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Floods are amplified and attenuated by features and processes across spatial scales, defined here as flood dynamics. We review and synthesise these influences at the catchment, river network and reach scales as a means of integrating understanding of controls on flood dynamics and identifying key questions that arise because of differences in techniques of investigation and disciplinary emphases between spatial scales. Catchment‐scale influences include catchment area, topography, lithology, land cover, precipitation, antecedent conditions and human alterations such as changing land cover. Network‐scale influences on flood dynamics include network topology, longitudinal variations in the geometry of successive river corridor reaches, lakes and wetlands and human alterations including flow regulation and cumulative changes in channel‐floodplain connectivity in multiple reaches across a network. Reach‐scale influences on flood dynamics include water sources, river corridor geometry and connectivity and human alterations such as artificial levees, channelisation, bank stabilisation, changes to floodplain land cover and drainage, dike operation, process‐based river restoration and urban stormwater management. Our review and synthesis of relevant literature suggest that the relative importance of these multiple influences on flood dynamics varies across spatial scales. Hillslope response may dominate hydrograph characteristics in smaller catchments, for example, whereas network geometry and flow dynamics exert progressively stronger influences on flood dynamics with increasing catchment size. Scale‐specific advances in understanding flood dynamics, including rainfall‐runoff analyses of water movements from uplands into channel networks (catchment‐scale), analyses of flow dynamics along networks of multiple channel reaches (network‐scale) and investigations of biophysical feedbacks and the influences of river corridor geometry and hydraulic roughness (reach‐scale), have largely contributed to understanding flood dynamics, but there remain important disconnects between these diverse bodies of research and outstanding questions related to the cumulative effects on flood dynamics across scales. 

Wide, low-gradient segments within river networks (i.e., beads) play a critical role in absorbing and morphologically adapting to disturbances, including wildfires and debris flows. However, the magnitude and rate of morphological adjustment and subsequent hydraulic conditions provided by beads compared to pre-disturbance conditions are not well understood. This study analysed trajectories of river morphology, flood attenuation and hydraulic fish habitat following the 2020 Cameron Peak Fire and July 2022 debris flow and flood at Little Beaver Creek, Colorado, USA. Using repeat aerial imagery, ground-based surveys and hydrodynamic modelling, we assessed morphological changes in a 600-m-long bead of Little Beaver Creek. Metrics of floodplain destruction and formation and channel migration greatly increased in magnitude after the first post-fire runoff season but returned to the historical range of these metrics three years after the fire. The 2022 flood deposited sediment, infilled side channels, reduced pool area and increased the area of bars and islands. Flood wave attenuation and hydraulic habitat conditions did not show clear improvement or impairment despite more rapid changes in system geometry, geomorphic unit abundance and geomorphic unit location. The ability of the site to attenuate peak flows changed minimally and inconsistently over the studied floods. Various lotic habitat conditions changed—namely a reduction in floodplain access and deepening of certain pools—but the overall flow-type diversity of the system was not largely impacted. The resilience of the active channel of Little Beaver Creek to the fire and flood disturbances while retaining key services demonstrates the importance of river beads for enhancing river-floodplain resilience to large disturbance events and highlights river beads as key areas for preservation and restoration. 

Abstract Despite the numerous hydrological, geological, and ecological benefits produced by floodplain landscapes, floodplains continue to be degraded by human activities at a much higher rate than other landscape types. This large-scale landscape modification has been widely recognized, yet a comprehensive, national dataset quantifying the degree to which human activities are responsible for this degradation has not previously been evaluated. In this research, we analyze floodplain integrity for the contiguous United States by spatially quantifying the impact of anthropogenic stressors on almost 80,000 floodplain units. We demonstrate the prevalence of human modifications through widely available geospatial datasets, which we use to quantify indicators of floodplain integrity for five essential floodplain functions of flood attenuation, groundwater storage, habitat provision, sediment regulation, and organics and solute regulation. Our results show that floodplain degradation is spatially heterogeneous and that the integrity of nearly 70% of floodplains in the United States is poor. We highlight that quantifying the integrity of spatially explicit floodplain elements can allow for restoration efforts to be targeted to the areas in most desperate need of preservation. 

This analysis of artificial levee impacts on floodplains in the contiguous United States reveals some unexpected results. 

Abstract Wood accumulations influence geomorphic, hydraulic, and ecologic functions within a river corridor, but characterizing these accumulations presents challenges across a range of field and remote sensing methodologies. We evaluate the ability of handheld lidar scanners, specifically lidar‐scanning capabilities of a fourth‐generation iPad Pro, to collect three‐dimensional wood accumulation data, which can be used to inform measurements of wood volume, porosity, complexity, and roughness. We discuss the potential and limitations of this novel methodology for river research and management. We found that handheld lidar presents a cost‐effective input for data‐processing workflows that field measurements of wood accumulation dimensions cannot as easily replicate including (1) a user‐friendly means of data collection and visualization; (2) accurate comparisons of wood volume over time; (3) integration into workflows to measure porosity parameters; and (4) potential use in informing hydraulic and morphodynamic models. Consideration of study area constraints and intended use of scans are prerequisites to using handheld lidar as an effective tool. We identified some specific limitations of using handheld lidar scanners in wood‐rich river corridors, including (1) scanners perform poorly when wood is under water or surrounded by dense vegetation; (2) scanners require physical access to areas of interest at distances less than 5 m; (3) scans need to be manually georeferenced; and (4) scans require manual measurements for any dimensional data, which still have associated user time and error. Handheld lidar as a scientific tool is rapidly developing and there is substantial room for expansion of applications, utilization, and advances in the use of this tool in river research and management. 

Abstract Floodplains provide important ecological, hydrological, and geomorphic functions within river corridors. During overbank flows, complex hydrodynamic conditions occur as water exits and re‐enters the channel and interacts with hydraulically rough floodplain vegetation. However, the extent to which floodplain vegetation influences channel‐altering hydrodynamic forces and thus bedform topography and sediment transport is poorly understood. We address this knowledge gap and present the results of flume experiments where we measured bedform topography under varied floodplain vegetation conditions at two overbank flow relative depths. The experiments were conducted in a 1‐m wide meandering compound channel inset in a 15.4 long, 4.9‐m wide basin. The channel bed was a mobile sand‐and‐gravel mixture with a median sediment size of 3.3 mm, and sediment transport occurred only within the channel. We tested bare and vegetated floodplain conditions with 2.7‐cm diameter rigid emergent vegetation elements at spacings of 3.0 and 12.1 units m⁻². We performed a moving‐window analysis of topographic surface metrics including skewness, coefficient of variation, and standard deviation, as well as topographic patch analysis of area and contagion to measure changes in bedform heterogeneity as flow depth and vegetation density were varied. Our results show that both greater density vegetation and larger flows can increase bedform topographic heterogeneity. These findings suggest that floodplain vegetation and natural hydrologic regimes that include overbank flows can enhance stream habitat complexity. Designing for the effects of established vegetation conditions and prioritizing floodplain vegetation planting may be useful for river managers striving to achieve successful biomic river restoration.