How landforms and geology affect the structure of riparian areas

Abstract

Current riparian zone assessments focus on the morphological features of the stream channel and ocular vegetation measurements. This procedure fails to address the hydraulic features responsible for the floristic structure and composition of the riparian zone. We looked at how the geology and landforms function as drivers of groundwater and surface-water exchange. These can mitigate watershed processes via groundwater availability to shape riparian processes; e.g. discharge, seasonality. We hypothesized that groundwater surface-water exchange is a first order process and that it dictates riparian water availability and that the underlying geology and landform can serve as a tool to gain greater understanding of a properly functioning riparian ecosystem. We tracked groundwater surface-water exchange using wells, piezometers, water temperature, conservative tracer injections and solute conductivity on an alluvial fan in the Gallatin valley. Southwestern Montana Conservative tracer injection indicated 3% tracer losses over the 1.5km reach. Through spring and summer 2011 groundwater wells and piezometers indicated flashy transient shallow groundwater. Significant late growing season stream discharge (~300 l/s) and standard riparian monitoring assessments would suggest a larger floristic community than what is present at the site. These metrics together suggest a disconnection between the surface-water and groundwater ultimately limiting the extent of the riparian vegetation community. We further determined that the surface water is disconnected from the local groundwater table. We conclude that the lack of surface water - groundwater connectivity drives the floristic structure and character of the riparian zone. Skewed or inaccurate views of riparian functionality may occur because; the current assessment fails to address surface water - groundwater connectivity.