MAKING THE GEOLOGIC MAP OF THE OWYHEE RIVER, OR: CONFLATING MODERN AND TRADITIONAL METHODS TO CHARACTERIZE A FASCINATING FLUVIAL SYSTEM

The Quaternary evolution of the Owyhee River, OR, has involved a fascinating array of geologic processes. Foremost among these are multiple, voluminous influxes of basalt lava that have repaved the channel for 10s of km, elevated its bed by 10s of meters, and dammed it for >10,000 yrs (in at least some cases). Additionally, the river has experienced multiple valley-scale landslide events that are, in part, spatially and temporally interrelated with the lava flows. In nearly every case, the lava incursions ultimately resulted in large lateral displacement of the river and transposition of its channel against valley slopes that are susceptible to failure. Failure of those slopes pushed the river back toward the intracanyon lava flow margin resulting in more landsliding. The landslides are point- to reach-scale sources of large volumes of rock material that produce short-lived dams that often fail catastrophically. Finally, the Owyhee River has also been subject to at least one large lake-overflow flood event that was routed down one of its major tributaries. It is likely that this type of event also contributed to landsliding or reactivation of existing landslides, but the geologic details are unclear. This combination of processes has resulted in a bewildering array of composite landforms and overlapping geological deposits locally. The only way to adequately characterize this complex geologic system is through geologic mapping. I combined traditional methods of extensive field reconnaissance and observation with modern methods to create a geospatial dataset of the river corridor’s geology and geomorphology. In particular, aerial photography and LiDAR were used to help create the map in GIS; a GPS unit was used to record every traverse and waypoint over 4 years of field work; a laser rangefinder was used to measure stratigraphic sections and key elevations throughout the map area; and more than 4000 digital photographs and 9 gigapan photographs were collected in the field. Nearly all of the photographs were geotagged with the GPS data collected on each traverse. All of the data collected have been used to support the development and refinement of the geologic map and provide a rich foundation for additional study and thematic representation.