FLUVIAL DISCONTINUITIES AND ECOLOGICAL CHANNEL CLASSIFICATION AT A REGIONAL SCALE, APALACHICOLA-CHATTAHOOCHEE-FLINT RIVER SYSTEM

Discontinuities in channel characteristics along a stream network can define boundaries of ecologically relevant classification units. Here we present development of a hierarchical, multivariate channel classification for the network of the Apalachicola, Chattahoochee, and Flint (ACF) River system in the Southeastern United States. The ACF basin has an area of over 50,000 km² with 1,300 m of vertical relief from the Blue Ridge Mountains to the Gulf of Mexico. The principal rivers and their tributaries flow through three physiographic provinces, the Blue Ridge, Piedmont, and Coastal Plain, and support a variety of freshwater fish and mussels. The basin’s topography and geology are diverse, including high gradient streams that flow on resistant metamorphic and crystalline rocks in the Blue Ridge and Piedmont, and low-gradient streams in the Coastal Plain that flow through karst terrain and unconsolidated sediments. Land-use changes and reservoirs in the basin complicate network connectivity. Our regional classification clusters channel reaches with similar combinations of channel width (modeled from regional channel geometry relationships), channel gradient, and valley width from 30-m digital elevation model data. We validated the regional classification by comparing it with an analysis of high-resolution LiDAR elevation data available for Potato Creek, a 600 km² basin within the Piedmont section of the ACF Basin. Channel and valley dimensions extracted from the LiDAR provide detailed information that can be used to link regional-scale classifications to ecological characteristics of streams. The spatial distributions of classes indicate where substantive discontinuities occur on the landscape and geologic maps provide partial explanation for why the discontinuities exist. Other reasons for discontinuities in the network include topographic position and both pre-and post-industrial landform history. A key challenge to developing this classification is the need to reconcile scales of available data with the need to classify channels at a scale relevant to ecological processes. Geologic maps provide independent information to delineate discontinuities; however, inconsistencies in scale, resolution and definitions of map units across boundaries limit usefulness of geologic information.