High River Could Trigger Second Spillway Opening

With water from the springtime snowmelt and weather systems upriver heading downstream, gages along the Lower Mississippi River have started creeping up again, continuing an unusually long high water season that stretches all the way back to November of last year.

As of May 2, the Mississippi River at the Carrollton Gage in New Orleans, La., was at 16.5 feet. According to the National Weather Service, the river at the Carrollton Gage is forecast to reach 16.7 feet by May 12 and hold there for a week, followed by a slow fall.

At 16.7 feet or above, the river in New Orleans will be close to requiring the U.S. Army Corps of Engineers to operate the Bonnet Carré Spillway, 33 river miles upriver from the city. The Bonnet Carré diverts river water from the Mississippi northeastward to Lake Pontchartrain. The trigger point for opening the spillway is a flow rate of 1.25 million cubic feet per second (cfs.) at the Carrollton Gage, which typically corresponds to a river stage of 17 feet, but the relationship of flow rate and river stage fluctuates.

“The forecast from the National Weather Service has the river getting close to the trigger point of 1.25 million cfs.,” said Matt Roe, public affairs officer for the New Orleans District. “We are watching the forecast and conditions across the Mississippi Valley for any changes.”

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If the river triggers operation of the Bonnet Carré Spillway, it will be the first time in the structure’s 88-year history that it’s been opened twice in a single year. Bonnet Carré has already had a historic decade. For most of its history, the Bonnet Carré has averaged one opening per decade. However, the Corps has opened Bonnet Carré five times since 2008, including three of the last four years.

According to the National Weather Service, longterm forecasts indicate the river could finally drop below 16 feet at Carrollton near the end of the month.

The last time the Carrollton Gage was below 12 feet was December 13, 2018.