REMOTELY OPERATED VEHICLE (ROV) VIDEO INVESTIGATION OF TWO LARGE SEAFLOOR MOUNDS IN SOUTHERN HOOD CANAL, WASHINGTON

Two large seafloor mounds in southern Hood Canal, Washington, located directly off the mouths of the Dewatto and Little Dewatto Rivers, have previously been interpreted as dredge spoils or as drumlins. It was previously shown that they were embedded too deeply in the substrate to be dredge spoils, and we demonstrate that their shapes are inconsistent with Puget Lowland drumlins, which are larger and more streamlined than the Dewatto mounds. Suggestions that the mounds may be natural gas seeps are dispelled by the coarseness of the observed mound material and the lack of vent features. We made direct visual observations on these two mounds by using a remotely operated vehicle (ROV) operated from a small boat navigated by the Washington Department of Fish and Wildlife. In addition, we have been mapping the adjacent uplands aided by high resolution LiDAR imagery.

The traversed mounds are mostly composed of well-rounded, cobble-sized clasts that appear much like compact Olympic provenance outwash gravels and tills found just above sea level in the bluffs of Dewatto Bay and in the submarine scarp at the mouth of the bay. While most other streams that empty into Hood Canal have deltas, both Dewatto River and Little Dewatto River lack deltas, suggesting submarine collapse of their deltas and implying relative recency. The inferred headwalls of both mounds are very steep, leaving little lateral support of the shoreline, that may be subject to future subaerial or submarine failures. One such failure has been mapped on the south shore of DeWatto Bay. These features are at the west margin of the Seattle and Tacoma fault uplift and the faults in the vicinity of Price Lake, all of which have been active in the last thousand years. We suggest that these features may be seismically induced and compare them to other inferred earthquake-induced landslides in the area.