Terrebonne Basin Dynamics
The Terrebonne Basin covers approximately 1,712,500 acres of southern Louisiana, including about 728,700 acres of wetlands (figure 22). About 96% of the wetlands in the Terrebonne Basin are privately owned. The USFWS recently established the 4,618-care Mandalay National Wildlife Refuge located in the Lake Hatch area of central Terrebonne Basin. State-owned land is represented by wildlife management areas (WMA's) and refuges covering about 28,244 acres in the southeastern basin. The state leases additional lands, which it manages as WMA's.
The USACE has constructed and maintains navigation channels in the Terrebonne Basin, which cross sensitive wetland areas. Vessel traffic in the channels is a major source of erosion in wetland areas. These channels also provide an avenue for saltwater intrusion into fragile wetland areas, thereby changing the salinity and nature of these wetlands and leading to deterioration and conversion to open water.
Subsidence occurs at different rates throughout the inactive deltaic plain as unconsolidated sediment dewaters and compacts. Subsidence in the Terrebonne Basin is among the highest in Louisiana at 0.42 inches/year (Penland et al. 1989). As subsidence occurs, flooding in wetlands increases, contributing to marsh loss. Subsidence also impacts the Terrebonne Basin's barrier island chains (Isles Dernieres and Timbalier Islands) that potentially provide protection to fragile inland wetlands. These islands absorb the impact of wave action from the Gulf of Mexico and potentially inhibit erosion of inland shorelines. As these islands shrink from subsidence, inland wetlands may become more vulnerable to the erosive forces of the Gulf of Mexico. Hurricane Andrew in 1992 had a severe impact on these islands, and without restoration, Louisiana's barrier islands will disappear. The extent of protection to interior areas by barrier island is currently being modeled through the Louisiana Barrier Shoreline Feasibility Study.
An abundant supply of fresh water and sediment is an important component to the health of wetlands in the Terrebonne Basin. These resources are supplied to the northern and western areas of the basin by the Atchafalaya River. The formation of the deep organic soils of this basin is a result of vegetative deposition, typically below ground with very limited mineral matter (Nyman et al. 1992, 1993a, 1993b, 1993c, 1994). The primary source of fresh water to the Timbalier subbasin (in the basin's southeast region) is precipitation, which averages 65 inches/year in this area. On average, precipitation is greater than evaporation; however, in the summer months evaporation exceeds precipitation. Sediment input into the southeast Terrebonne Basin occurs only when the Atchafalaya River stage is high and river waters flow down the Houma Navigation Canal. These inputs are small relative to the substantial influence of saltwater intrusion and high subsidence rates in the area. Overall, the southern basin has the most limited freshwater resources and sediment influx in the entire inactive deltaic plain. The absence of overflows from the riverine sources accounts for these freshwater and sediment deficits.
The hydrology of the Terrebonne Basin has been severely influenced by construction of canals and levees. As a result saltwater intrusion has occurred and has led to erosion and ultimate conversion of many areas from fresh marsh to salt marsh or open bodies of water. Barrier islands have also been impacted by erosion. As these islands have absorbed the wave energy of the Gulf of Mexico, they have continued to erode away.
Since 1932, the Terrebonne Basin has lost approximately 20% of its wetlands (Dunbar et al. 1992, figure 27). Current loss rates range from approximately 4,500 (Dunbar et al. 1992) to 6,500 (Barras et al. 1994) acres/year. This loss amounts to up to 130,000 acres during the next 20 years. One-third of the Terrebonne Basin's remaining wetlands would be lost to open water by the year 2040. Losses would be concentrated in the lower basin, where Timbalier Bay could become open to the Gulf of Mexico and the existing shoreline could retreat as much as 10 miles north (LCWCRTF 1993).