Linda F. Benedict, Keim, Richard
Catahoula Lake is a Ramsar Wetland of International Importance in central Louisiana and one of the most important habitats for waterfowl in the Lower Mississippi Valley. In recent decades woody vegetation has been increasing at the lake and degrading herbaceous plant communities, which are critical food for waterfowl. LSU AgCenter researchers are investigating the hydrologic mechanisms of this change, and they are working with the Louisiana Department of Wildlife and Fisheries to develop water management strategies that better mimic the natural variability in the lake that originally created this important wetland. The lake has a large annual fluctuation in water level controlled by the Little, Tensas, Black, Red and Mississippi rivers. River management for navigation and flood control has changed lake sediments and water levels in the lake, and the expansion of woody vegetation has accelerated during the era of management. Management to restore the lake will require innovation because of the loss of natural hydrologic variation.
Catahoula Lake, which covers about 30,000 acres in central Louisiana, is one of the most important habitats for water birds in the Lower Mississippi Valley. Because of this importance, the lake is recognized as a Wetland of International Importance under the Ramsar Convention of 1971 and receives special attention from agencies responsible for managing it, including the Louisiana Department of Wildlife and Fisheries, U.S. Army Corps of Engineers and the U.S. Fish and Wildlife Service.
The lake has a highly unusual hydrologic regime that is responsible for the quality of water bird habitat. The lake fluctuates from nearly dry to up to 20 feet deep each year, from deep water in the late winter through early summer to a mud flat in the later summer and fall when herbaceous plants important to wildlife habitat can grow. The position of the lake at the margin of the Mississippi Alluvial Valley and proximity to the Mississippi, Atchafalaya, Red, Ouachita, Tensas and Black rivers means the lake is subject to backwater flooding that creates the high water conditions (Figure 1). The lake also receives all of the flow of the Little River, which drains much of north central Louisiana, so times of high local rainfall can also cause the lake to rise. However, during the late summer and fall, the lake usually loses almost all of its water because of drier conditions.
Extensive modification of rivers surrounding the lake for flood control and navigation has changed the natural flooding patterns responsible for creating this unusual lake. Levees throughout the Lower Mississippi Valley have reduced overland flow between large rivers and the lake, and locks and dams have changed flows in channels that feed and drain the lake. The most important change at Catahoula Lake was a set of locks and dams in the Black River completed in 1972, which would have permanently inundated the lake.
For the seasonal drawdown, an 18-mile canal was constructed to allow the lake to drain below the locks, and a water control structure was added to allow control of water in the lake. Management of this structure has been according to a strict prescription for annual fluctuations in water levels developed by the agencies concerned and is presently accomplished as a cooperative endeavor by the Louisiana Department of Wildlife and Fisheries and the Army Corps of Engineers. The goal of the plan is to manage water to produce the best habitat for water birds and waterfowl hunting, based on the historic fluctuations that define the lake. The focus of the management is on reproducing the predictable wetting and drying, but eliminating random variations – especially those that conflict with hunter success. Adherence to this plan has reduced overall variability in water levels, which is a common problem in wetland management that frequently results in plant community changes. However, it is not clear what role water level variability may play at Catahoula Lake.
Although the lake is surrounded by forest and ringed by baldcypress, water-elm, swamp privet and buttonbush, the majority of the lake bed has remained almost totally free of trees for centuries. In recent decades, however, woody vegetation has been increasing at the lake and degrading the important herbaceous plant communities. The Louisiana Department of Wildlife and Fisheries has implemented woody vegetation removal programs for several decades to fight this trend. However, the cost of this program is substantial, and the continual need for it begs the questions: Why was Catahoula Lake historically free of woody vegetation? Can water in the lake be managed to better mimic historic hydrologic processes and naturally control woody vegetation?
To address these questions, the Louisiana Department of Wildlife and Fisheries funded research in the LSU AgCenter School of Renewable Natural Resources. Researchers addressed the motivating questions with four linked research projects:
(1) Use tree rings and aerial photos to quantify occupation of the lake by trees over the past 70-100 years.
(2) Measure sediment quantity and chemistry to identify whether sedimentation may be contributing to vegetation change.
(3) Model historic hydrologic processes controlling lake levels and compare current regimes to historic regimes to identify whether flood stress on trees may have changed recently.
(4) Model wave disturbance of the lake bed and assess whether recent bed disturbance may have decreased in the recent past.
The combined investigation of historic aerial photos and tree rings revealed that woody plants have been encroaching into the lake bottom for at least 60 years, but that the rate has increased in the past 40 years (Figure 2). Woody vegetation covered 29 percent of the lake bed in 1940, 30 percent in 1966, and 38 percent in 2007, despite removal efforts that began in the 1950s and intensified in the 1990s. Thus, despite greater efforts to remove woody vegetation recently, coverage is increasing at a greater rate than in the past. This acceleration in the era of the Diversion Channel suggests that managed water levels may be fundamentally different from pre-modification water levels in a way that makes an ecological difference. However, previous work has shown that water-elm, the principle woody species expanding at the lake, is extremely resistant to flood stress and that simple flooding is insufficient to control its spread. Therefore, we need to learn more about the nature of flooding changes in the lake and how they might be affecting habitat for water-elm.
Using concentrations in lake bed sediments of a radioactive isotope deposited globally during nuclear testing in the 1950s and 1960s, AgCenter scientists found that sediment deposition rates have not been appreciably higher in the past 60 years as compared to the previous two centuries (using rates determined from other sediment tracers in previous work). Thus, it appears that shallowing of the lake is likely not a major reason for increased woody vegetation. However, recent sediments are lower in pH and contain higher concentrations of zirconium and titanium than older sediments, which suggests a change in the source of sediments from mixed coastal-plain and Mississippi-alluvial sources to being dominated by coastal-plain sediments via the Little River. This change coincides in time with the decrease of connectivity between the lake and larger rivers by locks, dams and levees, although we do not yet know whether chemical changes are the cause of increased woody vegetation.
Using a sophisticated wave model modified from an ocean wave model in collaboration with researchers at the University of Miami, AgCenter researchers found the strength of wave forces on the lake bed varies by water depth and by seasonal variations in wind. Although still in the experimental phase, early results of this work indicate that locations most subject to wave stresses coincide – at least roughly – with locations with the lowest rates of woody vegetation encroachment. This modeling capability will be combined with the developing model of historic lake water level variability to estimate the effect of water management on wave action and thus woody plant establishment.
Management to restore the lake will require innovation because of the loss of natural hydrologic variation caused by extensive modification of the Lower Mississippi Valley. The LSU AgCenter is working with the Louisiana Department of Wildlife and Fisheries to develop experimental water level management schemes to both learn more about how water levels and lake ecology are related and to plan for better ecosystem-focused water management in the future. For example, there have been recent deviations from the strict water management guidelines in an effort to increase variability in the lake water regime. These deviations have been accompanied by complaints from lake users (e.g., waterfowl hunters and oil companies) who have grown accustomed to predictability in lake level. The LSU AgCenter and the Louisiana Department of Wildlife and Fisheries are communicating with stakeholders as planning continues.
Catahoula Lake is one of the most unusual geographic features in Louisiana and one that relatively few residents are familiar with or understand. This lack of understanding of how water levels in the lake control its ecosystem and the significance of the lake for wildlife habitat combine to increase the importance of work to ensure its conservation.
Richard Keim is an associate professor in the LSU AgCenter School of Renewable Natural Resources. His areas of expertise are hydrology of forested wetlands and watersheds and management of bottomland and coastal forests.
The summer 2015 issue of Louisiana Agriculture magazine includes articles on a variety of topics that affect Louisiana’s agriculture industry and the environment – water management at Catahoula Lake, 4-H youth wetland programs, artificial reefs for water conservation, corn nitrogen management in saturated soil conditions, and more. 36 pages