PORTLAND, Ore. — A wet spring and a heavy snowpack have combined to fill Northwest reservoirs and push some rivers to high levels as temperatures begin to rise and boost the mountain runoff, forecasters said Wednesday.

A flood warning was in place Wednesday for the Columbia River in the Portland area. Minor flooding was expected in some lowland areas, such as the river banks in Vancouver, Wash., on Sauvie Island north of Portland, and at some camping areas, such as Government Island between Portland and Vancouver.

The National Weather Service said the river was just below its 16-foot flood stage Wednesday but was expected to gradually rise above that stage by Thursday and remain slightly above for the rest of the week.

The high water on the Columbia has caused some traffic problems by increasing the number of bridge lifts on Interstate 5 between Vancouver and Portland to accommodate maritime traffic.

Between May 15 and noon Wednesday, the Oregon Department of Transportation raised the lift spans on the I-5 bridge 42 times — nine times last Saturday alone — due to reduced clearance for maritime traffic, said ODOT spokesman Don Hamilton.

But under an agreement with the Coast Guard, no lifts are allowed during rush hour from 6:30 a.m. to 9 a.m. and 2:30 p.m. to 6 p.m. Monday through Friday, Hamilton said.

The duration of an average bridge lift is 8 minutes for a sailboat and 12 to 25 minutes for a tugboat with a barge. Times can vary depending on the size and height of the vessel, the stream flow, and whether the vessel is going upstream or downstream, Hamilton said.

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In Eastern Oregon and Washington, a small stream flood advisory was issued Wednesday for Grant, Umatilla, Union and Wallowa counties in Oregon, including the cities of Pendleton, Milton-Freewater and La Grande. The weather service advisory, in effect through 12:30 p.m. Thursday, included Walla Walla County in south central Washington, the city of Walla Walla, and Columbia County in southeast Washington.

"We're looking at high water for two to three weeks at a minimum and we're probably only one or two good rainstorms away from some high flood levels in some areas," said Steve King, a senior hydrologist at the weather service's Northwest River Forecast Center in Portland.

The cool spring helped the mountain snowpack build up and stay in place at middle to higher elevations longer than usual, he said.

"It's just a fantastic snowpack out there, and that's really set things up for a very strong runoff season," King said.

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