SEDIMENT RETENTION IN A BOTTOMLAND HARDWOOD WETLAND IN EASTERN ARKANSAS

One of the often-stated functions of wetlands is their ability to remo ve sediments and other particulates from water, thus improving water q uality in the adjacent aquatic system. However, actual rates of suspen ded sediment removal have rarely been measured in freshwater wetland s ystems. To address this issue, suspended sediment dynamics were measur ed in a 85-km(2) bottomland hardwood (BLH) wetland adjacent to the hig hly turbid Cache River in eastern Arkansas during the 1988-1990 water years. A suspended sediment mass balance was calculated using depth-in tegrated, flow-weighted daily measurements at wetland inflow and outfl ow points. Over the three-year period, suspended sediment load decreas ed an average of 14% between upstream and downstream sampling points. To test the idea that the suspended sediments were retained by the adj acent wetland and to determine what portion of the BLH forest was most responsible for retaining the suspended sediments, concurrent measure ments of sediment accretion were made at 30 sites in the wetland using feldspar clay marker horizons, sedimentation disks, the (137)cesium m ethod, and dendrogeomorphic techniques. Sedimentation rates exceeding 1 cm/yr were measured in frequently flooded areas dominated by Nyssa a quatica and Taxodium distichum. Maximum sedimentation rates did not oc cur on the natural levee, as would be predicted by classical fluvial g eomorphology, but in the ''first bottom,'' where retention time of the water reached a maximum. Multiple regression was used to relate sedim entation rates with several physical and biological factors. A combina tion of distance from the river, flood duration, and tree basal area a ccounted for nearly 90% of the variation in sedimentation rates.