Doctoral Dissertation Research: Responses of Gully Erosion and Sedimentation to Environmental Change in the Upper Republican River Basin, Nebraska

中部大平原的许多河流系统具有华丽的泄流制度和高产沙量。因此，它们的沉积过程对不断变化的水文条件极为敏感。具有高分辨率沉积单元的挖填阶地和冲积地层学的证据表明，整个全新世都发生了水文和沉积调整。目前活跃的沟壑和山谷填充物的形态特征也表明，当代沟壑侵蚀速率对现代环境条件的变化很敏感。这一博士论文研究项目的总体目标是量化内布拉斯加州西部上游共和河流域的沟壑侵蚀和山谷淤积速率，并确定这些速率如何随环境变化而变化。这项研究将考察沟壑系统是如何(1)在20世纪末在十年时间尺度上被激活和稳定；(2)在全新世的气候变化中被切割和加剧；(3)在晚第四纪向下延伸以形成流域地貌。这项研究将整合野外、实验室和地理信息系统的分析。历史上的冲沟侵蚀将通过检查航空照片来记录，以确定自20世纪50年代以来十年时间尺度上的冲沟逆行迁移的规模。将使用137-Cs沉积物示踪技术，确定从主动水头下游的沟渠衍生的沉积物的空间分布。全新世河谷沉积速率将根据冲积填土和阶地中有机质的放射性碳年龄测定来估计。对这些填充物的沉积学和土壤学分析将有助于解释沉积过程中活跃的沉积和土壤形成过程。将利用土壤地层学和地貌地层学证据对水系网络的扩展进行调查，以推断在整个晚第四纪不同时间的水系网络的前缘界限。这些界限可以定义将在地理信息系统环境中分析的网络平面，以确定在千年时间尺度上具有重要水文意义的排水网络参数的变化。这项研究的结果将有助于记录半干旱河流系统对环境变化的响应。随着科学对未来气候变化在大平原等敏感半干旱地区的潜在影响越来越关注，对过去水文变化的现实评估变得越来越有必要。这项研究将通过研究其对沟壑侵蚀和山谷淤积的影响来加深对这种变异性的理解。通过研究从几十年到几千年的时间范围内的这些过程，该项目将建立对潜在过程速率的完整范围的更好理解，并将记录可能支配这些速率变化的许多因素。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。

Many fluvial systems of the central Great Plains have flashy discharge regimes and high sediment yields. As a result, their sedimentary processes are extremely sensitive to changing hydrologic conditions. Evidence in the form of cut-and-fill terraces and alluvial stratigraphy with high resolution depositional units suggests that both hydrologic and sedimentologic adjustments have taken place throughout the Holocene. Morphologic properties of currently active gullies and valley fills also indicate that contemporary gully erosion rates are sensitive to changes in modern environmental conditions. This general objective of this doctoral dissertation research project is to quantify gully erosion and valley aggradation rates in the upper Republican River basin of western Nebraska and to determine how these rates have varied in response to environmental change. This research will examine how gully systems have (1) activated and stabilized on decadal time scales in the late 20th century; (2) incised and aggraded in response to climate changes of the Holocene; and (3) extended headward to shape watershed morphology during late Quaternary. This research will integrate field, laboratory, and GIS analyses. Historical gully erosion will be documented by examining aerial photographs to determine the magnitude of headward gully migration on a decadal time scale since the 1950s. The spatial distribution of gully-derived sediment downstream from active headcuts will be established using a 137-Cs sediment tracing technique. Holocene valley aggradation rates will be estimated on the basis of radiocarbon age determinations of organic matter within alluvial fills and terraces. Sedimentologic and pedologic analyses of these same fills will permit interpretation of the depositional and soil-forming processes active during aggradation. Extension of drainage networks will be investigated by using soil-stratigraphic and morpho-stratigraphic evidence to infer the headward limits of networks at various times throughout the late Quaternary. These limits can define network planforms that will be analyzed in a GIS environment to determine changes in hydrologically significant drainage network parameters over millennial time- scales.Results of this research will help document the response of semi-arid fluvial systems to environmental change. As scientific concern mounts over the potential impact of future climate change in sensitive semi-arid regions like the Great Plains, realistic appraisals of past hydrologic variability become increasingly necessary. This research will improve understanding of that variability by examining its impact on gully erosion and valley aggradation. By investigating these processes over temporal scales ranging from decades to millennia, the project will establish a better understanding of the complete range of potential process rates, and will document the many factors that may govern changes in these rates. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.