[1] The effectiveness of gravel augmentation as a river restoration strategy depends on the extent and duration of the topographic and bed texture changes created by the pulse of added sediment. Previous work has emphasized the strong tendency for natural sediment waves to propagate primarily by dispersion; however, pulse translation may occur for gravel additions to armored channels downstream of dams where added sediments are finer than the preexisting bed material. Here we report results of a laboratory investigation in which we created an immobile, armored bed and documented the spatial and temporal evolution of the bed topography and bed texture in response to gravel pulses of various volumes and grain sizes. The introduced sediment waves evolved by a combination of translation and dispersion, with a significant translational component. Pulse translation and dispersion can be readily discerned on a graph of the time evolution of the downstream cumulative distribution of elevation differences from the preexisting bed topography. Translation was most evident for smaller volumes of added sediment. Pulses of finer-grained gravel moved through the flume more rapidly, resulting in a larger magnitude but shorter duration of bed fining. More work is needed to understand the influence of bar-pool topography and flow magnitude and duration before the grain size and volume of gravel additions can be selected to achieve optimal patterns of pulse propagation.

[1] Additions of sand to gravel beds greatly increase the mobility and flux of gravel. However, it is not known how additions of finer gravel to coarser gravel beds will affect the mobility of bed material. Here we examine the effect of fine gravel pulses on gravel bed material transport and near-bed flow dynamics in a series of flume experiments. Bed material refers exclusively to sediment in the channel prior to the pulse introduction. The observations indicate that fine sediment pulses tend to migrate downstream in low-amplitude waves. As the waves pass over the gravel bed, the interstitial pockets in the bed material surface fill and coarse gravel particles are entrained. This increases bed material transport rates and causes a distinct shift from a selective mobility transport regime where particles coarser than the bed material median (8 mm) make up <30% of the load to an equal mobility transport regime where bed materials coarser than 8 mm and finer than 8 mm are transported in equal proportions. The only possible source for this coarser bed load material is the sediment bed, suggesting that portions of the coarse surface layer are being mobilized. Observations of near-bed velocity and turbulence suggest that fine gravel pulses cause fluid acceleration in the near-bed region associated with a reduction in the level of turbulence produced at the sediment boundary. This accelerated fluid at the bed increases drag exerted on coarse surface layer particles, promoting their mobilization. Our findings suggest that, in general, finer bed sediment (not just sand) can mobilize coarser sediment and that expressions for the influence of sand on bed mobility need to be generalized on the basis of grain ratios.

Sediment supply to gravel bed river channels often takes the form of episodic sediment pulses, and there is considerable interest in introducing sediment pulses in stream restorations to alter bed surface grain size distributions and bed mobility. A series of laboratory experiments was conducted in order to examine how sediment pulse grain size and volume affects the mobility of bed material in gravel bed channels. Pulses used in the experiments were composed of either the fine tail or the median of the subsurface bed material grain size distribution. Bed material refers to sediment in the channel prior to the pulse introduction exclusively. Both types of pulse were finer than the bed material surface median. Two pulse sizes were used, which were either equivalent to the volume of sediment required to cover the entire bed one median subsurface bed material grain diameter deep (full unit) or 1/4 of this volume (1/4 unit). The latter was designed to produce a transitory pulse. With the exception of the 1/4 unit coarse pulse, introduction of the sediment pulses to the channel caused dramatic increases in the bed load flux. The coarse sediment pulses fine the bed surface and coarsen the bed load. Finer pulses also fine the surface, but the bed load fines while the bed material load (that excludes pulse material) coarsens. The greatest effects on the fractional transport occurred during the full unit fine pulse where the pulse covered the greatest bed surface and effectively smoothed the bed, increasing near bed velocity and mobilizing the coarse particles. Overall, the coarse pulses were not very effective at mobilizing bed material. The large fine pulse mobilized ∼35% of the bed material surface (∼35% of its input weight) and was most effective at mobilizing the surface. However, the small fine pulse mobilized 50% of its input weight as it passed through the channel, making it the most efficient at mobilizing the bed material.

[1] We investigated how channel morphology, flow complexity, and habitat characteristics in a meandering gravel bed river evolved over time from a simple, reconfigured initial condition. Using a time series of topographic data, we measured rates of channel migration and morphologic change, documented patterns of sediment storage, and estimated rates of sediment supply. We constructed, calibrated, and validated hydrodynamic models to quantify how the evolving morphology influenced hydraulic conditions, flow complexity, and habitat suitability for Chinook salmon spawning and rearing. For a series of meander bends with constant curvature, similar bank materials, and an identical flow history, sediment supply and bar storage directly influenced channel migration rates. Habitat modeling indicated that the availability of Chinook salmon spawning habitat increased over time, whereas the majority of the reach continues to provide only low-to medium-quality rearing habitat for juvenile salmonids, primarily because of a lack of low-velocity refuge zones. However, other metrics of flow complexity indicate that areas of favorable flow conditions gradually expanded as point bars developed along the inner bank of each bend. These results indicate that although sediment supply can stimulate channel change and diversify river morphology, which acts to promote flow complexity and provide spawning habitat, these sediment-driven morphological changes might not create bioenergetically favorable habitat for juvenile salmonids.

Ecosystem restoration often aims to recreate the physical habitat needed to support a particular life-stage of a focal species. For example, river channel reconstruction, a common restoration practice along the Pacific coast, is typically used to enhance spawning habitat for adult Chinook salmon, a species experiencing large population declines. These restoration efforts rarely consider, however, that altering spawning habitat could have indirect effects on other life-stages, such as juveniles, which might occur if, e.g. reconstruction alters the benthic food web. To determine how channel reconstruction impacts benthic macroinvertebrates, juvenile Chinook's primary prey, we conducted two studies at a restoration site in the Merced River, California. We asked (1) has gravel enhancement altered invertebrate assemblages in the restored reach compared with an unrestored reach? and, if so, (2) can shifts in the invertebrate community be explained by increased substrate mobility and by reduced heterogeneity that results from restoration? We show that invertebrate abundance and biomass were lower in the restored reach and that these changes were accompanied by a shift from dominance by filter-feeding caddisflies (Hydropsyche) in the unrestored reach to grazing mayflies (Baetis) in the restored reach. Using an in situ manipulation, we demonstrated that this trend was driven by increased substrate mobility that reduces the abundance of Hydropsyche and by decreased substrate heterogeneity that reduces the abundance of Baetis. Our studies suggest that geomorphic changes typical of reconstructed rivers can alter food webs in ways that may have important implications for supporting the focal species of restoration efforts.