Characterizing alluvial benches and incipient floodplains in a piedmont stream and investigating biogeomorphic feedbacks as a possible mechanism for their occurrence

Abstract

In-channel benches are bank-attached, planar and narrow, fine-grained sediment deposits occurring between the river bed and the floodplain. Benches are often vegetated and sometimes paired and are formed predominately by suspended load deposition. These unique in-channel features are referred to are referred to by several different terms, such as berms, inner berms, shelfs, inset floodplains, and incipient floodplains which raises questions as to their formative processes and functionality within the fluvial system. As indicated by the broad terminology used to label benches, these features have considerable variation in characteristics. Two main classification systems have been developed for benches, one based on the elevation of the bench surface above the channel and the other based on the position of the bench along the channel. Several theories exist as to the formation and persistence of in-channel benches that are related to several different overarching controls, including: hydrologic variability related to drought and river regulation, accelerated sedimentation caused by anthropogenic disturbance, and rapid vegetation establishment under low-flow conditions. The geomorphic function of benches is a matter of some debate. In some cases, benches appear to function as incipient floodplains. In other locations benches are means of bank reconstruction in overwidened channels, and in some cases, benches are short-lived in-channel sediment storage feature This research aimed to better understand bench stratigraphy in order to interpret the instances in which benches act as incipient floodplains and to investigate what information about benches can be obtained by applying rudimentary dendroecological techniques. The research was conducted at several locations within Talladega Creek, Alabama. Five benches and their adjacent floodplains were cored to allow comparison of bench/floodplain stratigraphic and organic matter composition that would facilitate identification of incipient floodplains. In addition to sediment cores, tree cores were obtained for a minimum age analysis of bench surfaces and used to investigate historic hydroclimatic conditions that might be conducive to bench development. A tree species inventory of the benches was also recorded to compare to a theoretical model of riparian vegetation recently proposed by Corenbilt et al. (2007) that seeks to characterize biogeomorphic feedbacks. Stratigraphic interpretations based on particle size analyses indicate some differences exist between benches and floodplains. The benches were generally finer than the floodplains. Both benches and floodplains showed general fining upward, with some exceptions. Variations, however, are site specific, with both benches and floodplains exhibiting differences in organic matter content and median particle size by site. Tree cores revealed that in all cases, the establishment date of the oldest tree sampled at each site coincided with a drought period. The species inventory suggests some differences with the riparian vegetation model proposed by Corenbilt et al. (2007). The main conclusions that resulted from this research are that bench stratigraphy and organic matter content is determined by site-specific hydrogeomorphic and vegetation conditions rather than by broader influences. Based on the dendroecological analyses, it would appear that hydrologic variability associated with drought dominated hydroclimate is a factor in the formation of in-channel benches in Talladega Creek, Alabama. This is supported by the tree establishment dates and climatological data. The tree species inventory was not consistent with the Corenbilt et al. (2007) model, the species compositions indicating that as an environment for vegetation, in-channel benches function more like floodplains than in-channel features such as bars.