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Browsing by Author "Marshall, Anna E., author"

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    Islands in the stream: spatial and temporal patterns of logjam-induced river corridor dynamics
    (Colorado State University. Libraries, 2024) Marshall, Anna E., author; Wohl, Ellen, advisor; McGrath, Dan, committee member; Morrison, Ryan, committee member; Rathburn, Sara, committee member
    Spatial and temporal variations in water and sediment fluxes moving within the river corridor drive changes in the three-dimensional geometry of channels and floodplains. In forested river corridors, pieces of large wood (> 10 cm diameter and 1 m length) and logjams (≥ 3 pieces of large wood) become an integral part of the interactions among water, sediment, and the resulting river corridor form and function. The net effect of logjams stored at least temporarily in the river corridor is to increase spatial heterogeneity, or patchiness, via processes such as channel avulsion and formation/abandonment of secondary channels, increased channel-floodplain connectivity, and greater instream aggradation. The importance of spatial heterogeneity, logjams, and secondary channels/islands to river corridor function has been well documented, but a lack of existing quantitative underpinning creates knowledge gaps in the processes driving island formation and persistence, the role of wood in facilitating these processes, and the complex interactions between flow, sediment, and wood in dynamic river corridors. This dissertation addresses some of the existing knowledge gaps around how logjams interact in a river corridor to create heterogeneity at different spatial and temporal scales by characterizing the patterns, processes, and interactions occurring in a naturally dynamic system. The topics explored here focus on research primarily conducted along the Swan River in the Northern Rocky Mountains of Montana with mention of sites in the Southern Rocky Mountains of Colorado. These locations represent some of the few remaining river corridors in the contiguous U.S. with natural flow, sediment, and wood regimes, but represent former widespread conditions. In the work that follows, Chapter 2 explores the processes driving spatial patterns in bifurcations induced by logjams. I find that logjam-induced bifurcations exist as a continuum of different patterns and the position of a river segment along this continuum correlates with the ratio of erosive force to erosional resistance. Chapter 3 builds on this by investigating how accretionary and avulsive processes shape bifurcations over time, emphasizing a temporal progression of logjam-induced features using 14C and tree ring data. I find that islands tend to grow through upstream migration – the presence of buried logs with contemporary trees growing on them indicates this process – and lateral accretion. Chapter 4 dives deeper into the interactions between process and form, demonstrating the relationship between channel dynamism, logjam presence, and spatial heterogeneity at larger temporal and spatial scales. I find that logjams and channel movement through time interact in a cascade of processes and feedbacks that foster increased spatial heterogeneity. Wood preferentially accumulates in more geomorphically heterogeneous portions of the river corridor that provide sites capable of trapping and retaining wood. Logjams can then drive greater total sinuosity and the formation of secondary channels that result in further wood trapping, greater heterogeneity of floodplain vegetation, and ideal habitat for beaver that further modify river corridor heterogeneity. I also find that bifurcations and spatial heterogeneity persist even after logjam is no longer present. These results have implications for river management. If sections of the river corridor with more logjams and more beaver meadows display higher spatial heterogeneity, creating and protecting wood-rich heterogeneous retention zones within a river corridor is an important component to emphasize for river resilience. If physical effects persist even after a logjam is no longer present, than wood reintroduced to the river corridor as individual pieces or engineered logjams does not have to be anchored in place to facilitate formation of geomorphic heterogeneity within the river corridor. By dissecting the complexities of processes governing naturally dynamic river corridors, this work adds quantitative insight to the diverse functionality of heterogenous river systems in forested or historically forested regions and provides a launching point for future river management aimed at fostering river corridor function and resilience.
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    Wood-induced stream channel complexity as a driver of transient storage
    (Colorado State University. Libraries, 2021) Marshall, Anna E., author; Wohl, Ellen, advisor; Morrison, Ryan, committee member; Rathburn, Sara, committee member
    Rivers are naturally diverse, heterogeneous systems. This spatial heterogeneity is driven by inputs into a river corridor including water, sediment, large wood, coarse particulate organic material (CPOM), and dissolved loads, all of which interact with the river valley to create distinct forms of geomorphic complexity. Spatial heterogeneity within a river corridor drives surface exchange of water, sediment, and nutrients moving downstream and into transient storage zones. Transient storage is increased by features that enhance bed heterogeneity and surface flow separation, such as large wood. This thesis explores the role that wood-induced spatial heterogeneity has in facilitating zones of surface transport and storage. Specifically, I look at how discharge and logjam characteristics drive transient storage, as reflected in the movement of salt tracers and CPOM. Logjams alter gradients in hydraulic head and create zones of flow-separation and low velocity along channel margins, which enhance storage of organic matter and solutes. Although research has shown that a single logjam in a channel increases transient storage, limited work has been done to understand the characteristics of logjams that enhance transient storage. Scientific gaps remain in understanding whether decreasing the downstream spacing of logjams or the logjam porosity drives an increase in transient storage. Here, I designed experiments for two constructed flume systems—one with a change in downstream logjam spacing and one with a change in logjam porosity—to understand the effects of logjam characteristics on transient storage at different flows. More closely spaced logjams resulted in slower advection down the flume channel at low flows. At high flows and with changing porosities, there was no consistent effect of jam spacing or jam porosity on advective travel times. This likely points to the influence of a low Damkohler number, where high velocity and/or short reach length resulted in only a small amount of flow-path exchange with storage zones. In the future, a modified flume configuration with exaggerated scaling of discharge and porosity may be necessary to better distinguish the study effects. Surface transient storage is also a physical control on CPOM transport and deposition. The details of surface transient storage determine when and if CPOM is deposited and remobilized. Yet, no studies have looked at how CPOM transport and storage vary in relation to shorter (diurnal) as well as longer (seasonal peak flow) variations in discharge. I physically sampled CPOM moving along the bed and in-suspension at stream reaches above and below a logjam as well as a location with no logjam. I sampled CPOM masses throughout a seasonal hydrograph and on a 24-hour diurnal timespan to examine how transport and deposition changed with flow. The majority of CPOM was transported in suspension following a clockwise hysteresis along the seasonal hydrograph. CPOM stored in the channel and overbank areas is more likely to be mobilized as stage rises and snowmelt runoff enters the channel, whereas the supply of CPOM is depleted as the snowmelt hydrograph continues. A similar hysteresis in CPOM transport did not occur during 24-hour diurnal fluctuations in discharge except at peak CPOM movement. CPOM peaked before discharge peaked on both the diurnal and seasonal hydrographs. CPOM transport in a logjam backwater occurred at a significantly lower rate than in reaches without logjams, suggesting logjams provide storage zones for CPOM. Understanding the characteristics and processes of wood-induced spatial heterogeneity that facilitate zones of surface transport and storage has important management implications. Large wood and hyporheic restoration are increasingly used to enhance ecosystem services and functions in rivers, yet the specific characteristics of logjams remain poorly defined. Considering that CPOM is a primary energy source in the food webs of shaded forest streams, management designed to foster the sustainability of stream ecosystems can benefit from maintaining or creating features that enhance CPOM retention. Furthermore, designing logjams to restore a river reach with the goal of improving hydrologic function will provide greater value if the design incorporates reach-scale logjam characteristics that enhance transient storage.