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Sediment trapping along brown- and blackwater Coastal Plain riparian forested wetlands.
Hupp, Cliff*,1, King, Sammy, 1 U.S. Geological Survey, Reston, Virginia, USA
ABSTRACT- Sediment trapping is cited as a major function of riparian-forested wetlands. Few studies have compared local sedimentation processes on brown- versus blackwater systems. Sedimentation rates were estimated along the brown-water Cache River, Arkansas and the blackwater Coosawhatchie River, South Carolina along transects normal to streamflow, using dendrogeomorphic (tree ring) and clay-pad marker techniques. Rates were related to several physical parameters, woody vegetation, and its debris. Mean deposition ranged from 0.20 to 0.36 cm/yr and from 0.02 to 0.20 cm/yr on the Cache and Coosawhatchie, respectively. The Cache carries a suspended load (TSS) of about 100 to 350 mg/l, whereas the Coosawhatchie carries about 5 to 25 mg/l. Sedimentation patterns varied strongly with elevation and thus, vegetation patterns. Hydraulic connectivity (degree of flow-path connections to the main channel) appears to strongly affect sedimentation rates; greatest deposition occurs along sloughs with a direct flow path to the river. Whereas, low areas poorly connected to the channel have relatively low deposition rates, presumably due to diminished replenishment of suspended sediment during the hydroperiod. Sedimentation rates are highest in and near sloughs and their anabranches and lowest in areas unaffected by sloughs even though both floodplains are largely inundated during peak hydroperiod. Woody vegetation, including woody debris, may also play an integral part in concentrating flow paths. Low velocities facilitate deposition of fines, particularly organic material, however relatively moderate velocities may ensure a continuous supply of sediment-laden water, particularly mineral fines. The Cache (187.6 kg/ha/yr) and the Coosawhatchie (24.5/ha/kg/yr) trap substantial amounts of sediment, reinforcing the water-quality functions of these forested wetlands Fines deposited over the clay pads contained substantial amounts of organic material with a mean of 22 percent and 40 percent on Cache and Coosawhatchie, respectively; high concentrations of organic material may have important implications for global carbon cycling.
Key words: sedimentation, coastal, riparian, wetlands