Interactions of Logjams and Spatial Heterogeneity in Dynamic Rivers
River corridors can be dynamic through time and space, as indicated by geomorphic heterogeneity. Geomorphic heterogeneity is the spatial and temporal variability of geomorphic units or patches of the landscape classified by morphology, substrate, vegetation type, and/or other characteristics relevant to the ecogeomorphic processes that reshape the landscape (namely the fluxes of water, sediment, and wood, all mediated by biological activity). I am broadly exploring links between geomorphic heterogeneity, channel dynamics, and logjams in dynamic rivers over different spatial and temporal scales. I'm interested in thinking about this in the context of how rivers have evolved over human time scales as landscapes change as well as understanding mechanisms and processes that drive physically complex and resilient river systems in the face of climate change.
Interactions of Logjams, Channel Dynamics, and Geomorphic Heterogeneity within a River Corridor
In Marshall et al. (2024), we analyze 16 kilometers of the Swan River corridor in Montana to examine relationships between logjams (the total count, persistence over a decade, and distribution density), how much the main and secondary channels move with time, and how heteogeneous or patchy the river corridor is.
Illustration of a heterogeneous river corridor
Our results suggest that there is a positive relationship between channel movement and logjams. Second, higher flows correspond to greater values of logjams, but not to channel movement. Third, persistent values of river corridor patchiness are explained logjam density, beaver meadows, and channel movement. Our results reflect the complex interactions of water, sediment, and wood in rivers and the importance of spatial and temporal analyses of past and present river processes to understand future river conditions .
Forms of Wood-induced Aggradation and Erosion in the River Corridor
In Marshall et al. (in review), we add insight to current gaps in our understanding of processes and features associated with wood-driven aggradation and erosion. We focus on the temporal dimensions of accretion associated with avulsion and islands and introduce the concept of aggradation and erosion associated with wood levees.
As we characterize island and avulsion accretion, we focus on two tributaries to the Swan River in northwest Montana, USA. We use tree-ring and 14C dating to temporally constrain wood-induced island ages at five tributary islands. We interpret the three wood-induced forms of aggradation that we describe here as reflecting a temporal continuum. Wood levees have primarily non-woody vegetation and may be more transient features. Tributary islands appear to persist for a few decades. The size of trees at the floodplain avulsion site suggests a stand age of 100-200 years. Understanding the temporal dynamics of wood-induced features is important to understanding the formation and persistence of such features in natural channels. As efforts to restore natural processes are emphasized in river restoration, understanding both the spatial and temporal conditions that facilitate different types of wood-induced features has important implications for the broader physical and biological processes in a river corridor.
A Continuum of Wood-induced Channel Bifurcations
In Marshall and Wohl (2023), we compare types of wood-induced channel bifurcations across rivers and streams of differing size in the U.S. Rocky Mountains to understand the conditions that facilitate patterns of wood-induced bifurcations. We observed that a single piece of large wood or a logjam can facilitate differences in bifurcations based on accretionary-bar flow splitting or avulsive processes. A logjam can force overbank flow, bank erosion, and lateral bifurcation via avulsion. A logjam can also create lee deposition that is then stabilized by woody vegetation, forming a relatively short segment of split flow and a longitudinal bifurcation via accretionary-bar flow splitting processes. We use the ratio of erosive force to erosional resistance (F/R ) as an indicator of potential excess energy available for bank and floodplain erosion associated with channel bifurcations. We found that bifurcation types exist as a progressive continuum based on the ratio of F/R. At higher values of F/R, more channel avulsion is occurring and predominantly lateral bifurcations form. At lower values of F/R, banks are more resistant to erosive forces and wood-induced bifurcations are transitional or longitudinal with limited lateral extent.
Example of the continuum of bifurcation patterns observed in Rocky Mountain streams.
Plot of F/R vs. total sinuosity. We use total sinuosity as one metric to show the degree to which a river has bifurcated. The larger the value of total sinuosity, the more the river approaches the lateral bifurcation endmember above: smaller values indicate the longitudinal bifurcation endmember. The names you see here all correspond with various anastomosing streams and rivers across Colorado and Montana.
Understanding the conditions under which wood accumulations can facilitate different types of bifurcations and the geomorphic, hydraulic, and ecologic processes involved in facilitating these bifurcations is both significant and timely to river corridor science and management, particularly as the need for more resilient and connected river systems increases in the face of accelerated environmental change. We can use this understanding to inform river corridor restoration designed to enhance the formation of secondary channels, increase lateral and vertical connectivity, and promote a multithread planform.
Marshall, A. and Wohl, E. 2023. “The Continuum of Wood-Induced Channel Bifurcations.” Frontiers in Water. https://doi.org/10.3389/frwa.2023.1155623
Marshall, A., Wohl, E., Iskin, I., and Zeller, L. In Review. "Interactions of Logjams, Channel Dynamics, and Geomorphic Heterogeneity within a River Corridor". Water Resources Research
Marshall, A., Wohl, E., Erkkla, B. In Review. "Forms of Wood-induced aggradation and erosion in the river corridor". Earth Surface Processes and Lanforms