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One of the most important tools in the wetland restoration toolbox is river diversions. However, there seems to be a lot of misinformation and misunderstandings about the role and impacts of river diversions as a restoration tool.Louisiana’s wetlands were created from mud and sand being transported from areas outside Louisiana and deposited in shallow-water areas. The Mississippi River has had several channels over time, with the channel switching to a shorter route to the Gulf of Mexico whenever the older channel became too inefficient. Every time the channel would change, a new delta would form and the older delta would erode.
Were it not for the Old River Control Structure, the Atchafalaya River would have captured the majority of the flow of the Mississippi River by now. Congress has mandated that no more than 30 percent of the Mississippi River’s flow can be diverted into the Atchafalaya River, so for now, most of that bedload sediment coming down the Mississippi is being deposited off the continental shelf into deep water.
That 30 percent of the Mississippi River that flows down the Atchafalaya River is quite efficient and cost-effective at creating marsh in the Atchafalaya and the Wax Lake Outlet deltas.
The idea with river diversions is to try to get the sediment and nutrients in the river water out of the river before they are lost offshore. While the nutrients don’t build land, they certainly act as fertilizer for marsh plants.
At present, the following controlled diversions are currently up and operating: Davis Pond (10,500 cubic feet/second); Caernarvon (8,000 cfs); Naomi Siphons (2,000 cfs); and West Pointe a la Hache siphons (2,000 cfs).
Under the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA), smaller diversions into Bayou Lafourche and Hope Canal (near Lake Maurepas) are in the engineering and design phase.
CWPPRA is also paying for the creation and maintenance of uncontrolled diversions (crevasses) at numerous locations in the Mississippi Delta. Under that project, 18 crevasses were constructed and/or maintained in 1997, and a second phase of crevasse creation and maintenance will go to construction this summer.
Some of the biggest areas of misinformation and misunderstandings seem to be how effective diversions are at restoring wetlands, and their impacts on various fishery species.
In terms of effectiveness, certainly the larger the diversion, the more sediment is potentially deposited and the more land is potentially restored. Monitoring results from Caernarvon during the 1991 to 1998 time period suggest that more than 400 acres of marsh had been created in the affected area during that time frame. That doesn’t even consider the benefits the diversion has on slowing wetland loss rates.
Some people believe diversions could result in algae blooms and fish kills. The supposition is that nutrients in the water encourage the rapid growth of algae, which eventually die, and during their decomposition, use up the oxygen in the water.
While nutrients in Mississippi River waters are the cause of the large “dead zone” in Louisiana’s offshore waters, such has not occurred in inshore waters from past discharges at Caernarvon, or in Lake Pontchartrain from the occasional discharge at the Bonne Carre spillway. Generally, inshore waters are so shallow that they are well mixed by winds and tidal action, and are, therefore, highly oxygenated.
Finally, there is a lot of misunderstandings of the impacts of river diversions on commercially and recreationally important fishery species. Pre- and post-construction sampling by the Louisiana Department of Wildlife and Fisheries at 21 locations in the Breton Sound estuary indicate a lower catch-per-unit-effort (CPUE) for brown shrimp after Caernarvon went into operation, higher CPUEs for white shrimp, and no discernable difference for blue crab.
Statewide, young life stages of white shrimp appear to be partial to fresher systems than young brown shrimp. Young blue crab appear to be unaffected by a wide range of salinities.
While CPUE for brown shrimp is down, it appears that this reduction in harvest is a result of a geographic displacement of the fishery. That is, salinity reductions caused by Caernarvon have resulted in a seaward movement of the best harvest zones for brown shrimp.
Results are similar for finfish. Post-construction CPUE at Caernarvon increased for seven of eight finfish species studied. That is, there appeared to be more red drum, black drum, sheepshead, spotted seatrout, sand seatrout, Atlantic croaker, and gulf menhaden after Caernarvon went into operation.
And although it was an unexpected benefit, the increase in the largemouth bass fishery in the vicinity of the Caernarvon structure has been nothing short of incredible. Only spot, an important forage fish, decreased in abundance.
Often fishery trade-offs, politics and design issues can keep the diversions from being used to their maximum possible extent. Since 1992, Caernarvon has averaged only 15-20 percent of its flow capacity.
Due in part to some engineering issues needing to be addressed, Davis Pond has yet to flow at its maximum capacity.
And while diversions of large amounts of river water undoubtedly will have some displacement impacts on a few very important fishery species, not using one of our principal restoration tools to the maximum extent practicable will ultimately result in more drastic reductions in greater numbers of other important fisheries.
Because more than 90 percent of Louisiana’s fishery species are considered to be estuarine-dependent, the eventual loss of our wetlands will hurt all important fishery species, including spot and brown shrimp.
Richard Hartman is the leader of the Louisiana Habitat Conservation Division of NOAA Fisheries.
