(ST. LOUIS, AP) — Barge traffic along a key stretch of Mississippi River is about to be restricted as crews prepare to blast large rock formations that have threatened shipping on the drought-plagued waterway.

The Army Corps of Engineers says contractors will undertake urgent demolition of the submerged granite pinnacles near Thebes, Ill., as early as Tuesday. That means that portion of the river south of St. Louis then will be closed to shipping for all but eight hours each day.

Months of drought have left water levels up to 20 feet below normal along a 180-mile stretch of the river from St. Louis to Cairo, Ill. The problem worsened when the corps recently cut the outflow from the Missouri River, meaning far less water from the Missouri flowing into the Mississippi.