Doctoral Dissertation Research: A Hierarchical Modeling Approach to Simulating the Geomorphic Response of River Systems to Environmental Change

这篇博士论文研究了未来的环境变化如何影响河流的地貌或物理形态。 环境变化，包括气候多变性，被认为是影响河流流量的一个因素，特别是在山区流域，融雪对年流量有很大贡献。 这些流域的水文学可能发生的环境变化已通过水文模型进行了模拟。 最值得注意的是，这些影响包括温度升高导致冬季降水的比例增加，而不是降雪，从而增加冬季流量，降低春季融雪高峰，减少夏季流量。 然而，这些模拟研究很少预测与气候变化相关的水文变化如何影响地貌特征，如河道的宽度和深度，河道的形状和沉积物的运输。 这是对气候变化对河流系统影响的理解上的一个空白，因为河流形态对水文学有着动态的响应。 这项研究将有助于更好地了解河流系统对气候变化的地貌反应，通过开发一系列层次化的关联模型来调查气候变化如何影响水文，进而影响河流地貌。 这一模拟框架将应用于西北太平洋内陆三个以融雪为主的流域，其目标如下：1）开发缩小尺度的流域尺度气候变化情景，即对研究流域局部特定气候变量（如温度和降水）未来变化的预测; 2）应用流域尺度水文模型，预测研究流域的水文，包括河流流量的大小和时间，如何响应缩小尺度的气候变化情景而变化; 3）使用河段尺度地貌模型（可以比流域模型更详细地模拟单个河段），检查模拟的水文变化对研究河流的形态、河道形态、河流形状和沉积物输运的影响;（4）利用历史航空照片量化过去河流形态变化率，并与地貌模拟结果进行比较。 这种分层建模过程是一种创新的方法，将发生在多个尺度上的物理过程联系起来，从全球和区域气候到流域水文，再到当地地貌。 地貌过程显著影响河流的价值，包括其对受威胁和濒危物种以及人类用水的适宜性。 了解气候变化可能如何影响这些过程，将使水资源管理人员能够就如何应对未来的变化作出更明智的决定。 研究结果将在国家科学会议和同行评审期刊上传播。 作为博士论文研究改进奖，该项目将提供支持，使有前途的学生建立一个强大的独立的研究生涯。

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.