DNR and the Washington State Department of Fish and Wildlife (WDFW) joined forces recently to get a close-up look at a pair of large, dome-shaped mounds on the sea floor west of Dewatto Bay and Little Dewatto Bay in southern Hood Canal. Investigators wanted to see if the mounds—each 115 to 130 feet high and 1,150 to 1,475 feet wide—might be formations known as drumlins, deposits left by the glaciers that once covered this area but rarely seen under the waters of Puget Sound.
With seabed depths of 400 feet or more, the only practical way to get direct visual observations of these features was to use a small, submersible remotely operated vehicle (ROV) that the two agencies own. The ROV is equipped with a high-resolution video camera with locating lasers, and operated from a WDFW-owned boat.
Video observations sent back by the ROV revealed that the dome-shaped features are not likely drumlins (which tend to be ridge-shaped). Because of the coarseness of the deposits, they were definitely not caused by natural gas seeps. Nor were they determined to have been caused by geothermal venting.
The consensus was that the mounds are likely deposits from underwater landslides.
This joint venture focusing on geologic features in Hood Canal proved to be quite informative to WDFW biologists. Specifically, they got video documentation of several species of rockfish from an underwater location not easily accessible. WDFW marine fish scientists also gained valuable insight into the spatial distribution and habitat use by rockfish in Hood Canal that will be used to design future rockfish assessment surveys. Also observed were burrowing anemone (Pachycerianthus fimbriatus), white sea whips (Osteocella septentrionalis), marine worms, ratfish (Hydrolagus colliei), and several species of flatfish.
For the DNR geologists involved in the project, this investigation and mapping of seafloor geological hazards and geomorphologic conditions will lead to better understanding of the seafloor environment and its relation with habitat both in water and on land.
To download the report and see more photos and description of the project, visit the Publications of the Division of Geology and Earth Resources and look the link to OFR 2012-01: “Remotely Operated Vehicle (ROV) Video Investigation of Two Large Seafloor Mounds in Southern Hood Canal, Washington.“
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