MORGANZA, La. — After declaring that “public safety is our No. 1 priority,” officials from the Army Corps of Engineers opened the Morganza spillway, sending water from the engorged Mississippi River rushing into the pastures and cropland here to forestall potentially catastrophic damage farther downstream.

As Col. Ed Fleming, commander of the corps’ New Orleans district, had said beforehand, one bay of the giant structure was opened Saturday at 3 p.m. local time, sending water out at a predicted 10,000 cubic feet per second. If current forecasts hold, officials would eventually open enough bays to allow water out at 125,000 cubic feet per second, a quarter of the spillway’s capacity.

That diversion would relieve the already enormous pressure on the levee system as the river courses past Baton Rouge, New Orleans and a corridor of chemical plants and oil refineries. But it would also flood hundreds of thousands of acres of farmland and thousands of homes, as it pours out into the Atchafalaya River basin. Evacuations have been taking place for days in the towns and communities throughout the basin, along with large scale operations to protect these towns with sand bags and other barriers.

The Morganza, one of four floodways in the Mississippi River and tributaries flood control system, has only been opened once, in 1973. “We’re using every flood control tool that we have in the system,” said Gen. Michael J. Walsh, commander of the Mississippi Valley Division of the Corps of Engineers.