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Corps Completes Lower Snake River Dredging Plan, EIS

by Barry Espenson
Columbia Basin Bulletin - August 9, 2002

The U. S. Army Corps of Engineers officials in late July released its final report on a long-term "Dredged Material Management Plan and Environmental Impact Statement" on lower Snake River and McNary reservoirs.

The Corps' Walla Walla District prepared the 20-year plan for managing dredged material removed from the four reservoirs behind dams on the lower Snake River (Ice Harbor, Lower Monumental, Little Goose and Lower Granite) and McNary reservoir on the Columbia River.

The recommended alternative is a combination of maintenance dredging, levee raising and a beneficial use of dredged materials.

Among those beneficial uses will likely be the creation of fish habitat, the Corps says. That shallow-water habitat would be accomplished by using bottom-dump barges to transport and deposit the dredged material. Finer sands and silts will be used for a base and coarser sands, gravels and cobble would be placed over that base to provide a favorable substrate for juvenile salmonid rearing and resting.

The actual dredging activity would have "indirect, minor, short-term effects on the aquatic ecosystems by disturbing sediments and removing macroinvertebrate species" that are prey for resident and migratory fish.

"However, re-colonization of macroinvertebrates would occur relatively rapidly within both the dredging area and at the in-water shallow and mid-depth disposal areas," according to the report. The draft plan and EIS were released in November and was followed by a 45-day public comment period, according to Jack Sands, the dredging plan project manager.

"We didn't get a huge number of comments, but the ones we got were very detailed," Sands said. The report's appendix O includes includes Corps responses to comments filed by 23 federal, state, local and tribal entities and three individuals. The National Marine Fisheries Service issued a "no jeopardy" biological opinion regarding the plan's potential impact on salmon and steelhead listed under the Endangered Species Act.

The Corps maintains a 14-foot deep and 250-foot wide navigation channel through the reservoirs. These reservoirs are part of an inland navigation system that provides slackwater navigation from the mouth of the Columbia River near Astoria, Ore., to port facilities on the Snake and Clearwater rivers in Clarkston, Wash., and Lewiston, Idaho.

The Corps, in cooperation with the U.S. Environmental Protection Agency, developed the long-range plan for the maintenance of the navigation channel. Maintaining the reservoir above Lower Granite Dam in the Clarkston-Lewiston area included evaluation of the flow conveyance capacity due to the increasing sedimentation in the river. The Corps is proposing raising some levees to maintain recommended capacity.

"Extensive coordination would be completed before this work could begin," said Jack Sands, project manager.

The Corps' first dredging activity is currently proposed for winter 2002-2003 and includes dredging the navigation channel at the confluence of the Snake and Clearwater rivers, several port facilities in the Lewiston-Clarkston area, several recreation facilities in Lower Granite and Little Goose reservoirs, navigation lock approaches to Lower Granite and Lower Monumental Dams, and other areas.

Dredging in the area has been stalled since the winter of 1997-98, Sands said. The Corps put together an interim dredging plan in 2000. But the NNMFS did not agree with the Corps' findings that the interim dredging would have insignificant impacts to listed fish. A NMFS letter requesting further consultation expressed concerns about potential loss and disturbance of habitat from dredging and dredge disposal and water quality impacts caused by the "remobilization of sediments."

The interim plan -- intended to remove an estimated 244,269 cubic yards of sediment from the federal navigation channel at the confluence of the Snake and Clearwater rivers -- also drew the criticism of conservation groups and tribes concerned about water quality and impacts on fish.

The Corps decided last year to forgo an interim plan and concentrate on completing the 20-year EIS -- more comprehensive analysis than had been conducted for the interim process.

Sands said this week that the Corps feels it has answered the critics' questions. The agency will now review the document internally and expects to produce a formal "record of decision" in time for the winter dredging season. The work window is from Dec. 15 through the end of February.

"It's the time period in the year when we have the least change of having either adults or juveniles (salmon and steelhead) in the system," Sands said.

The Corps is proposing to use the dredged material to develop woody riparian habitat at the Chief Timothy Habitat Management Unit, located on the Snake River near Clarkston, Wash., and/or using in-water disposal to create fish habitat in Lower Granite reservoir.

"This beneficial use would create shoreline habitat in line with the goals of the Lower Snake River Fish and Wildlife Compensation Plan," said Sands. "Opportunities to use dredged material cannot always be anticipated, a process to continue the environmental coordination will go on through the next 20 years."

Future beneficial uses could also include Hanford (Nuclear Reservation) remediation and closure activities capping material, potting soil, riparian habitat restoration, fill at the Port of Wilma, fill on non-federal lands and fill for roadway projects.

For each future dredging activity, the Corps would identify potential beneficial uses and coordinate the uses with the "Local Sediment Management Group" that will be made up of state, federal, tribal and other interests. Impacts of each proposed project would be assessed through the LSMG and a public process and through consultation with federal managers.

Each reservoir, and port and recreation areas, requires some level of dredging periodically to maintain the navigation channel at the minimum authorized depth of 14 feet. Port officials and others dependent on the channel to transport wood and agricultural products and other goods down river complained that their economic viability was being challenged as each year passed and sediment accumulated in the channel. Barge loads were becoming limited as more and more high spots emerged along the channel, reducing the amount of draft for the vessels. Port officials said last year that recreational opportunities were also becoming hampered because of silted in ports, marinas and recreation areas.

The Corps for the past year had kept the reservoir levels about a foot higher that is considered optimal to allow the draft needed by the barges, Sands said.

Related Links:
US Army Corps of Engineers Dredged Material Management Plan/Environmental Impact Statement:

Barry Espenson
Corps Completes Lower Snake River Dredging Plan, EIS
Columbia Basin Bulletin, August 9, 2002

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