Dr. Theryn Henkel talks Caernarvon

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The changing landscape around the Caernarvon Freshwater Diversion from 1998 to 2015. Land gain was not visible until 2005. Prior to that Big Mar was filling in with sediment and a threshold was finally reached where land emerged above water and became vegetated. Also noticeable is the substantial land loss that occurred between 2004 and 2005 due to Hurricane Katrina. The delta continues to expand over time. Learn more about Caernarvon here. 

DIVERTING THE LAND-BUILDING POWER OF THE MISSISSIPPI


It is no secret that Louisiana's coastline is disappearing. One of the proposed solutions to the coastal land loss conundrum is building new land. However, constructing new land is easier said than done. To successfully restore an existing damaged marsh or to build one from scratch requires massive amounts of sediment.

The Mississippi River contains a surplus of sediment for engineers to borrow. The one problem.... sediment is really heavy. The cost to move metric tons of sediment is astronomical. In recent years coastal authorities have developed a solution to this problem, let the Mississippi River do the heavy lifting for them.

The 2017 Coastal Master Plan for developing to begin construction on two river diversion projects by early 2020. These sediment diversion projects are created by making a targeted breach in the levee and allowing water and sediment to flow into damaged wetlands.

Diverting the Mississippi River is not a new concept. The Caernarvon Freshwater Diversion has been harnessing the power of the Mississippi River for over 30 years for the purpose of controlling salinity.

 

LAND CHANGE IN BIG MAR


In southern Louisiana, land loss rates are high and areas of delta growth have generally been limited to the Atchafalaya Delta and the Bird’s Foot Delta. The Caernarvon Diversion was built to regulate surrounding salinity levels with fresh water to improve fishery productivity in the Breton Basin. Doing so required moving river water from the Mississippi into the local estuary. Because the original intention of the diversion was to regulate salinity, it was placed at an area of the Mississippi River unlikely to be laden with sediments and constructed to avoid sediment capture by drawing water off the surface of the river.

 

Despite this, new land is building. Since the construction of the diversion in 1991, sediment has been deposited in Big Mar Pond and beyond. Over time, there has been enough accumulation in some areas to permanently support emergent wetland plant life. In Big Mar Pond, prior to 2004, wetland growth was negligible, but since then wetland growth (or land gain) has been significant and appears to be increasing annually. Over 700 acres of land growth has occurred since the diversion came on line in 1991!

 

Here's a breakdown: From 2005 to 2012, there was 153 acres of land gain. From 2010 to 2011, there was significant wetland growth most likely built during the two months that the Caernarvon Diversion was open at maximum capacity (8,000 cfs) to combat the 2010 BP Oil Spill. Coincidentally, while open at maximum capacity, there were also three sediment spikes in the river that feeds the diversion allowing for more sediment deposition than expected. In 2014 and 2015, long periods of low water in the basin during the summer led to delta growth as vegetation expanded.

 

Over time we have seen a delta form with classic delta morphology of crevasse splays and distributary channels. The delta has quickly become vegetated with fresh marsh species and an extensive black willow forest.

You can track environmental changes in the Pontchartrain Basin by using our Hydrocoast Maps. The series is updated biweekly and you can receive it via email by signing up here. 

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