Semiannual patterns of erosion and deposition in upper Monterey Canyon from serial multibeam bathymetry

Recently acquired 3-m-resolution 244 kHz multibeam seafl oor bathymetry (0.5 m depth precision) reveals geomorphology at suffi cient detail to interpret small-scale features and short-term processes in the upper 4 km of Monterey Canyon, California. The study area includes the continental shelf and canyon features from 10 m to 250 m depth. The canyon fl oor contains an axial channel laterally bounded by elevated complex terrace surfaces. Sand waves with 2 m height and 35 m average wavelength dominate the active part of the canyon fl oor. The sand waves are strongly asymmetrical, indicating net downcanyon sediment transport in this reach. Terraces, including a broad 25-m-tall terrace complex near the head of the canyon, bear evidence of recent degradation of the canyon fl oor. Slump scars and gullies having a variety of sizes and relative ages shape the canyon walls. Serial georeferenced digital elevation models were analyzed to detect net changes in bathymetry or morphology occurring during both a six month period (September 2002 to March 2003) and a 24-h period (24 March to 25 March). Signifi cant changes over the six month period include: (1) complete reorganization of the sand waves on the channel fl oor, (2) local channel degradation creating new 2-m-tall erosional terraces on the channel margins, (3) local channel widening that laterally eroded older channel margin terraces, and (4) 60 m extension of one minor gully head on a steep canyon wall. There were no discernable changes in morphology during the 24-h study period. Raster subtraction of serial bathymetric grids provides estimates of sediment erosion and deposition that occurred between the canyon head and a point 2 km down canyon during the six month study. Erosion of 320,000 m3 (±80,000 m3 ) of sediment occurred mainly in the tributaries, along the margins of the axial channel, and in the lowest 700 m of the analyzed reach. This eroded volume was approximately balanced by 260,000 m3 (±70,000 m3 ) of sediment deposition that was concentrated in the nearshore region along the rim of the canyon head. There was no measurable sediment gain or loss during the 24-h study period.