The Missouri River is sinking.

Engineers aren't exactly sure why, and experts say it eventually could threaten billions of dollars in property development, levees and bridges, and make boat travel harder by exposing navigation hazards.

The problem is this: The Missouri River, in stretches from Rulo, Neb., to St. Louis, is losing elevation. As the river drops, it erodes stream banks and migrates up tributaries that flow into the main channel, causing similar problems there.

Because the water depth isn't really changing, the situation is nearly impossible to see by anyone looking at the river from shore. But for engineers, it's a costly headache.

Data indicate that in the worst areas, in Kansas City about 366 miles from where the Missouri River pours into the Mississippi River, the stream bed has dropped by about 12 feet over the last 50 years.

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