Undated (AP) _ The mighty Mississippi River was the setting for a great deal of American history. Now those same waters are threatening to wash away some of that legacy.

Scattered among the houses, businesses and farms menaced by the summer flooding were some of the Midwest's most treasured historic sites, left behind by explorers, pioneers and religious visionaries.

One of those was Nauvoo, Ill., population 1,100, an important place for the world's 8.4 million Mormons.

Volunteer crews from rival denominations joined to pile sandbags around one of the community's oldest buildings, the Old Nauvoo House across the street from the homestead and grave of Mormon founder Joseph Smith.

The Old Nauvoo House, originally planned in 1841 as a 300-bed hotel, now houses staff at the Joseph Smith Historic Center of the Reorganized Church of Jesus Christ of Latter Day Saints.

''It's just a few feet lower than the Joseph Smith homestead, but those few feet are critical,'' said Alma Leeder, director of the center's guide service.

''We've got a levee around it, but that river's out of control, and the levee's under siege. Then, too, the muskrats dig their dens in the levee, and that makes it leak.''

Leeder and the other volunteers from his Independence, Mo.-based church were joined by crews from the Salt Lake City-based Church of Jesus Christ of Latter-day Saints, which has its own historic sites a short distance north of Nauvoo - but on higher ground.

The Missouri- and Utah-based denominations split after Smith's murder at nearby Carthage, Ill., in 1844. The Utah denomination is known as the Mormon Church.

More sandbagging went on 125 miles downriver at Elsah, Ill., where about half the 851 residents are members of another American-born religion, Christian Science. The volunteers were trying to save a number of antebellum houses in the old river port. The oldest of the threatened buildings, the Riverview House, was built in 1846.

Charles Fyfe, who rents the house from Principia College, a nearby Christian Science school, said he would stay until either the levee breaks or the river drops.

Other pre-Civil War buildings in Elsah were lost last week.

Meanwhile, Missouri Gov. Mel Carnahan flew by helicopter Sunday to Ste. Genevieve, the oldest European settlement in his state, to watch efforts to save the historic community 50 miles south of St. Louis.

''I like the attitude,'' Carnahan said. ''It's the attitude that you're going to fight it back.''

Ste. Genevieve's 18th and early 19th century buildings include examples of the distinctive vertical-log construction of French Colonial architecture. The town was founded by French settlers in 1735.

The National Trust for Historic Preservation listed Ste. Genevieve last month as one of the most endangered historic places in the country, citing inadequate flood protection.

But as the river rose, so would it fall - eventually.

It was receding Sunday at Prairie du Chien, Wis., where the grounds of another historic site had been flooded. The water stopped six or eight feet below the Villa Louis mansion, built on the estate of fur trader Hercules Dousman, Wisconsin's first millionaire.