GRANTS PASS, Ore.—A 33-mile stretch of Interstate 5 in southern Oregon reopened Thursday evening after a sudden dump of snow caused a seven-hour closure, stranding untold numbers of ill-prepared motorists.

The highway was closed between milepost 99 at Canyonville and milepost 66 at Hugo in an area of steep mountain passes, the Oregon Department of Transportation said.

It reopened about 7 p.m. after crews dealt with the snow, vehicles that had spun out and motorists who had neither chains nor four-wheel-drive vehicles.

Heavy snow continued to fall in the area and Transportation Department spokesman Gary Leaming cautioned that travelers should expect congestion while the back-up cleared. Some travelers might want to stop for the night until conditions improved, he suggested.

The closure was one of a number of weather-related problems across the state as stormy weather temporarily closed a lane of I-84 in Eastern Oregon, overloaded a sewage system in the Portland area and sent landslides and high water onto coastal highways.

In Southern Oregon, 8 inches of snow fell suddenly in the mountainous region, and plows couldn't keep up, Leaming said.

Many people driving the main north-south highway between Canada and Mexico are not prepared for winter weather, driving too fast, not carrying chains, and not dressed for the cold, the spokesman said.

"We call that the I-5 mentality," Leaming said. "People think because they are on I-5, they can go 70 mph all the time. I was up there near Ashland, and there was a fellow in his shorts putting chains on."

Farther south, foul weather snarled traffic at the Siskiyou Summit where Interstate 5 crosses the California-Oregon border. The Transportation Department said tire chains were required to get over the summit.

In Eastern Oregon, high winds overturned at least five tractor-trailers east of Pendleton, and a stretch of I-84 was closed for much of the morning.

In the western part of the state, a landslide closed state Route 126 between Eugene and Florence at a point 24 miles inland. A detour was set up. Transportation officials said they hoped to bring in a contractor to clear the debris Friday.

High water Thursday morning on U.S. 101 south of Seaside temporarily restricted traffic to trucks, while landslides slowed traffic south of Tillamook. State Route 6, the Wilson River Highway, was closed for a time near Glenwood after two tractor-trailers jackknifed, but it was reopened later.

In Astoria, the city told three households to evacuate after two large trees fell across the street. One homeowner said it appeared the hillside where they were located was giving way. The city's public works director said crews would take the top off the trees in an attempt to keep them from damaging the houses.

The city of Gladstone said heavy rains overloaded its sewer system and raw sewage flowed into the Clackamas River near its confluence with the Willamette River. The city said people should avoid contact with the rivers for 48 hours.