CAERNARVON
A Case Study
The Caernarvon diversion has been moving Mississippi River water through wetland marshes since 1991.
The Caernarvon diversion, downstream from New Orleans, has been operating since 1991 with the goal of enhancing fish and wildlife habitat, especially for oysters, by diverting fresh water, nutrients and a relatively small amount of sediment into the Breton Sound estuary. The project was designed to create an optimum salinity gradient across the marshes between the Mississippi River and the Mississippi River-Gulf Outlet, utilizing a flow schedule characterized by periods of moderate to low flow and designed to re-establish the salinity levels known previously in the Breton Sound estuary.
Beginning in 2001, John Day of Louisiana State University and a group of colleagues began a study known as PULSES. The study tested the hypothesis that the flow of water through a reintroduction should mimic the natural seasonal flooding of the river system. In other words, periods of high discharge (pulses) should be followed by little to no flow.
The rate of water flow through the Caernarvon structure can be adjusted to optimize benefits to the marshes.
Working in cooperation with the Caernarvon project managers, the reintroduction schedule was modified to include times of greatly increased flow. Before and after each pulse, samples measuring sedimentation, nutrient uptake, chlorophyll and salinity were gathered. The resulting data clearly demonstrated that pulses have a major beneficial effect on the marshes.
- During times of high flow, the nutrient-rich and sediment-laden waters overflowed the channels and spread out, enriching wide areas of marsh.
- River water remained in the marsh for a longer period of time than during previous flows, allowing nutrient uptake to occur.
- Chlorophyll content increased, indicating that the introduced nutrients led to increased algal growth.
Other non-PULSES data collected at Caernarvon indicate that an excessive inflow of nutrient-rich fresh water and accompanying low salinities might harm oyster beds or shrimp populations and stimulate excess algal growth in the estuary. Nevertheless, the PULSES findings were so strongly positive that Caernarvon managers have adopted a temporary pulsed-flow management schedule before the PULSES study has been completed.
The PULSES study modeled the Caernarvon discharge (black) on the natural pattern of Mississippi River discharge (blue). The large pulses early in 2001 and 2002 reflect this approach. (Modified from Day et al., 2003)
The sediments and nutrients that are prevented from reaching the Gulf by the Caernarvon reintroduction now bolster marsh ecosystems instead of contributing to the nutrient load offshore. If the pulsing flow pattern employed at Caernarvon is used as a model for other reintroductions along the Mississippi, coastal wetlands and the Gulf will both benefit.