With an average annual rainfall of almost 60 inches, Mississippi is the third wettest state in the U.S. Much of that rain falls in late winter and early spring. While filling the rivers, streams and reservoirs that provide bountiful fisheries, the rains negatively affect fisheries by transporting erodible soils from the watershed into fishable waters.
The obvious effect of erosion is muddy water. The sediment-laden water reduces light penetration and limits the production of aquatic plants. Phytoplankton, the microscopic aquatic plants (algae) that are suspended in the water column, convert the sun’s energy into organic matter that fuels the food web in ponds, lakes and reservoirs. With less light, there is less phytoplankton production and, in turn, less production of aquatic animals including fish.
The muddy water also limits the growth of rooted aquatic plants that provide important habitat for fish and the invertebrates they feed upon.
When the rains stop and the flows slow, the water clears, but the sediment continues to damage fisheries as it settles to the bottom of the river, stream or lake. Areas of gravel and hard bottom that provided important foraging and spawning habitat for many fishes are smothered with silt and become unproductive. The quality of the fishery declines.
While sediment can quickly damage fisheries, long-term effects of erosion can be even more disastrous as the sediments accumulate, converting deep water to shallow and shallow water to dry land. The Tenn-Tom Waterway in northeastern Mississippi is a dramatic example of this process.
In the 1970s and early 1980s, a series of dams with navigation locks were constructed to convert the Tombigbee River into a waterway usable for commercial navigation — the Tenn-Tom Waterway. A study at Mississippi State University analyzed aerial photographs of the Tenn-Tom Waterway taken in 1985, shortly after the waterway was completed, and in 2003.
In 1985, the total aquatic area of the Mississippi portion of the waterway — from Bay Springs Lake in the north to Aliceville Lake in the south — was 27,228 acres. By 2003, the total aquatic area had declined to 25,903 acres, a loss of 1,325 acres in only 18 years.
The greatest change in water area occurred in the expansive downstream lakes — Aberdeen, Columbus and Aliceville. In 1985, Aberdeen was 2,817 acres, Columbus was 5,249 acres and Aliceville was 6,044 acres. By 2003, each of these three lakes lost between 8 and 11 percent of their aquatic habitat.
The largest loss of habitats occurred in backwaters, embayments and cutoff bendways — former channels of the Tombigbee River that were bypassed by man-made channels to allow commercial barge traffic to navigate the waterway.
Backwater areas decreased 33 percent in Aberdeen Lake, 24 percent in Columbus Lake and 53 percent in Aliceville Lake.
“Embayment areas decreased 16 percent in Aberdeen Lake, 11 percent in Columbus Lake and 9 percent in Aliceville Lake.
“Areas of cutoff bendways connected to the navigation channel at their upstream and downstream ends decreased 28 percent in Aberdeen Lake, 33 percent in Columbus Lake and 52 percent in Aliceville Lake as the upstream ends of these bendways filled in and blocked water flow and angler access.
These three categories of habitats — backwaters, embayments and bendways — are important habitats because these are the aquatic habitats where most sport fishes and forage fishes live, grow and reproduce. And these are the habitats where most anglers fish.
One summer day a couple years ago, I returned to a cove in Aliceville Lake to fish a creek channel where I had caught a nice limit of bass the previous summer. Sediment had claimed the creek channel and had so filled in the back of the cove that I couldn’t even get my boat within casting distance of the little channel. On Columbus Lake, sediment accumulation now blocks access to several backwater ponds where I have enjoyed good catches. Many anglers on the waterway tell similar stories.
The process of erosion from the watershed and sedimentation in water bodies is not unique to Mississippi. Nationwide, four billion tons of soil erode from disturbed lands each year, and many fisheries biologists throughout the country consider sedimentation one of the biggest threats to healthy fisheries.
The McClellan-Kerr Arkansas River Navigation System converted the Arkansas River into a navigable waterway by construction of a series of locks and dams, as was done on the Tombigbee River in Mississippi and Alabama. From 1973 when the navigation system began operation to 1999, the area of aquatic habitat in 12 pools of Arkansas River waterway shrunk 10 percent. As in the Tenn-Tom Waterway, the loss of aquatic area occurred primarily in the backwater and embayment habitats, the habitats off the main river channel that are important for fish production and fishing.
Sediment transport from uplands to rivers, streams and lakes is unavoidable, and sedimentation will gradually fill reservoirs. Indeed, engineers who design reservoirs consider sediment accumulation when they estimate the lifespan of reservoirs.
Although watershed erosion and sedimentation are inescapable, the rates can be reduced by best management practices on the watersheds and in the riparian zones — the strips of land that flank the stream bank or the shore of the lake. Controlling bank erosion also reduces sediment accumulation. Where sedimentation has already claimed fishing areas or access to them, organized anglers have been able to successfully navigate political channels to restore fisheries.
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