Very low flows in the Apalachicola River, defined as flows less than 6,000 cubic feet per second (ft3/s) at Chattahoochee, Florida, occurred in 15 of the 74 years of record from 1922 to 1995. At a river flow of 5,000 ft3/s, an estimated 260 acres of floodplain streams and lakes is connected to the main river channel. Most of these areas have shallow waters with no flow and are located in the middle and nontidal lower reaches of the river. These connected aquatic habitats comprise a very small percentage (0.3 percent) of the entire floodplain at very low flows, yet they serve as important refuges for fishes from the deep, swiftly flowing waters of the main channel. In the upper reach of the river, entrenchment that occurred after construction of Jim Woodruff Dam lowered bed elevations and river levels. Many perennial streams in the upper reach that were accessible to main channel fishes at low and very low flows prior to entrenchment are now inaccessible because of waterfalls or very shallow water at their mouths. About 4,000 acres of isolated aquatic habitat, mostly tupelo-cypress swamps with standing water less than 3 feet deep, is also present in the floodplain at very low flows. A review of the literature indicates that many species of fishes inhabit the quiet, shallow waters typically found in isolated swamps.
Low flows (6,000-10,000 ft3/s at Chattahoochee, Florida) occur in most years. The median annual 1-day low flow for the period of record is 8,490 ft3/s. About 740 acres of aquatic habitat in the floodplain is connected to the main channel at a river flow of 8,000 ft3/s. Most of these areas are tributary lakes, which are open bodies of water having a linear conformation and little or no flow except during floods. Large tributary lakes in the middle and lower reaches of the river, such as Iamonia Lake and River Styx, support diverse fish communities. In a previous study, 44 fish species were collected by the Florida Game and Fresh Water Fish Commission in tributary lakes during low flows.
Medium flows (10,000-20,000 ft3/s at Chattahoochee, Florida) occur every year. At the median flow for the period of record, which is 16,400 ft3/s, approximately 8,300 acres (10 percent of the floodplain) is connected aquatic habitat. Most of these areas are tupelo-cypress swamps bordering streams and lakes in the middle and nontidal lower reaches that are inundated by backwater from the main channel. Flowing-water habitats in more than 200 miles of streams and lakes are also connected to the main channel at the median flow. The amount of vegetative structure in connected aquatic habitats is much greater during medium flows than during low flows, because water is no longer contained in the beds of floodplain streams, but is covering vegetation and woody debris on streambanks and in adjacent swamps. Vegetative structure in aquatic habitats provides food sources, protective cover, and reproductive sites for fishes.
Medium-high flows (20,000-50,000 ft3/s at Chattahoochee, Florida) occur every year. An estimated 40,700 acres, which is approximately one-half of the floodplain, is connected aquatic habitat at 32,000 ft3/s. Nearly all aquatic habitat in tupelo-cypress swamps that is isolated at lower flows is connected to the main channel between flows of 20,000 and 40,000 ft3/s. High flows (greater than 50,000 ft3/s) occur in most years. At the median annual 1-day high flow of 86,200 ft3/s, about 78,000 acres (95 percent of the floodplain) is connected aquatic habitat. During high flows, water is moving through most of the floodplain in a general downstream direction. Many main channel fishes migrate into flooded forests where greatly increased food sources and abundant vegetative structure are available to them. Eighty percent, or 73 of the 91 fish species known to inhabit the Apalachicola River have been collected in river floodplains of the eastern United States and are probably present in the Apalachicola River floodplain during medium-high and high flows.
In evaluating the impacts of flow alterations, it is important to determine types and extent of habitat affected, address impacts on biotic communities, and make comparisons of altered to historical flows. In an example, effects on habitat as a result of flow regulation to create a navigation window for barge traffic in the fall of 1990 were examined. For 19 days during this period, there was approximately 590 fewer acres of connected aquatic habitat than there would have been if the navigation window had not been implemented. Effects of reduced aquatic habitat on fishes include reductions in the amount of food, protective cover, and spawning sites. A hydrologic event with flows similar to this period of reduced flows occurred once every 10 years on average (1922-95) and probably would not have occurred in 1990 if navigation windows had not been implemented.