The topology and ecohydrology of river corridors in mountain river networks

Abstract

River corridors are comprised of the river, the surrounding valley and riparian areas, and subsurface hyporheic zones. River corridors have the potential to regulate hydrological, biogeochemical, and ecological processes and patterns from reach to watershed scales. Within mountainous landscapes, narrow sections of the river corridor are often interspersed within wider, yet less frequent, river corridor sections. Reach-scale studies (i.e., 1 km) suggest that wide river corridors, also referred to as river-floodplain systems and river beads in this dissertation, have disproportionate impacts on river network behavior. In chapter one, I introduce the concept of river corridors, briefly review the history of the concept's development, the hydrologic and eco-geomorphic factors that drive functioning in these systems, and alterations driven by anthropogenic activities. In chapter two, as a first step to deepening understanding of the influence of river network valley morphology on watershed process, I quantify the spatial distribution of wide and narrow river corridor segments in twenty river networks in the Southern Rockies Ecoregion. I then characterize the spatial configuration of river beads including their frequency, abundance, and spacing. These results reveal variable network topology of river beads in the region and illustrate the need to consider network position when investigating functioning in these systems. I conclude that characterizing river bead configurations can improve river network understanding and aid decision making in prioritizing conservation and restoration efforts. In chapter three, I explore water-mediated linkages, termed hydrologic connectivity, that connect landscape components within an intact beaver mediated river-floodplain system in Rocky Mountain National Park. I evaluate surface water hydrologic connectivity using field indicators and develop a continuous connectivity metric that represents a vector strength between a source along the North St Vrain River to ten surface water target sites within the river-floodplain system. To measure this connectivity strength, I analyzed hydrometric, injected conservative tracers, and natural occurring geochemical and microbial tracers across streamflows in 2018. I developed empirical models of surface water hydrologic connectivity as a function of river stage to predict daily connectivity strength across multiple floodplain sites for 2018 and assessed the sensitivity of surface connectivity to inter-annual streamflow variability between 2016-2020. At the river-floodplain system scale, I found hydrologic connectivity always increased with streamflow while across-system variance in connectivity peaked at intermediate streamflow. At sites with intermittent connections to the river, river stage disconnection thresholds were variable and their connectivity dynamics were sensitive to inter-annual variation in streamflows, suggesting that future connectivity behavior under climate change will depend on how flow durations change across a range of flow states. These results suggest that the intermediate flows are critical periods for understanding seasonal connectivity within river-floodplain systems. Accordingly, our results suggest that alteration to connectivity regimes as dictated by future hydrologic change will be in part a function of the speed at which streamflow moves from peak to low flow states. In chapter four, I examine the spatial patterns in land cover within the Southern Rockies Ecoregion and assess the implications of wetland cover on river corridor productivity and the sensitivity of productivity to inter-annual climate variability across geographic and climatic gradients in the region. We found that wetlands, which comprise today only around a third of river corridor area, maintain high productivity even in river corridor segments within water limited landscapes. However, degradation in wetlands and the loss of woody cover create river corridors with high sensitivity to climate variability, particularly in areas with lower climatic water availability. Wetlands with woody cover were clustered in proximity to rivers and maintain relatively low climate sensitivity even in more water limited landscapes. Vegetation productivity and sensitivity patterns in river corridors without wetlands were largely driven by climatic water availability. Areas with high water availability generally contained forested cover with high productivity and low climate sensitivity while water limited areas generally contained shrub lands and grasslands cover with low productivity and high climate sensitivity. These results suggest that wetland loss and/or degradation have resulted in losses in productivity and climate resilience, particularly within more water limited portions of the region.