The Army Corps of Engineers has decided not to release additional waters from the Missouri River to help ensure that commercial navigation continues on the Mississippi River.

According to a letter sent by Assistant Secretary of the Army Jo-Ellen Darcy on Thursday, the Army Corps believes releasing waters would deplete storage reservoirs in the Missouri River that have already seen low levels from the drought earlier this year. Darcy cited several consequences of reducing storage, including impacts to wildlife, reduced drinking water availability for tribes and higher irrigation costs for farmers.

"The Corps has identified the potential for significant negative effects on the Missouri River system," Darcy said in the letter, which was a response to a request from Illinois Sen. Dick Durbin (D).

Darcy further said releasing the waters would be "insufficient" in the first place to continue navigation on the Mississippi River without more rainfall.

The release of waters had been opposed by several lawmakers led by Montana Sens. Max Baucus (D) and Jon Tester(D), who argued that such an act was outside the authority of the Army Corps.

In a Nov. 30 letter to Darcy, President Obama and Federal Emergency Management Agency Administrator Craig Fugate, the lawmakers urged the administration to reject calls to make an emergency declaration for the Mississippi River due to economic losses from reduced barge traffic.

"We can appreciate the situation that navigation users have faced this year on the Mississippi River, because we have also been impacted by the drought, but the relief requested is outside the authorities" contained in statute, the group of lawmakers wrote.

Also signing the letter were South Dakota Gov. Dennis Daugaard (R), Kansas Gov. Sam Brownback (R) and North Dakota Gov. Jack Dalrymple (R).

Farm groups, shipping organizations, oil industry groups and the U.S. Chamber of Commerce have all requested that the government take immediate action to ensure that commercial navigation can continue on the Mississippi River after this summer's drought reduced water levels.

In the letter Thursday, Darcy said the corps would continue to "undertake all measures within its authority and available resources that are necessary to maintain navigation on the Mississippi River."

Last week, Agriculture Secretary Tom Vilsack also indicated that the administration believed the Mississippi River was a priority.

"If the Mississippi shuts down for an extended period of time," Vilsack told reporters, "we're actually talking about millions of dollars of lost opportunity or additional expense to be able to shift stuff by rail or to a different port. We'll get it to market, but it'll be more expensive."

Discussions over the river are "ongoing," Vilsack added. Later this month, the Army Corps is expected to begin to remove rock pinnacles along the river that have made navigation unsafe with the low water levels.