Ted Jackson, Associated Press
Heavy rains prompted the U.S. Army Corps of Engineers to open a spillway north of New Orleans on Friday.

NORCO, La. (AP) — The Army Corps of Engineers opened a spillway near New Orleans for the first time in 11 years on Friday in an effort to spare the city any flooding from the swollen Mississippi River.

Heavy rain in the Mississippi Valley prompted the decision to open the Bonnet Carre Spillway, about 30 miles north of New Orleans.

The corps said the spillway could be open for a month. Gov. Bobby Jindal, who gave the cue to open the spillway with a thumbs-up to workers, said corps officials assured him levees should hold and should not be topped.

In addition to helping guard against flooding, opening the spillway is meant to ease pressure on levees and make it safer for ships and barges to navigate the river. The Mississippi River between Baton Rouge and New Orleans is one of the world's busiest commercial waterways.

The river is typically high in spring as a combination of northern snow melt and rain in the U.S. heartland turns it into a rushing torrent of muddy water.

Despite the annual rising of the river, the corps seldom employs the spillway. Friday's opening was only the ninth since the spillway's construction in 1931.

The spillway diverts river water to Lake Pontchartrain, and from there into the Gulf of Mexico. The runoff from the river is not expected to significantly raise the level of the lake, but it is expected to affect plant and marine life as fresh river water pours into the brackish lake. The state plans to monitor its effects.

The spillway has 350 gated bays, each of which holds 20 large timbers — "needles" — that are removed by cranes to allow river water to flow through. The corps planned to open 38 bays initially, though Lt. Col. Murray Starkel said more could be opened if conditions warranted.

Corps engineers also opened some spillways Friday to release water into the already soggy White River basin from lakes swollen by recent rain in Arkansas and Missouri.

The corps began letting water out from Beaver, Table Rock and Norfork lakes, which dump water into the White River. The river rises in northwest Arkansas, flows north into Missouri, and then back south through Arkansas on its way to the Mississippi River.