2011 USGS / EPA Elwha River Diver Survey in the Strait of Juan de Fuca

EPA divers with USGS work to baseline benthic conditions at the mouth of the Elwha River before dam removal.

Scuba surveys to assess effects of Elwha dam removal on shallow, subtidal benthic communities
Steve Rubin, Ian Miller, Nancy Elder, Reg Reisenbichler, Rusty Rodriguez, Jeff Duda, Rob Pedersen, Mike McHenry, and Matt Beirne; USGS Western Fisheries Research Center, Seattle, WA (SR, RR, RR, JD), UC Santa Cruz, Santa Cruz, CA and WA Sea Grant, Port Angeles, WA (IM), USGS Western Fisheries Research Center Marrowstone Marine Station, Nordland, WA (NE), USEPA Region 10 Environmental Cleanup Office, Seattle, WA (RP), Lower Elwha Klallam Tribe (MM, MB).
The impending removal of the Elwha River dams will affect marine habitats when sediments that have accumulated behind the dams for nearly 100 years are transported to the Strait of Juan de Fuca. Deposited and suspended sediments can have a variety of effects on marine plants and animals including burial, inhibition of propagule settlement, light reduction affecting plant growth, damage to animal gills and filter feeding structures, and alteration of interactions among species with different tolerances and responses to sediment. The initial large sediment influx from release of accumulations in the reservoirs may stress nearshore communities, but in the long run communities may benefit from reestablishment of sediment delivery from the upper watershed. Scuba surveys were initiated in 2008 to characterize nearshore biological communities prior to dam removal. Community structure in the Elwha nearshore (west Freshwater Bay to the base of Ediz Hook) was partly controlled by substrate composition and seafloor relief. Kelp density was low in sandy areas, intermediate on gravel-cobble substrates, and highest on bedrock or boulder reefs. Taxa richness (number of kelp, invertebrate, and fish taxa) was more strongly associated with seafloor relief than with substrate. Relief was usually provided by scattered boulders perched on top of mixed sand and gravel-cobble substrate. On average, 12 (59%) more taxa occurred where boulders were present compared to areas lacking boulders but with similar base substrate. The presence of boulders allowed species adapted to rocky reefs to coexist with species that inhabit sand and gravel-cobble substrates, thereby increasing species richness. These results highlight the importance of seafloor characteristics and suggest that different habitats and associated communities may respond differently to sedimentation. Elwha nearshore communities will continue to be surveyed during and after dam removal. Measuring community responses to short and long term changes in deposited and suspended sediments offers an unprecedented opportunity to gain insight relevant to managing these important marine resources.

For more information on the EPA Region 10 Dive Team see: www.epa.gov/region10/dive

Category:

Science & Technology

Tags:

• epa dive team
• scientific diving
• usgs dive team
• elwha river
• elwha dam removal
• benthic survey