Drainage-basin-scale geomorphic analysis to determine reference conditionsfor ecologic restoration - Kissimmee River, Florida

Major controls on the retention, distribution, and discharge of surface wat er in the historic (precanal) Kissimmee drainage basin and river were inves tigated to determine reference conditions for ecosystem restoration. Precan al Kissimmee drainage-basin hydrology was largely controlled by landforms d erived from relict, coastal ridge, lagoon, and shallow-shelf features; wide spread carbonate solution depressions; and a poorly developed fluvial drain age network. Prior to channelization for flood control, the Kissimmee River was a very low gradient, moderately meandering river that flowed from Lake Kissimmee to Lake Okeechobee through the lower drainage basin. We infer that during normal wet seasons, river discharge rapidly exceeded L ake Okeechobee outflow capacity, and excess surface water hacked up into th e low-gradient Kissimmee River. This backwater effect induced bankfull and peak discharge early in the flood cycle and transformed the flood plain int o a shallow aquatic system with both lacustrine and riverine characteristic s. The large volumes of surface water retained in the lakes and wetlands of the upper basin maintained overbank now conditions for several months afte r peak discharge. Analysis indicates that most of the geomorphic work on th e channel and flood plain occurred during the frequently recurring extended periods of overbank discharge and that discharge volume may have been sign ificant in determining channel dimensions. Comparison of hydrogeomorphic relationships with other river systems identi fied links between geomorphology and hydrology of the precanal Kissimmee Ri ver. However, drainage-basin and hydraulic geometry models derived solely f rom general populations of river systems may produce spurious reference con ditions for restoration design criteria.