GEOMORPHIC SIMILARITIES BETWEEN UPPER MONTEREY SUBMARINE CANYON AND TERRESTRIAL RIVER SYSTEMS: A LOOK AT NEW HIGH RESOLUTION, 3-D, SERIAL IMAGERY

Recently acquired 3 m resolution 3 kHz multibeam seafloor bathymetry (0.1 meter depth precision) reveals geomorphic details at a scale sufficient to interpret small-scale, short-term geomorphic processes in the upper 4 km of Monterey Submarine Canyon, California. In addition to interpreting the general geomorphic processes at work, we use serial georeferenced images to discern the net changes occurring in the canyon walls and floor over both six-month and 24-hour periods. Features shared by terrestrial rivers and the upper Monterey Submarine Canyon include: meandering channel, seasonal “pool” erosion at the outsides of meander bends, terrace development and destruction, canyon-wall slumping, advection of slump material to regain grade, and sand wave morphology in high-energy sandy channels. The main changes over the six-month period were 1) net removal of canyon fill, locally leaving new 2 m tall erosional terraces on the channel margins while locally removing older terraces, 2) storage of 8800 m3 of sediment above one head tributary channel through shelf-slope break progradation, and 3) 60 m extension of a minor gully head. Changes observable over a 24 hour period include both up-canyon and down-canyon sand wave crest migration. The dominant long-term processes shaping the canyon are canyon-wall slumping, leading to widening, and channel downcutting that gradually advects the slumped material down canyon. The canyon wall slumping is likely both earthquake induced and triggered by channel undercutting. Sediment is transported by gravity flows, deep tidal flow, and “annual flushing” events. Despite the present high sea level, the upper canyon is able to efficiently transport over 500,000 m3/yr of sediment and remove slump debris without net long-term aggradation or degradation. Our data indicate that the upper canyon has not experienced net Holocene filling is presently approximately “at grade.”