Understanding Diversions: The Water Column

A water column illustrates a top-to-bottom cross section of a body of water, plus its sediment particles - something like a rock core sample for water. In the case of the Mississippi River, the body of water perhaps most important to Louisiana's coastal wetlands, a water column contains billions upon billions of sediment particles.

Historically, this river changed course periodically, forming new land in different locations. Each year, during spring flooding, water levels overtopped the river banks, depositing sediment into the adjacent coastal wetlands. Today, the river's course is fixed and levees have replaced the river banks, preventing sediment from reaching the wetlands.

As the Caernarvon Freshwater Diversion Structure has shown, diverting fresh water before it gets to the Gulf is a highly productive method of protecting and restoring coastal marsh. But understanding how and why Caernarvon works, and applying that knowledge to future diversions, hinges on understanding the water column.

Using the Column in Freshwater Diversions

In freshwater diversions, like Caernarvon and the Davis Pond Structure currently under construction, water is diverted from the upper portions of the water column. This approach limits the amount of sediment entering the diversion outfall area while lowering salt levels and strengthening nearby freshwater marsh and plant life. The added growth, death and decay of plants does contribute to a base for new marsh to grow upon.

Using the Column in Sediment Diversions

In a sediment diversion, sediment and water are both diverted from lower parts of the water column. A greater amount of sediment enters the outfall area. As the diverted sediment accumulates (or accretes), new marsh is formed. The accompanying fresh water helps offset saltwater intrusion into the new marsh and provides a steady stream of nutrients to the newly formed marsh. Sediment diversion is a much more expensive process than freshwater diversion.

Freshwater Layers

This area of the water column is typically free of larger sediment particles. While sediment exists in the upper parts of the column, it is finer material like silts and clays -- materials with extremely small diameters.

It's important to note that the river's turbulence promotes mixing between the layers. As a result, there is never an even gradation between layers.

Sediment Layers

The larger particles tend to be located in the lower parts of the column and are predominantly sand. The concentration of sand within the column is generally lowest at the top of the column and highest just above the river's bottom (the riverbed).

Riverbed

In the bottom of the river, the "bedload" travels toward the Gulf of Mexico, where most of it is deposited in deep water beyond the continental shelf.