Undated (AP) _ Six Coast Guard boats sailed into West Alton, Mo., to help evacuate residents Wednesday after crews failed to shore up levees around the town on a narrow peninsula and both the Mississippi and Missouri rivers flooded in.

And a nearby lock and dam construction site was purposely flooded to protect it from the Mississippi's force.

Both rivers were bloated by water draining from Midwestern states ravaged by nearly two weeks of record flooding that forced an estimated 55,000 people to flee their homes.

Elsewhere, pumps slowly lowered the water in East St. Louis, Ill., which was swamped under 4 1/2 feet of water when a flood gate failed. In Arkansas, the Arkansas River flooded mostly farmland after inundating cities in Oklahoma.

At West Alton, a community of about 450 people, a levee north of town on the Mississippi side of the peninsula was breached about midnight Tuesday and sandbagging to keep out the Missouri on the south failed Wednesday. Deputies had earlier warned residents to be prepared to move to higher ground.

Coast Guard Lt. Chris Smith had warned that the Mississippi was expected to rise an additional two feet before cresting. ''When the river levels are this high, you're talking about an awful lot of water,'' he said.

About 10 miles west of West Alton, the Missouri opened a two-mile gap in a levee Tuesday and surged in a quarter-mile-wide channel 12 miles across the peninsula dividing it from the Mississippi north of St. Louis, isolating the small town of Portage des Sioux, Mo.

Coast Guard and National Guard helicopters were used to help evacuate the Portage des Sioux area.

Experts discounted the possibility that the flooding would permanently change the Missouri's course.

The Missouri crested Tuesday at nearby St. Charles, which is protected by levees, at a record for this century of 37.5 feet, 12.5 feet above flood stage. The record there was 40.1 feet in 1844.

The Missouri has broken through or poured over 73 levees along a 200-mile stretch between Glasgo and St. Charles since Oct. 1, the Army Corps of Engineers reported.

The National Weather Service predicted the Mississippi would crest Thursday at about 40 feet at St. Louis, more than 10 feet over flood stage. In downtown St. Louis, the Army Corps of Engineers closed 20 of the 38 flood gates that protect the city.

Army Corps of Engineers spokesman Clyde Wilkes said workers began flooding a 23-acre steel coffer dam surrounding the $950 million Alton Locks and Dam construction project in an effort to protect it from the river.

He said damage caused by filling the construction site with 20 feet of water would cost about $500,000 to repair, but the cost would be much greater if the dam gave way under the river's pressure. The Mississippi was at 33.9 feet Wednesday at Alton, 12 feet of above flood stage, and was lapping at the top of the coffer dam, Wilkes said.

Upstream at Grafton, Ill., where the Illinois River meets the Mississippi, nearly 500 families were evacuated because of flooding, and Police Chief Ritchie Caldieraro said traffic was halted because of water over roads. One person died of a heart attack after moving his belongings, the chief said.

Nearly five feet of water swamped the streets of East St. Louis, Ill., and Mayor Carl Officer said Tuesday it could be weeks before the 1,200 people forced to flee can return. A broken floodgate allowed the rising Mississippi to flow into the city.

Walter Greathouse, director of the Metro East Sanitary District, said emergency crews had the gate ''95 percent closed'' and pumps were lowering the water in the flooded area at the rate of about 1 1/2 inches per hour.

President Reagan granted federal relief Tuesday to northeastern Illinois, where flood damage was estimated at $35 million. Estimated flood damage in the East St. Louis area rose to $18 million.

Most water was going down in Oklahoma, but the Arkansas River was still rising at the tiny community of Moffett, where about half of the 250 residents had fled. Most of the 500 homes flooded in Miami still contained water Wednesday, but police said water had receded from about half of the 800 flooded homes in Bartlesville.

In Arkansas, Joe Bray of the Office of Emergency Services said only farm land was flooded Wednesday.

Michigan rivers continued receding, but problems persisted in Macomb County's Harrison Township, where about 2,000 homes have been flooded by high water in canals leading from Lake St. Clair and along the shoreline.

Sandbagging efforts continued in the township for a fifth day Wednesday, said deputy township supervisor Mary Cunningham. ''I've really lost count at this point,'' he said. ''We ordered 30,000 more (sandbags) yesterday.''

Elsewhere, Montana's Milk River, which has flooded a 150-mile stretch during the past two weeks, was reported stable Wednesday at Nashua after cresting Tuesday at nearly six feet above flood stage. The town was protected by dikes.

''The river travels over 100 miles within the confines of our county,'' said Valley County emergency coordinator Terry Mercer. ''At one end, the flood is receding. At the other end, it hasn't even begun yet. It's a slow-moving flood.''