Omaha, Neb. ? Several hundred thousand acres of rich Midwestern farmland and even some urban areas near the Missouri River are at risk of flooding this summer during months of historically high water that experts fear will overwhelm some levees, especially older ones.

Engineers who have studied past floods say the earthen levees in rural areas are at greater risk.

“Most of the levees are agricultural levees. They’re not engineered. They’re just dirt piled up,” said David Rogers, an engineering professor at Missouri University of Science and Technology.

So far, most levees have held along the 811 miles the Missouri travels from the last dam at Gavins Point in South Dakota to its confluence with the Mississippi River near St. Louis. The flooding thus far has covered more than 560,000 acres of mostly rural land, including nearly 447,000 acres of farmland. The water has forced some evacuations, but the extent of the damage may not be clear until it recedes.

That’s not expected to happen until the fall as the Army Corps of Engineers says it needs to continue releasing substantial amounts of water from upstream reservoirs inundated with heavy spring rains and melt from an above average Rocky Mountain snowpack.

The Corps predicts that the river will eventually rise high enough to flow over some 18 to 70 levees, mostly in rural areas of southeast Nebraska, southwest Iowa and Missouri. Other levees will become saturated, and water can erode their foundations, seep underneath or find other flaws to exploit.

A saturated levee may lose stability, potentially causing it to crumble, as one did in June near Hamburg, Iowa, allowing floodwater to cover several miles of farmland and threaten the town. Flaws in levees, such as animal burrows, can allow water to flow through and eventually destroy the structure.

“At times like these, this is when we find out where the weak spots are,” said Erik Loehr, an associate professor of engineering at the University of Missouri.

Rural levees, experts say, are likely to be older, privately maintained and not tall or strong enough to stand up to such a long-running flood.

Corps officials and engineering experts are more confident that city areas such as Omaha, Kansas City and St. Joseph are well protected by substantial floodwalls that have been maintained.

Omaha’s main floodwall, for instance, is built to contain a river 40 feet deep — which is 4 feet higher than the river is expected to reach. But several of the rural levees in northwest Missouri are more than 2 feet shorter than the river’s expected crest, and some have already been exceeded.

Even stronger urban floodwalls and levees can falter against the destructive force of floodwaters. As a precaution, officials in Omaha and across the river in Council Bluffs, Iowa, have developed plans to evacuate roughly 40,000 people from areas near the river in case a levee fails.