Ghosts of landuse past? How have milldams and their legacies influenced riparian zone structure, functions, and processes?

Abstract:

Recent work has shown that milldams have dramatically altered the geomorphology of streams and their valley-bottoms. Milldams raised stream water levels, backed up water, reduced flow velocities, and resulted in accumulation of large amounts of sediments in valley-bottoms, also referred to as a legacy sediments. While most of the milldams are gone/breached, many still exist and are now being actively removed to enhance aquatic habitat. To date, much of the research focus has been on investigating how milldams and their removals impact sediments and nutrients stored within the stream network. Very few studies have explored how milldams influence riparian zones and their structure and functions. We hypothesize that milldams, existing and breached, have a strong influence on hydrologic and biogeochemical conditions and processes in riparian zones and these legacies persist for a long time. We are particularly interested in the cycling and fate of nitrogen (N) in riparian zones. Where dams have been breached, riparian zones are perched above the stream level and are hydrologically disconnected from streamflow. Such dry and oxic riparian soil conditions likely favor processes like nitrification and the potential for N leakage from riparian systems. In contrast, elevated water levels upstream of existing milldams likely promote saturated, wet, and anoxic conditions that favor processes like denitrification. Denitrification is an important beneficial process that contributes to the function of riparian zones as water quality buffers. However, it is unknown if the continuously-wet and stagnant hydrologic regime upstream of milldams stimulate and maintain elevated denitrification rates in soils, or, if a more dynamic regime is favorable. Sediment deposition upstream milldams also results in buried organic horizons in stream beds and riparian zones. These buried organic horizons could be important carbon sources for stream and riparian N processes. We address these important questions and unknowns using preliminary data from our riparian sites on existing and breached milldam locations in Delaware and Pennsylvania. Understanding how milldams and their legacies affect riparian functions will help us make better management decisions and provide important guidance on dam removals.