Graduate Thesis Or Dissertation


Construction, Degradation, and Restoration of Wet Meadow Ecosystems in Semi-arid Landscapes, Northern Great Basin, USA Public Deposited

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  • Wet meadow ecosystems are a useful natural laboratory in which to explore feedbacks between biogenic and geologic controls on valley bottom landforms. Characterized by flat valley floors flanked by higher-gradient hillslopes, these meadows are singular both in that they represent depositional features in what is primarily an erosional environment, and maintain high water tables in an otherwise dry environment. As such, they can maintain high species richness relative to their surrounding landscapes, and are often important agriculturally. In the last two hundred years, many such wet meadows have rapidly incised, leading to broad successional shifts towards sparser, drought-adapted species commonly found on the uplands. This has prompted a great deal of interest and funding towards their restoration, and speculation as to the potential of their restoration to augment late season baseflow. There are limited data documenting these restoration projects, leaving open the question both of whether such projects are capable of augmenting late season baseflow and, more fundamentally, whether these ecosystems can be restored. At the heart of the latter question is an ongoing debate in the geomorphic community as to whether incision is the result of anthropogenic (extrinsic) actions, or if the processes controlling incision are coded into the landforms where it occurs (intrinsic). In the case of the former, restoration could, presumably, undo human actions to restore the processes by which the surface formed. In the case of the latter, incision may be an inevitable byproduct of geomorphic history and processes. The objective of this work is to better understand the mechanisms by which wet meadow ecosystems form and degrade, and provide geologic and hydrologic context to the feasibility of current activities aimed at their restoration. Using a combination of near-surface geophysics, stratigraphic analyses, geomorphic mapping, vegetation surveys, hydrologic modelling, and systematic literature review, this interdisciplinary work represents a discourse analysis with low-order valleys in semi-arid landscapes. A rigorous geophysical characterization of three variably incised wet meadows in the Silvies Valley suggests side-valley fans creates slope breaks that both induce aggradation of sediment comprising valley fill and generate knickpoints that propagate through valley fill, causing incision. Unusual wavy structures with a high aspect ratio (1:1) are observed in geophysical data underlying meadow profiles, potentially indicating biogeochemical coupling between surficial features and deeper bedrock weathering processes. Physically-based hydrologic modelling delves deeper into the geomorphic controls on bank discharge, and demonstrates geometrically that many wet meadow restoration projects aiming to decrease depths of incision will likely decrease streamflow. Finally, a new framework – inverted expectation mapping – is presented and employed to synthesize disparate types, sources, and records of data in an attempt to better understand the capacity of a new restoration strategy, beaver-related restoration, to achieve a variety of objectives for which it is employed. This analysis indicates that beaver should not be expected to reliably behave in ways that meet human expectations of restoration, and that while beaver-related restoration program should not be expected to increase streamflow, such projects often reliably increase groundwater levels in the near-channel area provided structures remain structurally sound. This last result reveals a common misunderstanding; the term “streamflow” is commonly used to describe visual increases in available water, when the physical quantity that has increased is more commonly groundwater or surface water storage. More broadly, this work aims to explore the ways in which the material and shape of the earth’s surface influences the ability for specific life forms to exist on it, and the thresholds of surficial adjustment that exert biologically significant change. A better understanding of these so-called “critical processes” are not only intrinsically satisfying but, as will be argued, must be central to any human endeavor to purposefully alter those same materials and shapes in service of desired outcomes.
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