FARGO (AP) - The drought gripping the Red River Valley is harming more than just farmland and lawns - it's hurting the ground itself.

One of the driest summers in more than a decade is agitating the clay material underneath Fargo and neighboring Moorhead, Minn.

The instability can crack sidewalks, driveways and streets, said Donald Schwert, a geology professor at North Dakota State University. The drought also might accelerate the erosion of banks along the Red River, he said.

Schwert said that, in Fargo, about 105 feet of expansive clay left over from glacial Lake Agassiz blankets the firmer glacial till below. The Red River constantly cuts against the clay and weakens the banks.

Construction of riverfront homes puts added pressure on the weak clays, speeding up the erosion process. The result can be an underground "clay flow" that leaves the upper ground unsupported, which causes it to slump, Schwert said.

The potential cracking of sidewalks, driveways and streets occurs when the clay material underneath shrinks because of a lack of water, Schwert said. Properly watering lawns might help prevent those types of ground failures.

"I'm a big fan of watering your lawn, especially where your lawn meets with your house, driveway or sidewalk," Schwert said. "This keeps the clays fully expanded, and homes and structures fully supported."

The Fargo Park District has lost facilities, trails and land to riverbank sliding. Crews removed an old barn at Trollwood Park in north Fargo about 10 years ago because it was sliding down the river embankment.

One road in Trollwood is opened only during plays because of slumping in the area, said Roger Gress, Park District executive director.

The district also is spending $2.2 million on a new clubhouse and pro shop at Edgewood Golf Course, partly because of slumping. The existing clubhouse and pro shop are about 50 feet from the Red River, and both are at risk of sliding down the embankment because the land is eroding.

Cass and Clay counties have bought out some homes that were sliding into the river. In the past two years, efforts were made to help homeowners avoid having to sell or move homes because of slumping.