JEFFERSON CITY, Mo. - Barring tremendous rainfall, water levels will start dropping soon on the Missouri River as the Army Corps of Engineers tries to store more water in upstream lakes.

The corps on Wednesday began decreasing the amount of water released from Gavins Point Dam in South Dakota, the farthest downstream dam on the 2,714-mile river that winds from Montana to Missouri.

It typically takes about 10 days for those lower water levels to reach the mouth of the Missouri River, where it dumps into the Mississippi River near St. Louis.

Based on the current river conditions, the reduction in reservoir releases could translate to a 3-to-4 foot drop in downstream water levels, said Steve Mahfood, director of the Missouri Department of Natural Resources. But river levels also could vary depending on rainfall.

The corps, which manages the dams on the Missouri River, announced this summer the commercial barge navigation season would be shortened by 47 days because of new water conservation measures in its revised operating manual.

The Missouri River barge navigation season, which typically ends Dec. 1, is expected to stop by Oct. 15 in St. Louis, said Paul Johnston, a corps spokesman based in Omaha, Neb.

River levels have been a point of contention for years between downstream states such as Missouri and upstream states such as North Dakota and South Dakota. When reservoir levels drop, it can hurt the recreation and fishing industries of upstream states. But when the corps holds back more water for the lakes, it can hurt the downstream barge industry and potentially affect river intakes for drinking water systems and power plants.

The water flow at Gavins Point Dam was 23,500 cubic feet per second before the corps began a gradual reduction on Wednesday, Johnston said. By Sunday, the flow is scheduled to drop to 12,000 cubic feet per second. If that level causes no problems for downstream drinking water systems or power plants, Johnston said, the flow would be reduced to 11,000 cubic feet per second for the next month. Eventually, the flow could drop as low as 10,000 cubic feet per second during the winter season, he said.

Johnston said the water conservation measures should help slow - or temporarily reverse - the dropping water levels on the three largest upstream reservoirs.

Fort Peck Reservoir in Montana is 34 feet below average, he said. Lake Sakakawea in North Dakota is 25 feet below normal, and Lake Oahe in South Dakota is 30 feet below average, he said.