Explanation of river flooding terms

BATON ROUGE, LA (WAFB) - With the rising waterways, there are words being used some people may not be familiar with. This is just for some terminology and clarification.

"River stage" is the height of the river at any given time. For most of our south Louisiana sites, "a stage of 0 feet" would mean sea-level, and the stages we get are reported relative to sea level. (The direct association to sea-level is not used for Mississippi River sites north of Red River Landing.)

"Flood stage" is a 'relict' term that we continue to use and can be confusing. This is the level at which the river would produce flooding, if there were no levees.

"Crest" is the peak/highest level of water expected.

"Levee height" is rather obvious, and THIS is the number to compare to the river stage and forecasted crest.

For example, for the Mississippi River at Baton Rouge, as of Tuesday evening (May 3):

Flood Stage (FS): 35.0 ft (flood stage NEVER changes)

River Stage:  35.4 ft (at 7 p.m., the river is "officially" in flood because that exceeds FS)

Forecast Crest:  47.5 ft on May 23 - note that the date is nearly as important as the crest value

"Volume" or "CFS" is cubic feet per second. Rather than measure the height of the water (river stage), this tells how MUCH water is moving. While there is an obvious relationship between "stage" and volume (or sometimes referred to as rate), the Army Corps of Engineers really uses critical "cfs" values (that must be actively measured) to determine when to open the Bonnet Carre and Morganza Spillways (aka, Floodways).

River Control Structure/Complex (RCS):  A floodgate system well north of Baton Rouge designed to divert approximately 30 percent of the Mississippi River flow down the Atchafalaya River (as mandated by Congress). The RCS is managed by the Army Corps of Engineers, and must be adjusted routinely as the flow in the Mississippi River changes. It is always operating, allowing greater and greater volumes of water down the Atchafalaya River as the Mississippi River gets higher and higher.

Bonnet Carre and Morganza Spillways:  Two additional floodgate systems designed to divert water from the Mississippi River when it is exceptionally high and the Army Corps of Engineers deems there to be a threat to the levee system.

The Bonnet Carre Spillway was built after the Great Mississippi Flood of 1927. It diverts Mississippi River water into Lake Pontchartrain. It has been opened nine times, most recently in 2008. The Morganza Spillway sends water down the Atchafalaya River, in addition to that already sent down the river by the RCS. It has been opened only once, in May 1973, which led to the flood-of-record for Morgan City.

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