Dr. Caamaño-Avendaño, Diego

Topographic variation within fluvial systems is essential for providing a mosaic of physical habitats and supporting the dynamic hydraulic, geochemical, and biological processes that determine both aquatic and riparian ecosystem function. In highly-modified rivers through both urban and rural settings, the physical heterogeneity of alluvial channels has been diminished by anthropogenic activities. As riparian areas are increasingly under pressure from agricultural and urban development, identifying the geomorphic controls on physical heterogeneity through these environments is critical. In this study, we use the bed coefficient of variation (CV) extracted from a high-resolution bathymetric LiDAR survey as a dimensionless metric for topographic variation and physical heterogeneity over 100 km of the Boise River corridor that spans an urban-rural gradient. Our CV results for both the streambed and channel demonstrate that the average topographic variation of reaches in urban areas is 22–25% lower than reaches located in rural areas along the same river. While these results initially support the application of the urban stream syndrome hypothesis, CV values had similar magnitudes in both urban and rural reaches suggesting there is a dominant control on topographic variation that was not directly related to urban land use. Analysis of CV values relative to normalized levee width indicates that the causative driver of morphologic simplification in the channel was lateral constraints from levees. In the Boise River, topographic variation increased linearly with normalized levee widths that ranged between 50% and >300% of the average channel width. Further, topographic variation was maximized in reaches where flow expansion during high discharge inundated between 1 and 2 times the average channel width (approximately 65–70% of the available floodplain). Our simple and objective watershed-scale approach leverages highresolution topography data to identify reaches of high physical heterogeneity for river conservation, as well as help guide environmental flow releases in managed rivers.

Restoration of an alluvial wet-meadow system was conducted in the late 1990s to reestablish hydraulic interactions between the river, floodplain, and groundwater to support aquatic–riparian ecosystem function. A single-discharge approach sized the bankfull channel dimensions to the effective discharge (Qe) and three degrees of channel widening relative to the Qe design were explored to identify which design attained dynamic equilibrium in the shortest time. The three experimental channel designs were implemented with bankfull widths of 96%, 157%, and 191%, respectively, of the Qe geometry. Response trajectories were documented for channel dimensions, sediment mobility, channel morphology, floodplain connectivity, and riparian vegetation for the three channel designs, and the efficacy of a single-discharge approach for restoration was examined. Analysis of 20 years of monitoring data and hydraulic modeling revealed that each design responded differently to the imposed initial channel conditions and evolved at substantially different rates. The design with bankfull dimensions most closely approximating Qe reached dynamic equilibrium within four years of restoration, whereas the moderately over-widened channel (57% larger) exhibited slower responses toward dynamic equilibrium for some metrics and did not fully attain the Qe design dimensions within the monitoring period. The extremely-over-widened channel (91% larger) mainly induced slow rates of bed deposition that are projected to take nearly 300 years for the bankfull dimensions to narrow to the Qe width. All reaches had low bed mobility (bankfull Shields stress < 0.03) 14 years after restoration, demonstrating the challenge of reducing the drivers of channel widening, while maintaining sufficient competence for bedload transport and a sustained supply of coarse bed material for salmonid habitat. Restoration that sizes the channel to Qe can provide rapid dynamic equilibrium, but is a first-order simplification of 1) channel dynamics and 2) the range of flows needed for restoring physical and biological processes in wet-meadow systems.