As part of the STRATAFORM project, a high-frequency (300 kHz) multibeam sonar survey was conducted to investigate the inner shelf of the Eel margin in August 2000.  Data were acquired with a hull-mounted Simrad EM 3000D multibeam echosounder, which simultaneously collected depth and backscatter information in a swath that was typically 8 times the water depth.  The sonar imagery was supplemented with nearly 100 grab samples in order to understand the backscatter patterns and their relationship to surface sediment distribution.

Our coverage extends from approximately 8 m water depth up to 65 m water depth, and shows the general form of the inner shelf as well as the presence of distinct features.  The larger primary Eel River delta extends offshore from 25 m to over 60 m water depth.  A distinct bathymetric bulge north of the Eel River mouth, at 15-30 m water depth, appears to be a separate nearshore depositional area.  Offshore of this deposit, the shelf flattens suggesting that this area has been a site of sediment accumulation on an otherwise gently sloping bottom.  Other distinct bathymetric features observed in our survey include dredge spoil deposits near the entrance to Humboldt Bay.  Significant backscatter variation was observed in our survey since sediments ranged from coarse sand and gravel to muds.  In addition, bedforms, typically sand waves with wavelengths of approximately 1m were observed in the sonar images, and may contribute to broad patterns in backscatter, particularly in the nearshore region.  A directional dependence of backscatter, which may be related to the presence of these bedforms, was also noted during our survey.

A multibeam survey conducted in 1995 in this region used a lower frequency system (95 kHz) and extended from the slope to the mid-shelf.  The outer limit of our survey overlaps the inner limit of the previous survey providing the opportunity to identify temporal variation on the mid-shelf.  In some areas our backscatter data agree fairly well with the previous survey, but in other places there are significant differences.  In particular, a zone of higher backscatter was noted trending NE-SW near the Mad River in the 1995 multibeam data.  However, our data show that the sediments in this region are characterized by lower backscatter than the surrounding sediments.  This change in backscatter may be related to the relocation of the Mad River mouth in 1999 when it shifted 5 km southward.  Overall, our study suggests that inner-shelf multibeam surveys provide the means to link the processes and patterns on the mid-outer shelf environment, which has been a primary focus of STRATAFORM, with the inner shelf and near-shore regions.