically than at present. Little is known about the native vegetation as historical accounts are limited and commonly general in nature. Therefore, pollen and charcoal samples were collected from older deposits to evaluate the type and distribution of the prehistoric vegetation. Analyzes of these samples suggest an ecosystem dominated by conifers, grasses and sedges. A significantly reduced presence of riparian species was also recognized. In addition, several species found in the paleobotanical record are not found in the immediate area today. Based the sedimentological characteristics of undisturbed alluvial deposits and the prehistoric vegetation records, it appears that areas of the Middle Fork John Day River supported relatively extensive wet meadow complexes that are largely absence today.