Stuck in the Mud
Growing Flood Risk, Growing Costs with Keeping the Lower Snake River Dams
BACKGROUND
Faced with rising costs and risks associated with maintaining dams—including public safety and environmental impacts —many communities around the nation are choosing instead to remove them. Northwest residents are faced with similar decisions with four dams on the lower Snake River in eastern Washington.
Fishing businesses, conservationists, and scientists have called for the removal of these dams for years as a necessary action to restore wild salmon to the Columbia and Snake Rivers. But new concerns have arisen about the flood risks, rising costs, and declining benefits associated with keeping these four dams in place.
The Northwest needs to weigh the costs—declining salmon, public safety concerns, mounting expenses—against the limited benefits—primarily 1,200 average megawatts of energy annually and 140 miles of barge transportation—and consider whether removal of these dams would be most cost effective and beneficial to local communities and the region.
SEDIMENT MOUNTING BEHIND LOWER GRANITE DAM
The lower Snake River dams in eastern Washington were built between 1960 and 1975. Farthest upstream, Lower Granite Dam was the last one built. The towns of Clarkston, Washington, and Lewiston, Idaho, flank the reservoir at the confluence of the Snake and Clearwater Rivers, about 40 miles upstream from the dam. A series of levees line the waterfront.
From the beginning, the U.S. Army Corps of Engineers (Corps) acknowledged that significant sediment would accumulate behind Lower Granite Dam, particularly at Clarkston/Lewiston where the Clearwater joins the Snake River, slows its speed, and drops sediment, adding to the massive silt already fl owing downstream in the Snake from thousands of miles of streams. This large drainage includes the Salmon River, which is one of the longest undammed rivers and which provides the largest intact block of salmon habitat left in the lower 48 states.
FLOOD RISK GROWS AS SEDIMENT BUILDS
As the sediment has displaced water in the reservoir, the river level has risen. Downtown Lewiston now sits lower than the height of the river, protected only by the levees. These levees originally were built to allow the reservoir to be longer, rather than for flood control for local communities.
The Corps studied the sediment issue extensively in a 2002 Environmental Impact Statement.1 Sediment build-up has happened at a much faster pace that the Corps predicted. Approximately 3 million cubic yards of sediment accumulates in Lower Granite Reservoir every year. The reservoir is 55 percent full of silt, leaving less room within its banks.
The growing sediment has reduced channel capacity and the ability of the levee system to hold back water in flood events. The levees were originally built to have 5 feet of “freeboard,” the height of the levee above the water level. With rising waters, the freeboard is now less than 3 feet and as low as 1.5 feet in places. The Corps estimated in the 2002 EIS that nearly $2 billion worth of buildings and infrastructure sit in the Clarkston/Lewiston area floodplain, facing a growing threat of damage.
CORPS’ PREFERRED SOLUTIONS: COSTLY, UNREALISTIC, OR BOTH
The Corps is looking at several options: 1) raising levees, 2) dredging sediment, 3) reducing incoming sediment, or 4) a combination of these. The costs in the 2002 EIS for combined dredging and levee-raising ranged from $2.27 million to $916.35 million.
While the Corps downplays the risks of flooding in the next 20 years, its own studies note that the risk of catastrophic outcomes greatly increases over time. Over the long term, managing for flood risk will require increasingly higher levees and more dredging. It is unlikely that the ultimate cost of addressing these problems will remain on the low end of the Corps’ projected range.
Levee-Raising Disruptive and Costly
If Lower Granite Dam remains in place, levees will need to be raised. In the 2002 EIS, the Corps studied raising levees as little as 3 feet and up to 12 feet. A 3-foot raise will disrupt recreation trails and require some highways to be raised. If the levees require a raise to four feet or beyond, the costs and disruptions dramatically rise. In addition to highways, several bridges will need to be raised, including two four-lane structures, and significant property, private homes, buildings, and parks will become “sacrifice zones” and be inundated. Other buildings will require emergency flood protection measures (sandbagging, for example) during high water events.
In addition to raising and extending existing levees, under all scenarios additional levees will need to be built. The Corps has raised concerns about the structural integrity of both modified levees and new levees attached to existing ones, saying any potential overtopping of these levees in a flood event will render them structurally unsound and may cause them to fail completely. The Corps’ EIS estimated the cost to the federal government--and ultimately taxpayers—at more than $2.273 million to raise the levees 3 feet and more than $87.661 million for 12 foot levees. The Corps has only estimated levee needs through 2074, the designated project “life” of the Lower Granite Dam. Of course, the flood risk and associated costs will continue as long as the dam remains in place.
Dredging is Expensive and Unrealistic
The 2002 EIS studied the impacts of a variety of dredging scenarios for Lower Granite Reservoir: simply maintaining the navigation channel; dredging 3 million cubic yards annually; dredging 1 million cubic yards annually for 10 years and 325,000 cubic yards every year until 2074; or dredging 2 million cubic yards annually for 20 years and then 725,000 cubic yards annually through 2074.
With more than 3 million cubic yards of sediment depositing behind Lower Granite Dam every year, it is unrealistic to think such a large amount of material can be dredged and disposed of every year safely and affordably. The sediment problem goes well beyond keeping the navigation channel operational, as sediment is accumulating in the marinas and at the Port of Lewiston, and sediment deposited throughout the reservoir adds to the growing flood risk.
Sediment Reduction is a Laudable but Futile Goal
The Corps has also proposed an ambitious program to reduce the sediment fl owing into Lower Granite Reservoir. Reducing sediment is a laudable goal, especially in eastern Washington where agricultural practices cause extensive soil erosion. But the sediment fl owing into Lower Granite Reservoir primarily comes from a massive landscape of 32,000 square miles, large portions of which are relatively intact habitat. In fact, more than 25 percent of the land is in designated wilderness, where farming erosion is not an issue and sediment control is impractical or unnecessary.
Furthermore, the Corps of Engineers has no ownership or jurisdiction over these lands. To rely on cooperative efforts among private landowners and multiple federal and state land agencies to control erosion in an effort to reduce downstream flood risk is unrealistic.
DAM REMOVAL IS A COST-EFFECTIVE ALTERNATIVE
The Corps has refused to consider removal of Lower Granite Dam (and the other 3 lower Snake River dams) as a possible alternative to address the sediment and flood risk issues. The Corps should consider dam removal as an alternative. They need to weigh both the benefits to salmon recovery and the saving of taxpayer dollars on dam operations and maintenance costs.
The Corps has estimated that taxpayers and Northwest ratepayers pay about $33.7 million annually to operate and maintain the lower Snake dams. And additional salmon recovery costs with the dams in place could run hundreds of millions of dollars a year. These funds would be better allocated to much-needed repairs on other dams in the federal hydrosystem, including John Day Dam on the Columbia which needs $1 billion worth of repairs, and Dworshak Dam on the Clearwater, which has a poor safety rating and would wreak havoc in multiple towns downstream were it to fail.
SUMMARY
The costs of maintaining the four lower Snake River dams now outweigh the benefi ts they provide. The economically sensible solution is to remove the four dams, replace their benefits, and invest in the transition to a free-fl owing river. Citizens and decision-makers should direct the Corps of Engineers to evaluate dam removal as an alternative in the Corps’ ongoing planning process. The Corps plans to release a draft environmental impact statement in 2008-9 if sufficient funding is secured.
For more information contact Sam Mace at 509-747-2030 or sam@wildsalmon.org
