Assessing Potential Controls on River Bead Functionality in Mountain Streams

We evaluated flux attenuation potential, referred to as functionality, in laterally extensive, storage-dominated river reaches known as ‘beads’. Bead functionality was evaluated as a relationship between driver variables, which directly measure or measure proxies of geomorphic and biotic system inputs, and response variables, which are proxy variables believed to influence flux travel time and storage magnitude, assuming that functional beads contribute to higher travel times and storage magnitudes at the network scale. Geomorphic driver variables include drainage area, catchment slope, elevation, land cover and precipitation metrics, which represent water inputs into the stream corridor, as well as delta normalized burn index (dNBR) and catchment slope, which further represent sediment inputs into the stream corridor. Biotic driver variables within each bead include wood load, beaver modifications, and type of riparian vegetation. Response variables include normalized difference vegetation index (NDVI), normalized difference water index (NDWI), patch density, and total sinuosity. Driver and response variables were measured through a mixture of fieldwork and remote data for 52 beads in 27 catchments in the Colorado Front Range, USA. Statistical analyses examined relationships between drivers and responses and the effectiveness of grouping the beads in different ways (by dominant vegetation and by elevation). Analyses suggest that bead functionality is most strongly linked to bead ratio, or the ratio of bead size to catchment size. Functional beads are larger relative to catchment size. In addition, beads can be efficiently grouped by dominant vegetation; these different types of beads reflect significant differences in catchment geometry, geomorphic inputs, and biotic inputs, and display significant differences in bead geometry. Although functionality is the complex result of numerous factors and may require case-by-case assessment efforts, restoration of channel-floodplain connectivity and facilitating greater retention of water will enhance river restoration by increasing the width of the active floodplain. Investigating drivers of functionality provides a crucial link between system inputs, restoration action, and desired response, allowing plans to be tailored to address targets. Because bead position and geometry cannot be feasibly modified, the functionality framework can be used to identify sites with the greatest potential for restoration.本研究针对被称为"串珠（beads）"的广侧向分布、以储水为主的河道河段，评估了其通量衰减潜力——该潜力亦被称为功能度（functionality）。串珠功能度的评估基于驱动变量与响应变量之间的关联：驱动变量直接测量或替代表征地貌与生物系统的输入要素，响应变量则为被认为会影响通量传播时长与储水规模的替代变量，本研究假设功能度更高的串珠河段在河网尺度上具备更长的传播时长与更大的储水规模。地貌驱动变量包括流域面积、流域坡度、海拔、土地覆被与降水指标（这些变量表征输入河道廊道的水量），以及delta归一化燃烧指数（dNBR）与流域坡度，二者进一步表征输入河道廊道的沉积物输入量。每个串珠河段内的生物驱动变量包括木质残体负载量、河狸活动改造特征与河岸植被类型。响应变量包括归一化植被指数（NDVI）、归一化水体指数（NDWI）、斑块密度与总弯曲度。本研究针对美国科罗拉多锋面地带（Colorado Front Range）的27个流域内的52个串珠河段，通过野外调查与遥感数据结合的方式获取驱动变量与响应变量。统计分析探究了驱动变量与响应变量之间的关联，以及以不同方式（按优势植被类型、按海拔）对串珠河段进行分组的有效性。分析结果显示，串珠功能度与串珠比（bead ratio，即串珠河段规模与流域规模的比值）关联最为紧密。功能度更高的串珠河段，其规模相对于流域规模而言更大。此外，可依据优势植被类型对串珠河段进行高效分组；不同类型的串珠河段在流域几何形态、地貌输入要素与生物输入要素上均存在显著差异，且其自身的几何形态也呈现出显著区别。尽管功能度是多种要素共同作用的复杂结果，且可能需要逐案开展评估工作，但通过恢复河道-洪泛区连通性、提升水体滞留能力，可通过拓宽主动洪泛区宽度来优化河流修复效果。探究功能度的驱动要素，可在系统输入要素、修复行动与预期响应之间建立关键关联，从而能够针对修复目标定制修复方案。由于串珠河段的位置与几何形态难以通过人工手段改造，因此可借助功能度框架识别出最具备修复潜力的河段。