DYNAMIC RIVERSCAPES: CHANNEL MIGRATION ZONE MAPPING FOR THE MCKENZIE RIVER, OREGON

The McKenzie River represents a dynamic fluvial system in the southeastern end of the Willamette River basin, Oregon. Despite significant anthropogenic changes during the 20^th century, including construction of large dams, levees, revetments, and other confining structures, more than 40 km of the banks of the McKenzie River have migrated laterally by dozens if not several hundred meters since the late 1960s, threatening people and infrastructure.

The Oregon Department of Geology and Mineral Industries (DOGAMI) produced channel migration zone (CMZ) maps for the lower 108 km of the McKenzie River in eastern Lane County, Oregon. The components that comprise these CMZ maps include: the active channel; historical migration area; high, medium, and low erosion hazard areas; areas subject to avulsion; and flagged stream banks. The method used was primarily based on the interpretation of historical aerial photographs and lidar-based topographic, geologic, and flood inundation maps. This study shows that modern channel migration is concentrated in the lower 42 km of the river where avulsions and bank retreat 100 to more than 300 m have been recently observed.

CMZ maps are designed to aid in community planning, raise awareness of flood and erosional hazards, and inform decisions about emergency management and land use. The maps may be used to identify which buildings, critical facilities, transportation infrastructure, and utility lines are potentially at risk from channel migration and to prioritize areas for pre-disaster risk reduction. CMZ maps can also be used to identify areas where the modern active channel has moved beyond the Federal Emergency Management Agency’s regulatory floodway in their Flood Insurance Rate Maps.

These CMZ maps are the first of their kind published for the McKenzie River. Consequently, they serve as an important baseline from which future morphological changes may be assessed. For example, these datasets can be used to determine if erosion rates are changing through time, or for evaluating how channel migration responds to events, such as large floods, wildfires, or the removal of dams. Finally, these data provide critical information to better understand the impact of climate change and land use modifications.