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Regional methods for evaluating the effects of flow alteration on stream ecosystems




Wilding, Thomas K., author
Poff, N. LeRoy, advisor
Bledsoe, Brian, committee member
Sanderson, John, committee member
Steingraeber, David, committee member

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Three stand-alone chapters explore the development and implementation of regional flow-ecology methods. Ecohydrology is an interdisciplinary field that brings together specialized research in hydrology, hydraulics, geomorphology and ecology. My dissertation reflects the need for interdisciplinary knowledge, tackling issues as diverse as low flows for trout (Chapter 2) to peak flows for cottonwood (Chapter 3). A regional-scale view unifies these investigations, with Chapter 1 establishing the scientific foundation and management objectives for regional flow-ecology methods. Summary Chapter 1 - To balance the benefits of dams and water diversions against society's expectations for the natural environment, flow managers require scientific advice on the ecosystem response to flow alteration. The methods selected to investigate the ecosystem effects of flow alteration (e.g., PHABSIM - Physical Habitat Simulation) should reflect the scale of flow management and the information requirements of flow managers. In addition, a hierarchical habitat framework provides an ecological foundation for the development and implementation of flow-ecology methods, because ecosystem response to flow is constrained by large-scale processes. This can be put into practice using hydrogeomorphic classification to define the higher-level physical processes (e.g., sediment transport, disturbance) that dictate the mechanisms of biotic response to flow. Regional flow-ecology methods provide a vehicle for incorporating prior knowledge and hydrogeomorphic processes into flow management at both regional and local scales. Chapter 2 - Changes in hydraulic habitat (depth and velocity) with flow can be predicted using intensive reach-specific methods, such as PHABSIM. I used existing PHABSIM data to develop GHMs (Generalized Habitat Models) that predict trout habitat-flow curves for unsurveyed streams of the southern Rocky Mountains. Predicted habitat was significantly correlated with the abundance of large brown trout (P<0.01), but not smaller trout (using Colorado Division of Wildlife monitoring data). The rapid-survey GHM (from channel width) represents a major reduction in survey effort compared to a full PHABSIM survey, and produced better predictions of observed habitat than the desktop GHM (from mean annual flow). Chapter 3 - Cottonwood trees are valued members of riparian ecosystems and, in the drier areas of North America, their recruitment depends on high flow events. To help plan for anticipated increased water demand, the ELOHA framework was used to develop flow-ecology relationships for three basins in Colorado (total area 53,000 km2). Existing data revealed a negative relationship between the abundance of plains cottonwood (Populus deltoides Bartram) and reduced peak-flows. The hypothesis that this flow constraint would also apply to a second species, narrowleaf cottonwood (Populus angustifolia James), was not supported because four reaches (out of the 39 surveyed) had abundant and reproducing narrowleaf forest, despite pronounced flow alteration (>40% flow reduction). Historic photographs revealed that narrowleaf in the Middle Park area (Colorado) have increased in abundance since dam closure, colonizing previously bare gravel bars. That narrowleaf appear less sensitive to flow alteration than plains cottonwood could reflect different species traits (e.g., alternative sources of disturbance for root suckering by narrowleaf), together with the many physical transitions from plains to mountains that are associated with the species transition.


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