Skokomish Tribe, USGS map seafloor topography and habitats of Skokomish estuary

If the waters of Hood Canal were drained from the Skokomish delta today, the exposed seafloor would show a complex network of channels within the delta that leads to steep ridges on the bottom of Hood Canal.

The Skokomish Tribe is working with U.S. Geological Survey (USGS) to map the topography, morphology and habitats of the delta and the tribe’s 1,000-acre estuary in southern Hood Canal.

The goal is three-fold: to determine the amount and condition of existing nearshore habitats; to better understand how the river’s sediment transports to the delta; and how sediment movement effects ecosystems, including eelgrass, tidal flats and food resources for salmon.

A map of the Skokomish delta that shows preliminary findings after USGS scientists used sonar to read the topography of the seafloor.
A map of the Skokomish delta that shows preliminary findings after USGS scientists used sonar to read the topography of the seafloor. Photo: USGS

“We’re mapping what’s in between the nearshore and deeper waters,” said Eric Grossman, the project’s USGS principal investigator. “It’s an area that’s been studied sporadically since the 1950s but is lacking data on elevation, substrates and physical processes, such as sediment movement.

“We’d like to see what habitats exist for salmon, such as eelgrass and the ridges, in order to determine how they may change with actions in the river, climate change and the estuary restoration work,” Grossman said.

Preliminary findings include discovering ridges up to 90 feet tall at the base of the delta; tidal channels transporting sediment from the river to the canal; and eelgrass beds.

“The slope of the delta front is one of the steepest I’ve seen in our mapping of Puget Sound deltas,” Grossman said. He also observed channels and slump features that suggest the delta is actively changing. Linear ridges at the base of the delta stand 45 feet to 90 feet tall with a wave length of 300-450 feet and up to a half-mile in length.

These features provide complex habitat that have been found around the world to be important for fisheries and ecosystems, Grossman said. With the updated information, the tribe hopes to further study the large ridges.

“We would like to better understand the age and substrate of these ridges and how they formed,” said Shannon Kirby, the tribe’s habitat biologist.

The USGS team used sonar to map the depth of the seafloor and tidelands over approximately 7 square miles. They also measured the thickness of recent sediment layers on the seafloor, penetrating between 120 feet and 240 feet below the seafloor.

The sonar data will be merged with LiDAR data, collected by plane, to create a seamless onshore-offshore map and digital elevation model showing the elevations and morphology of the shorelines, marshes, tidal flats, and deeper offshore regions of the delta, Grossman said.

“This will provide the first comprehensive model of the entire Skokomish Delta and allow the tribe and partners to better assess coastal resources and model potential changes owing to climate and land-use change,” he said.