Ecogeomorphic interactions of Russian olives (Elaeagnus angustifolia) and point-bar morphology along Powder River, Montana, USA

Abstract

Point bars are crucial alluvial features that help sustain meandering river ecosystems. They are influenced by interactions among physical processes and biological components, such as riparian vegetation. Understanding the mechanisms underlying these interactions can inform management decisions. The meandering Powder River in southeastern Montana, USA, provides a valuable study system for fluvial interactions because it has no major human alterations. Riparian stands of non-native Russian olive trees ( Elaeagnus angustifolia ) became noticeable after ~50-year flood in 1978. Fortuitously, 20 channel cross-sections had been established in 1975 and 1977 and have been surveyed periodically ever since. This temporally extensive dataset was coupled with a spatially extensive dataset of 85 point-bar cross-sections derived from high-resolution LiDAR that enabled evaluation of interactions between Russian olives and point-bar morphology through time and space. Based on published literature and field observations, we hypothesised that an increase in the width of Russian olive stands would result in (1) a decrease in the overall point-bar slope, (2) an increase in point-bar heights and (3) in point-bar toe-slopes. The results indicated that point-bar slopes did decrease with an increase in stand width, supporting the first hypothesis ( R 2 = 0.87). However, point-bar heights decreased with an increase in stand width ( R 2 = 0.63), contradicting the second hypothesis and there was no relation between point-bar toe-slopes and stand width ( R 2 < 0.1). The reverse hypothesis that point-bar morphology controls Russian olive stand widths makes more biophysical sense. Thus, Russian olive stand width increases as the point-bar slope and height decrease ( R 2 = 0.79 and 0.50), which provide a greater area of inundation for colonisation. This study demonstrates that geomorphology is controlling non-native vegetation colonisation along Powder River, but that the subsequent stabilisation associated with Russian oliveexpansion may in turn upset the critical geomorphic balance between bank erosion and point-bar aggradation necessary for a stable meandering system.