Research Technical Report

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

Smith, D.P., G. Ruiz, R. Kvitek, P.J. Iampietro (September 2005)

Geological Society of America Bulletin 117(9):1123-1133

ABSTRACT

Recently acquired 3-m-resolution 244 kHz multibeam seafloor bathymetry (0.5 m depth precision) reveals geomorphology at sufficient 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 floor 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 floor. The sand waves are strongly asymmetrical, indicating net down-canyon 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 floor. 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). Significant changes over the six month period include: (1) complete reorganization of the sand waves on the channel floor, (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 m super(3) (+/-80,000 m super(3)) 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 m super(3) (+/-70,000 m super(3)) 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.