DUBUQUE, Iowa (AP) — Work on Mississippi River locks has caused a sudden drop in river levels in Iowa, surprising marinas and leaving some boats high and dry.

Crews at Lock and Dam No. 10 in Guttenberg closed the dam's gates Tuesday, decreasing the river's flow, the Telegraph Herald reported.

The water level at the Dubuque railroad bridge had dropped about 1.5 feet (0.46 meters) by Wednesday, said Scott Pettis, hydrologic technician for the U.S. Army Corps of Engineers' Rock Island District.

Bobby Luhm, general manager of American Marine in Dubuque, said his docks were damaged and a few rental boats at the marina were sitting on dry land.

"We've never actually seen this happen," he said. "It's something we really can't plan for."

Dips in the river level are normal when water flow is lower, but such a steep drop is unusual, Pettis said.

"It usually isn't quite as drastic as this one because I've had other people call me (Wednesday) and say, 'I haven't seen this in a while,'" Pettis said.

The drop started in Wisconsin on Sunday and reached Guttenberg by Tuesday, said Nathaniel Anderson, civil engineer for the Corps' St. Paul District.

Workers usually have to close the dams when the weather is drier to maintain navigation channels, though crews at Lock and Dam No. 10 had to do so more aggressively than usual.

"We did see a rapid reduction in flows and just have to adjust for that all the way down the river as it comes to each lock and dam," Anderson said.

The decreased flow forced workers in Dubuque to close Dam No. 11 to keep water levels steady, Pettis said.

"I know it's quite an inconvenience to boaters, but it's quite a challenge to us to try to regulate the dams with a big dip in it like this," Pettis said.

He said Wednesday that he expects water levels to start returning to normal within a few days.

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Information from: Telegraph Herald, http://www.thonline.com