Geomorphic and Sedimentologic Analysis of Four Streams in Southeastern Alabama: Channel and Habitat Response to Changes in Land Use

Abstract

Streams of the Southeastern US have varying degrees of degradation due to the long history of agricultural land use, reflected in incising and eroding banks with aggradation of fine grained legacy sediment. These effects of land use alteration result in a reduction in the physical and biological function of the stream, and thus inherently the biotic populations of the stream. Building upon biological and geomorphic data collected in 2011 we quantified sediment dynamics and channel and habitat response to changes in land use in four drainages in the Tallapoosa Basin, Alabama. Known land use alterations have occurred in these drainages since 2011 with deciduous forest conversion to conifer (silviculture operations) or to urban development. The data shows the more urbanized drainages have the lowest fish diversity (Shannon H’) and richness, and since 2011 fish diversity and catch has decreased across all sites. Also, at all sites there has been a decline in macrobenthic diversity, and a change in feeding group distributions from collector/filterer to collector/gatherer. The crayfish populations have also declined since 2011. Furthermore, there was also marked differences in biotic populations across sites due to their land uses and drainage areas. The more urbanized drainage had the greatest suspended sediment concentration, but average suspended sediment concentration was less than 10 ppm for all sites. At all sites, the banks are composed of medium silt, and bank failure/slumping is evident in the urbanized drainages. From 2011 to 2016 there has been a coarsening of bed substrate, with the exception of one site, and increase in exposure of bedrock. The width to depth ratio (WDR) and shear stress has increased at all but one site since 2011. These results suggest that changes in land use, even on a short time scale of five years, has had marked effects on habitat quality and morphology of the channel reaches. Changes in bed substrate could be a result of increases in stream power, bank erosion appears to be the primary source of sediment delivery to the stream, and the increasing WDR is a symptom of channel widening due to bank stability. These data paired with the geomorphic and sedimentological characteristics of each site can inform restoration efforts and address the potential impacts of these alterations on local biotic populations if forest conversion continues in the Alabama Piedmont.