Numerous studies exist on the hydraulics of woody debris jams and the mechanisms driving their geomorphic influence. While most hydraulic studies treat jams as single, solid objects, jams are clearly not individual cylindrical logs but rather an accumulation of pieces ranging in size from leaves and twigs to entire trunks. Here we treat debris jams as complex and porous accumulations of heterogeneous material to understand the relative importance of the different size fractions comprising a jam. We systematically dismantled three deflector debris jams in four stages, removing a total of 17,783 individual wood pieces, to experimentally manipulate jam porosity. We measured the surrounding velocity, shear stress, and drag force (FD). The assumption of nonporosity can result in a 10−20% overestimation of FD. Back‐calculated values of the combined drag coefficient and frontal area term (CDAF)calc represented the drag characteristics of natural debris jams, whereas separating frontal area (AF(emp)) and drag coefficient (CD) contributions in natural jams is misleading. Values of (CDAF)calc for each jam at each stage of removal captured the effects of size and composition of the jam. Wood piece size in debris jams dictates the surface area to volume relationship. This association in turn determines the rate at which FD and (CDAF)calc change with the addition of material. Only low‐porosity jams produce the geomorphic and hydraulic characteristics commonly associated with deflector jams. Our results on natural debris jams also illustrate the importance of employing variable wood size fractions when using woody debris jams for river restoration.

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Flow and sediment regimes shape alluvial river channels; yet the influence of these abiotic drivers can be strongly mediated by biotic factors such as the size and density of riparian vegetation. We present results from an experiment designed to identify when plants control fluvial processes and to investigate the sensitivity of fluvial processes to changes in plant characteristics versus changes in flow rate or sediment supply. Live seedlings of two species with distinct morphologies, tamarisk (Tamarix spp.) and cottonwood (Populus fremontii), were placed in different configurations in a mobile sand-bed flume. We measured the hydraulic and sediment flux responses of the channel at different flow rates and sediment supply conditions representing equilibrium (sediment supply = transport rate) and deficit (sediment supply < transport rate). We found that the hydraulic and sediment flux responses during sediment equilibrium represented a balance between abiotic and biotic factors and was sensitive to increasing flow rates and plant species and configuration. Species-specific traits controlled the hydraulic response: compared to cottonwood, which has a more tree-like morphology, the shrubby morphology of tamarisk resulted in less pronation and greater reductions in near-bed velocities, Reynolds stress, and sediment flux rates. Under sediment-deficit conditions, on the other hand, abiotic factors dampened the effect of variations in plant characteristics on the hydraulic response. We identified scenarios for which the highest stem-density patch, independent of abiotic factors, dominated the fluvial response. These results provide insight into how and when plants influence fluvial processes in natural systems.

The natural tendency of woody debris to accumulate into complex debris jams has been adapted by the restoration industry because of the morphological and ecological benefits of these structures. While much work has been done on woody debris, there is a lack of understanding of the dynamics of debris jams including the controls on their formation and the associated changes in hydraulics. Treatment of jams as static structures, whose hydraulics may be described by that of a single-solid object, prevents optimal success of wood-based restoration projects. This paper reviews the state of the science on the initiation and accumulation of wood forming a debris jam. This review is used to develop a conceptual model of the evolution of a single debris jam focussing on the relationship between the structure and hydraulics and the feedback that exists between them. The proposed mechanisms behind debris jam evolution are supported by a case-study of three natural jams. Incorporation of this model into restoration and management plans will result in more successful and cost-efficient projects.

[1] Improved understanding of the connection between riparian vegetation and channel change requires evaluating how fine-scale interactions among stems, water, and sediment affect larger scale flow and sediment transport fields. We propose a spatially explicit model that resolves patch-scale (submeter) patterns of hydraulic roughness over the reach scale caused by stands of shrubby riparian vegetation. We worked in tamarisk-dominated stands on the Yampa and Green Rivers in Dinosaur National Monument, northwestern Colorado, USA, where questions remain regarding the role of vegetation in inducing or exacerbating documented channel changes. Hydraulic roughness patterns were derived from patch-scale measurements made with detailed terrestrial laser scan (TLS) data that were extrapolated to reach scales based on correlation with light detection and ranging (LiDAR) (ALS) data. Two-dimensional, patch-scale, hydraulic models were used to parameterize the stage dependence of hydraulic roughness of typical patch types (i.e., sparse, moderate, and dense patches). We illustrate the value of using this approach to characterize vegetation roughness by applying our results to a two-dimensional hydraulic model of flow for one of our study sites. Results from this work predict that the roughness of vegetated floodplains increases with flow depth and is dependent on patch-scale stem organization. Geomorphically relevant patterns (i.e., areas of low or high shear stress that are likely to scour or fill during high flows) become apparent with the detail introduced by spatially explicit, depth-dependent roughness. To our knowledge, the multiscalar analysis presented here is the first to mechanistically account for shrubby riparian vegetation stand structure, and associated hydraulic roughness of vegetation patches, at the reach scale.