This doctoral dissertation research investigates how future environmental change may affect the fluvial geomorphology, or physical form, of rivers. Environmental change, including climate variability, is acknowledged as a factor influencing river flow, particularly in mountainous watersheds, in which snowmelt makes a large contribution to annual discharge. Potential environmental changes in the hydrology of such basins have been simulated by hydrologic models. Most notably, these impacts include higher temperatures leading to an increase in the proportion of winter precipitation falling as rain rather than snow, with consequent increased winter discharge, a lower spring snowmelt peak, and decreased summer discharge. Few of these modeling studies, however, have projected how hydrologic changes associated with climate change may affect geomorphic characteristics such as the width and depth of river channels, the shape of the river course, and the transport of sediment. This is a gap in understanding of climate-change impacts on river systems, because river morphology responds dynamically to hydrology. This research will contribute to better understanding of the geomorphic response of river systems to climate change through development of a hierarchical series of linked models to investigate how climate change influences hydrology, which in turn influences fluvial geomorphology. This modeling framework will be applied to three snowmelt-dominated watersheds in the interior Pacific Northwest, with the following objectives: 1) development of downscaled basin-scale climate change scenarios, which are projections of future changes in climate variables such as temperature and precipitation that are locally specific to the study basins; 2) application of a watershed-scale hydrologic model to project how the study basins' hydrology, including the magnitude and timing of river flow, may change in response to the downscaled climate change scenarios; 3) examination of the impact of the modeled hydrologic changes on the study rivers' morphology, channel form, river shape, and sediment transport, using a reach-scale geomorphic model that can simulate an individual river segment in greater detail than a watershed model; and 4) quantification of past rates of change in river shape using historic aerial photos to compare with the geomorphic modeling results. This hierarchical modeling process is an innovative approach to linking physical processes that occur across multiple scales, from global and regional climate to watershed hydrology to local geomorphology.
The results of this research will have implications for the management of river systems. Geomorphic processes significantly affect the value of rivers, including their suitability for threatened and endangered species and for human uses of water. Knowledge of how climate change may affect these processes will allow water resource managers to make more informed decisions about how to respond to future changes. The results will be disseminated at national scientific conferences and in peer-reviewed journals. As a Doctoral Dissertation Research Improvement award, this project will provide support to enable a promising student to establish a strong independent research career.
This doctoral dissertation research investigates how future climate change may affect the fluvial geomorphology, or physical form, of rivers. Climate change and variability is widely acknowledged as a major factor influencing river flow, particularly in mountainous watersheds in which snowmelt makes a large contribution to the annual discharge. Potential climate change-driven changes in the hydrology of such basins have been simulated by hydrologic models. Few of these modeling studies, however, have projected how hydrologic changes associated with climate change may affect geomorphic characteristics such as the width and depth of river channels, the planform or shape of the river course, and the transport of sediment. This is a gap in understanding of climate-change impacts on river systems, because river morphology responds dynamically to hydrology. This research contributes to better understanding of the geomorphic response of river systems to climate change through development of a hierarchical series of linked models to investigate how climate change influences hydrology, which in turn influences fluvial geomorphology. This modeling framework is applied to three snowmelt-dominated watersheds in the interior Pacific Northwest (Figure 1). The project includes the following objectives: 1) development of downscaled basin-scale climate change scenarios, which are projections of future changes in climate variables such as temperature and precipitation that are locally specific to the study basins; 2) application of a watershed-scale hydrologic model to project how the study basins’ hydrology, including the magnitude and timing of river flow, may change in response to the downscaled climate change scenarios; and 3) examination of the impact of the modeled hydrologic changes on the study rivers’ morphology – channel form, planform, and sediment transport – using reach-scale geomorphic models that can simulate an individual river segment in greater detail than a watershed model. This hierarchical modeling process is an innovative approach to linking physical processes that occur across multiple scales, from global and regional climate to watershed hydrology to local geomorphology. The first part of the research focuses on developing high-resolution climate change scenarios, using empirical estimates of the relationship between topography and climatic variables to elevationally adjust the output from coarser-resolution climate models (Figure 2). Qualitative and quantitative assessments of this approach indicate that it performs well for the study region of the northwestern United States, and it is likely to also be successful in other regions in which topography exerts a strong control on climate. Projections of future climate change for the study area developed using this method include consistent increases in both maximum and minimum temperature and variable changes in precipitation among different climate models (Figure 3). The second part of the project focuses on simulating the impacts of the climate change scenarios from Objective 1 on river discharge and suspended-sediment transport in the study rivers, using a watershed hydrology model. The simulated impacts include an increase in the amplitude of the annual cycle of river discharge, with increased winter discharge resulting from more winter precipitation occurring as rain rather than snow, a reduction in the magnitude of the spring snowmelt peak and its shift to earlier in the season, and a reduction in summer discharge resulting from the lower snowpack (Figure 4). Simulated changes in suspended-sediment load generally follow the changes in discharge. Finally, the third part of the project uses the scenarios of climatic and hydrological change from Objectives 1 and 2 to simulate changes in reach-averaged bedload transport on the study rivers using a one-dimensional model that implements sediment transport formulas, as well as a three-dimensional model of the river reaches that simulates changes in the spatial pattern of erosion and deposition and therefore in channel form. Simulated bedload transport as simulated by the two different models is comparable for all three rivers, which provides an independent verification of the models (Figure 5). Changes in bedload transport under the climate change scenarios are highly dependent on changes in the recurrence interval of critical discharge needed to mobilize bed sediments, which are a function of the climatic and geomorphic characteristics of each river. Simulation results of changes in erosion and deposition patterns reveal coherent patterns of change for two of the study rivers, but there is no discernible pattern of change in the third river, which is a lower-energy river with steep banks (Figure 6). The results of this research have implications for the management of river systems. Geomorphic processes significantly affect the value of rivers, including their suitability for threatened and endangered species and for human uses of water. Knowledge of how climate change may affect these processes will allow water resource managers to make more informed decisions about how to respond to future changes.
