By now you’ve probably seen the flooding in Louisiana and Mississippi, with the worst of it centered in Baton Rouge, La. With historic rainfall and swollen rivers, the flooding has reached beyond river banks and into neighborhoods and homes, resulting in the deaths of several people and the displacement of thousands.

Part of the reason the flooding has been so extensive is that the natural drainage of Baton Rouge and its environs has become overwhelmed. With the construction of the Mississippi River levees, the Comite (pronounced KOH-meet) and Amite (pronounced A-meet) rivers became the primary drainage flows to Lake Maurepas and eventually the Gulf of Mexico.

The historic rainfall has filled not only the Amite and Comite, but also their many tributaries, including bayous, creeks, streams, canals, and culverts. So as the Amite and Comite fill with water, the ability of those tributaries to pass the accumulated rainfall is greatly diminished, so the water backs up, much like a traffic jam caused by road construction does on the freeway, causing flooding upstream. This process is known as backwater flooding, and it’s responsible for much of the neighborhood flooding happening in Baton Rouge.

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Backwater Flooding: The Process

Water flows downhill; that’s one of the things everyone knows about it. So the water levels in the main river channels are usually lower than the tributaries that empty into them. The tributaries, in turn, are usually lower than the land that drains into them.

When large floods occur, water from the main channel backs up into the tributaries and spills over onto the land surface. This is “backwater” flooding. This backwater flooding is worsened when rain continues to fall on the land and more water piles up in the tributaries as those streams cannot quickly discharge the accumulated rainwater into the main channel.

In Baton Rouge right now, the main river channels have become so full of water that this water is backing up the tributaries. With the tributaries already swollen from the same rainfall, the inability for them to drain plus the addition of extra water from the main river channel can cause the tributaries to back up into the neighborhoods around them. Since those neighborhoods were designed to rely on the tributaries for drainage, the water has no place to go and so just sits.

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What Happens Next?

The good news is, as the Amite and Comite gradually drain into Lake Maurepas, their levels will go down enough to allow more of the water in the tributaries to flow out as well. That allows the natural drainage to resume and will take care of much of the backwater flooding. However, some of the areas where the water flooded into will need longer to drain, because they were not intended to flood.

USGS crews will be out monitoring water levels in the Amite River and surrounding area to keep track of the drainage and servicing the streamgages that report water levels here. In addition, USGS scientists will be flagging high-water marks to help efforts to forecast potential flooding during future events.