By David Bailey

MINNEAPOLIS (Reuters) - With the heaviest flooding yet to come, officials closed 260 miles of the Missouri River to boaters and residents from Iowa through Nebraska and Missouri were sandbagging on Wednesday to hold back the waters.

"It's going to happen, we know it's going to happen," said Pam Todd, who was fielding calls while Parkville, Missouri, city officials toured levees.

"Everyone remembers 1993 and so many volunteers have come out. Every entity you can think of is helping us," she said.

The outlook may have worsened as well with up to 5 inches of rain falling in Montana since Monday, increasing flooding on Missouri River tributaries and adding to stress on the system stretched by heavy May rains and a melting deep snowpack.

Overflows in Montana caused a fresh round of evacuations, including some 13 families when high water from a rapid rise in the Milk River threatened Nashua, which is downstream of the Fort Peck Dam in northeastern Montana.

Flood-battered Roundup, which had already been cutoff for some time this year, was cutoff again on Wednesday by waters up to 6 feet deep after the Musselshell River breached its banks.

"Anything could happen at any time; this is changing from hour to hour, minute to minute," said Monique Lay, spokeswoman for Montana Disaster and Emergency Services.

The U.S. Army Corps of Engineers said on Wednesday it had stepped up again planned water releases from the Fort Peck Dam to relieve pressure on the reservoir, which saw inflows double from Monday to Tuesday due to the rain storms.

The Corps said the additional releases would not alter its planned peak releases from the other five dams in the system.

Thousands of South Dakota and North Dakota residents have voluntarily left homes in communities protected by hastily built sandbag barriers and levees in the past two weeks ahead of planned stepped-up water releases from federal reservoirs.

On Wednesday, officials said levees in the Dakotas were holding against the rising waters, but cautioned that the record flows and sustained periods could still prove to be too much for the barriers.

STEPPED UP RELEASES IN MONTANA

The U.S. Coast Guard has closed the Missouri River to recreational traffic for 260 miles from where Iowa, Missouri and Nebraska meet to the Gavins Point Dam in South Dakota. The closing was intended to protect both new levees and boaters.

Water releases from Gavins Point are closely watched by communities all the way to the Missouri River's confluence with the Mississippi River at St. Louis because the river flows freely from there.

The Corps now plans to release 60,000 cubic feet per second of water from the Fort Peck Dam on Friday. It expects to reach 150,000 feet per second at its other five reservoirs by about mid June and hold at that peak until at least mid August.

The river already has pushed beyond its banks in many spots from Sioux City south, forcing residents and companies to encircle structures with sandbag barriers and earthen walls

In Bellevue, Nebraska, just south of Omaha, water reached up nine feet in some flooded areas, touching the nets on the basketball hoops in a city park along the Missouri River and submerging picnic tables.

Farther south, flood waters closed one of two eastbound lanes on Nebraska state Highway 2 near Nebraska City. Dirt and sand were stacked more than eight feet high around a gas station and fast-food restaurant in Nebraska City.

A partial breach in a levee near Hamburg, Iowa, was patched by sandbags dropped from a helicopter over the weekend, forcing contractors to build a secondary levee to protect the south side of the town of 1,200.

Federal officials expect the fix to hold until the backup levee is finished, but not to withstand the full might of the river's crest, threatening the town in the southwest corner of Iowa near Missouri and Nebraska.

Corps officials have warned that more breaches are likely on the lower Missouri River given the varying strengths of the flood protections and the height, speed and duration of planned releases of water from reservoirs.

Iowa officials expect low-lying parts of Interstate 29 that run parallel to the east bank of the river to closed as water flows peak later in June.

The Missouri River basin forms the northwest portion of the Mississippi River basin that stretches from Montana to western New York and funnels water south into the Gulf of Mexico.

(Additional reporting by Laura Zuckerman in Salmon, Idaho, Michael Avok in Bellevue, Nebraska, and Kevin Murphy in Kansas City. Edited by Peter Bohan and Carol Bishopric)