* Rain, rock removal, dredging to keep river open
* Corps of Engineers overly optimistic -Senators Durbin, Harkin
* Pitfalls if tap Missouri River reservoirs-Corps (Adds Senators skeptical that water will be deep enough)
WASHINGTON, Dec 7 (Reuters) - Fall rains and dredging will keep barge traffic moving on the middle Mississippi River through mid-December despite low water levels caused by a drought covering 60 percent of the continental United States, the government said on Friday.
More than 100 million tons of cargo, half of it corn and soybeans, are shipped annually through the stretch of the river south of St Louis. Shippers and farm groups fear shallow water will cut off traffic later this month.
Barge companies have told customers not to load barges as heavily as usual so they will not ride as deep in the water.
Two Democratic senators from Illinois and Iowa, the top corn and soybean-growing states, said the U.S. Army Corps of Engineers was overly optimistic about water levels.
The Obama administration has expedited removal of underwater rock pinnacles that are a threat to barges and it is mobilizing dredges to remove 21 shoals from the Middle Mississippi. Rock removal is expected to begin this month.
With that work and with rain in the forecast, the Corps of Engineers believes there will be enough water to sustain navigation through mid- to late December without tapping Missouri River reservoirs, said Assistant Army Secretary Jo-Ellen Darcy, who oversees the Corps.
Assistant Senate Majority Leader Dick Durbin of Illinois said one forecast indicated low water, below the 9-foot target for shipping, in the middle Mississippi for most of December. Durbin said he would arrange a meeting for the Corps of Engineers officials to brief business and local leaders on how it would maintain enough water in the shipping channel.
Iowa Sen. Tom Harkin said: "We have heard from industry officials, who rely on this waterway, and they are also concerned that normal barge traffic will not be sustainable later this month without considerable rain or releases of water on the Missouri River."
An array of Midwestern officials, agriprocessors and farm groups last month suggested diversion of water from the reservoirs to boost the middle Mississippi. In reply on Friday, Darcy said the Corps lacked authority to divert the water and the idea was unworkable.
Releases would deplete the reservoirs, already 20 percent lower than desired, by an additional 5 percent. Low reservoirs would threaten access to water by communities, for industry and for irrigation, as well as harm wildlife and tourism.
The releases would be "insufficient to maintain navigation on the middle Mississippi River without additional rainfall," wrote Darcy. Lowering the reservoirs now would leave less water available for the 2013 shipping season on the Missouri and Mississippi rivers.
The Corps of Engineers, which maintains inland waterways, is required to operate the six main-stem reservoirs on the Missouri River, which flows into the Mississippi at St. Louis, so they could withstand a 12-year drought. As part of its drought conservation plan, winter releases from the reservoirs would be 30 percent smaller than usual.
On Friday, the Corps planned to reduce releases from the Gavins Point Dam, the spigot at the bottom of the reservoir chain, to 14,000 cubic feet per second, on the way to the intended winter-season flow of 12,000 cubic feet.
Three nuclear power plants and the cities of Omaha and Kansas City draw water from the Missouri River. (Reporting by Charles Abbott in Washington; Additional reporting by Karl Plume in Chicago; Editing by Nick Zieminski and Gunna Dickson)