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1.12.09
Ice bridge can spell trouble for river
By: David Hendee, Omaha World-Herald
A nearly 5-foot drop in the elevation of the Missouri River raised anxiety levels among Nebraska and Iowa community water managers in December as water supplies were threatened.
The problem was an ice bridge that dammed the Missouri upstream of Ponca, Neb., for several days. The falling river created water supply troubles at Sioux City, Iowa, and threatened Omaha and Nebraska City intakes.
Omaha's drinking water supply wasn't diminished, but the Metropolitan Utilities District restricted its river intake at the north Omaha Florence Plant by 50 percent because of the falling river.
Upstream at Sioux City, the river dropped to a dangerous level at the MidAmerican Energy Co. coal-fired power plant, said Larry Murphy of the Army Corps of Engineers in Omaha.
Tom Wurtz, M.U.D. president, said the Omaha utility deals with fluctuating river levels year-round and the latest drop didn't cause serious problems because water demand is low in winter and M.U.D.'s two Platte River plants could easily take up the slack.
"But we were watching it," Wurtz said. "We needed more water (in the river)."
Ice bridges are created when chunks of ice floating downstream jam together, joining shoreline ice and creating a dam across the channel.
Watching forecasts for extremely low temperatures, corps river managers in Omaha increased water releases from Gavins Point Dam in northeast Nebraska by about 17 percent on Dec. 9 in anticipation of river ice troubles downstream, said spokesman Paul Johnston.
Additional water releases from dams help the river to slowly create a deeper channel under the ice, keeping water flowing.
By Dec. 15, the ice bridge formed downstream from Ponca.
"There's 80 miles of river from Sioux City and Gavins Point, and it was about 45 percent ice covered," Murphy said.
The corps increased Gavins Point releases to keep downstream intakes operable. Despite increasing releases a total of 33 percent, the ice bridge caused river levels to drop at Omaha and Nebraska City.
Gavins Point releases were gradually reduced beginning Dec. 22, as temperatures moderated and downstream river flows returned to more normal levels.
A similar problem left about half of St. Joseph, Mo., without water in the late 1980s when hundreds of miles of the Missouri froze upstream. St. Joseph's water intakes have been modified and the city hasn't had the problem again.
"It doesn't happen a lot," Murphy said, "but we've had some mild winters since 2000."
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