Water from Mississippi River reservoirs in Minnesota started heading downstream Friday in an attempt to help ease the threat of low river levels to barge traffic south of St. Louis.

The drawdown from Pokegama Lake, Lake Winnibigoshish, Leech Lake, Big Sandy Lake and Gull Lake will continue for three weeks. Water in those lakes had been held back and has been at "summer recreation" levels since Nov. 2 in anticipation of the current release, which is meant to cushion the effects of flow reduction from the Missouri River system, said Patrick Moes, spokesman for the U.S. Army Corps of Engineers-St. Paul District.

A Missouri River dam reduces its flow every fall, dropping water levels about 3 feet south of St. Louis, Moes said. That could be significant this year, with the river running at such low levels that dredging has continued through the summer to maintain adequate depth for barges. Dynamiting of underwater rock formations near Cape Girardeau, Mo., is also being considered.

A barge industry trade group has asked President Obama to intercede in order to keep the Missouri River water flowing downstream. Moes said the Mississippi water from Minnesota is likely to cut only 3 to 6 inches off the 3-foot drop expected from the Missouri River flow reduction.

"If something doesn't change and we don't get water in the next three, four or five weeks, navigation could be shut down," he said. "There won't be enough water to float a barge."

Barge traffic into and out of the Twin Cities generally ceases for the winter in late November or early December. A shutdown in the South, Moes said, would have drastic economic consequences, driving up shipping costs of all kinds of commodities. Barges there are already being required to carry less weight.

The Minnesota reservoirs, which are also popular recreational lakes, will be drawn down to customary winter levels only, Moes said.

The Associated Press contributed to this story.

Bill McAuliffe • 612-673-7646