Doctoral Dissertation Research: Multi-Scalar Geomorphic and Vegetative Feedbacks in the Colorado River Basin

利用地貌学模型的新研究表明，河岸植被在单线河道的形成和维持中起着重要作用。了解什么样的地貌、水力和水文条件促进了入侵的河岸植物对这些模型的发展至关重要。在犹他州立大学流域科学教授约翰·施密特的指导下，博士生丽贝卡·曼纳斯将在野外尺度上研究河岸植被与地貌过程之间的相互作用。这项研究的设计利用了科罗拉多河流域独特的地理和地貌，以及大量已发表的研究和长期的水文和物理数据集。了解地貌-植被相互作用的可处理尺度是小尺度，在小尺度上，水力和沉积环境可以被描述和量化。为了解释这些过程在更大尺度上的表达，本研究采用了多标量方法。选择了科罗拉多河流域的四个河段来捕捉水文、气候和沉积学控制的梯度，以及红柳入侵时间和流域水文变化的差异。这些河段用于描述河道形态与河岸植被之间的相互作用，以确定由物理驱动因素（水文和泥沙供应）和河岸植被（红柳入侵）变化引起的河道变化幅度。洪泛区地层学和四个河段中代表性地貌表面的二维流动模型将用于描述和量化非本地河岸植物的入侵如何诱导正反馈，从而导致河道加速变窄。水流模型作为研究植被/地貌反馈的工具的准确性将取决于不同植物群落在不同生命阶段的水力粗糙度的空间精确表征。粗糙度参数化将在单一物种（红柳）的生活史上进行检验。由于无法充分描述是什么决定了河道的形状和大小，我们在地貌学中预测河道对扰动的响应的能力仍然容易出错，而且存在很大的不确定性。在这个气候快速变化的时期，这是科罗拉多河流域的一个关键问题，因为该地区对积雪过程很敏感，而积雪过程是盆地高需水量的主要来源。美国国家公园管理局（NPS）认识到，河道变化会影响水生和河岸物种，并改变河流环境的娱乐价值，因此对确定在环境条件改变的情况下，河道将在何处以及如何变化表示了兴趣。这项研究的结果不仅将通过帮助分离和确定哪些环境变量导致了河道变化来进一步发展地理科学，而且还将帮助管理人员了解科罗拉多河流域的管理情况。具体来说，这项研究将致力于回答以下问题：河流系统的哪些区域对水文变化和非本地植物物种的入侵最敏感，以及在入侵植物物种的生活史中，它何时能有效地改变河道的形状。

New research using geomorphological models has recently shown that riparian vegetation plays a significant role in the formation and maintenance of single-thread channels. Knowing what geomorphic, hydraulic and hydrologic conditions promote invasive riparian plants is critical to the development of these models. Doctoral student Rebecca Manners, under the guidance of Professor John Schmidt in watershed sciences at Utah State University will examine the interactions between riparian vegetation and geomorphic processes at the field scale. The study design takes advantage of the unique geography and geomorphology of the Colorado River basin and its abundance of published studies and long-term hydrologic and physical datasets. The tractable scale of understanding geomorphic-vegetation interactions is the small scale, where the hydraulic and depositional environments can be described and quantified. To account for the expression of these processes at the larger-scale, this study takes a multi-scalar approach. Four reaches of the Colorado River basin have been chosen to capture the gradient of hydrologic, climatologic, and sedimentologic controls, as well as the difference in timing of tamarisk invasion and hydrologic alteration in the watershed. These reaches are used to describe the interactions between channel form and riparian vegetation to determine the magnitude of channel change caused by changes in physical drivers (hydrology and sediment supply) and riparian vegetation (invasion of tamarisk). Floodplain stratigraphy and 2-D flow modeling of representative geomorphic surfaces within each of the four reaches will be used to describe and quantify how the invasion of non-native riparian plants induces a positive feedback that causes accelerated rates of channel narrowing. The accuracy of flow modeling as a tool in examining vegetation/geomorphic feedbacks will depend on spatially precise characterization of the hydraulic roughness of various plant communities at different life stages. Roughness parameterization will be examined over the life-history of a single species (tamarisk). Our ability to make predictions in geomorphology about channel response to perturbations remains error prone with much uncertainty because of the inability to adequately describe what determines channel shape and size. This is a critical issue in the Colorado River Basin during this period of rapid climate change because this region is sensitive to snow accumulation processes that are the primary source of the basins high water demand. Recognizing that channel changes impact aquatic and riparian species as well as alter the recreational value of the river setting, the National Park Service (NPS) has expressed an interest in identifying where and how channels will change provided altered environmental conditions. Results from this research will not only further geographic science by helping to isolate and identify which environmental variables have contributed to channel changes, but also help inform managers regarding management of the Colorado River basin. Specifically, this research will work towards answering questions regarding which areas of a river system are most sensitive to hydrologic changes and invasion by non-native plant species and when in the life-history of an invasive plant species does it become effective in altering the shape of the channel.