INTERPRETATION OF TECTONIC, FLUVIAL AND EOLIAN LANDFORMS IN THE UPPER COACHELLA VALLEY, CALIFORNIA, USING AERIAL PHOTOGRAPHY, DEM AND LIDAR TECHNOLOGY

Competing late Quaternary tectonic, fluvial and eolian processes shaped the landforms in the upper Coachella Valley region of southeastern California. The upper Coachella Valley is bounded to the north and south by major strands of the San Andreas fault system. The largest fluvial landform in the area is an alluvial piedmont plain constructed by major drainages from the San Bernardino Range. Other important features include the Long Canyon alluvial fan, pressure ridges associated with major fault strands, and sand dune deposits derived from intermittent streambeds to the west. Recognition and interpretation of subtle landscape features are critical to quantifying the 100-year flood hazard and fault rupture hazards in this rapidly developing area. In this study, landforms around Desert Hot Springs, California, were first evaluated using modern and historical air photo coverage. Aerial photographs taken immediately after historical flood events in 1974 and 1991 help to identify small, stable overflow channels on the alluvial piedmont. Modern air photos show details of development since the floods. USGS DEMs provide a convenient and inexpensive source of digital elevation data for two-dimentional surface water flow analysis. These data are too coarse to render useful images of subtle landforms, however, and often do not provide adequate datapoint density for modeling surface flow in critical areas such as drainage divides. These issues are addressed by LiDAR technology, which provides datapoints on a 0.5-meter grid and yields a datapoint density 400 times greater than even a 10-meter DEM. Rendered into a three-dimensional “image,” LiDAR data permit the identification of extremely subtle landscape features, facilitated by simulation of any desired illumination angle or direction. LiDAR data can also be used to generate detailed topographic profiles with a 15 cm vertical resolution. LiDAR images of the alluvial piedmont provide evidence for a family of subtle, previously unrecognized, linear channels oriented subparallel to the nearby Mission Creek fault. To the southeast, the channels project toward aligned sand dunes on the younger Long Canyon fan. These channels may represent subtle tectonic landforms or possibly relict channels related to earlier drainage regimes.