Plans for opening a Mississippi River spillway in Louisiana have been cut back dramatically because the river is rising more slowly than expected.

The Army Corps of Engineers says it plans to release 20 percent of the water originally planned through the Morganza Spillway. Its opening is scheduled Sunday.

Spokesman Ricky Boyett says crews will release about 30,000 cubic feet per second. That's enough to fill the Boston Aquarium's biggest tank in less than one second.

The spillway has been opened twice.

It's not yet needed for flood control. However, the river is expected to get high enough to overflow the spillway unless some spillway bays are opened.

Water flowing over the top would prevent crews from opening it if the river becomes fast enough to threaten levees.