There is a strong scientific consensus that river corridors are badly damaged by societal impacts, costing the United States alone more than $76 billion yr-1 and harming many species beyond assignment of any dollar value. In response to this problem, society has called upon governments, scientists, and private consultants to rehabilitate rivers. A scientific consensus has emerged that process-based rehabilitation within a watershed-scale context is highly important for a successful outcome. Unfortunately, the science underlying linked abiotic-biotic processes in rivers is still rudimentary, relative to the complexity of the ecosystem to design and build process-based solutions with the same success evident in the practice of medicine or the manufacture of buildings and other civil structures. Many existing practices have been vetted and found by scientific experts to be largely ineffective. Multiple paradigms for rehabilitation based on different combinations of scientific complexity, universality, and comprehensivity are diverging and splintering the technical community. Lacking a consensus among academic scientists and private practitioners, it is too soon to establish professional standards or regulatory requirements. Within this context, geomorphologists have a key role in getting involved in all phases of river rehabilitation and bringing their perspective and capabilities to bear on the grand challenge of preventing environmental collapse. © 2013 Elsevier Inc. All rights reserved.

To determine whether large wood (LW, ≥1-m length, ≥10-cm diameter) plays a role in Chinook salmon (Oncorhynchus tshawytscha) redd (i.e. egg nest) placements in a regulated, Mediterranean-climate, medium-sized river (where channel width is less than the upper quartile of length of potential instream wood pieces), characteristics of 527 large wood pieces, locations of 650 redds, and mesohabitat delineations (riffle, run, glide, pool) were collected during a spawning season along a 7.7km reach directly below Camanche Dam on the Mokelumne River, CA. LW was regularly distributed across the study reach an average 70 LW pieces km-1. Some LW clustering was evident at islands and meander bends. Spawners built 85% of redds within one average channel width (31m) of LW. Spawners utilized LW within a 10m radius 36% of the time in the upper 3km rehabilitated reach, and 44% of the time in the lower 4.7km marginal habitat reach. A greater percentage of LW was utilized in riffles in the upper 3km reach where 90% of redds were built, while a larger percentage of spawners used LW in riffles in the lower 4.7km reach. LW-redd interactions occurred at greater rates than by random chance alone in the lower 4.7km reach, which implies that LW aids spawning in marginal habitats. River managers and salmonid spawning habitat rehabilitation (SHR) projects should take LW additions into consideration as an important component of river rehabilitation. © 2010 John Wiley & Sons, Ltd.

Abstract. This paper demonstrates a relatively new method of analysis for stage dependent patterns in meter-scale resolution river DEMs, termed geomorphic covariance structures (GCSs). A GCS is a univariate and/or bivariate spatial relationship amongst or between variables along a pathway in a river corridor. Variables assessed can be flow independent measures of topography (e.g., bed elevation, centerline curvature, and cross section asymmetry) and sediment size as well as flow dependent hydraulics (e.g., top width, depth, velocity, and shear stress; Brown, 2014), topographic change, and biotic variables (e.g., biomass and habitat utilization). The GCS analysis is used to understand if and how the covariance of bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel-cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges through a suite of spatial series analyses on 6.4 km of the lower Yuba River in California, USA. A key conclusion is that the test river exhibited positively covarying and quasi-periodic oscillations of bed elevation and channel width that had a unique response to discharge as supported by several tests. As discharge increased, the amount of positively covarying values of bed elevation and flow-dependent channel top width increased up until the 1.5 and 2.5 year annual recurrence flow and then decreased at the 5 year flow before stabilizing for higher flows. These covarying oscillations are quasi-periodic scaling with the length scales of pools, bars, and valley oscillations. Thus, it is thought that partially confined gravel-cobble bedded alluvial rivers organize their adjustable topography with a preference for covarying and quasi-periodic bed and width undulations at channel forming flows due to both local bar-pool mechanisms and non alluvial topographic controls.

Macro-roughness elements such as boulders and bedrock outcrops, collectively referred to as large bed elements (LBEs), are key features influencing hydrodynamics and morphodynamics in mountain rivers. Where LBEs are abundant and account for a substantial portion of total flow resistance, existing geomorphic theory, previous physical experiments, and limited field observations support the theory that LBE configurations adjust to maximize flow resistance. However, methods to explicitly map individual features along entire river segments are lacking, thus limiting analysis of LBE spatial structure in boulder-bedded rivers. In addressing these gaps, this study sought to develop a procedure for mapping LBEs from 3D point-clouds, explore LBE spatial structure in a real boulder-bedded river, and test the hypothesis that LBE configurations were organized to maximize flow resistance. The mapping procedure applied a ground classification algorithm to produce a roughness surface model, from which LBEs were extracted by a marker controlled watershed algorithm. Implementing the procedure, 42,176 LBEs were mapped in 13.2-km of the mountainous Yuba River (Northern California). Scale and discharge-dependent LBE concentration and spacing metrics were quantified for multiple laterally and/or hierarchically nested spatial domains and classified to differentiate three flow-resistance based hydrodynamic regimes: isolated roughness, wake interference, and skimming flow. Of these regimes, wake interference corresponds to a state of maximum resistance, so hypothesis testing involved determining if this regime was dominant. Results confirmed 25 of 28 segment- and reach-scale LBE concentrations were in the wake interference regime. However, spacing metrics classified 24 of these same spatial domains in the skimming flow regime. Concentration metrics, which quantify LBE density in a given spatial area, differ from spacing metrics, which represent LBE proximity to one another. While comparison of segment and reach-scale regime classifications by each metric concluded concentration was superior to spacing for regime classification purposes, these disparities leave open questions about this extremal model of geomorphic adjustment. Lastly, lateral variability of metrics across the river corridor had implications for discharge-dependent resistance.

Micro-scale (<0.1 channel widths) physical conditions within channels ultimately exert a strong control on habitat selection by fish species. Data are presented demonstrating that micro-habitat requirements of spawning Chinook salmon (Oncorhynchus tshawytscha) are strongly inter-related; fish utilized coarser substrate if associated with faster flow velocities. This was not observed to be controlled through a hydraulic sorting mechanism but, rather, related to the physical processes of spawning. Failure to consider the 'elastic' nature of Chinook salmon spawning requirements resulting from links between the physical parameters may have implications for river management practices such as restoration or setting environmental flows. © 2009 John Wiley & Sons, Ltd.

Despite studies showing that dams have significant effects on the sediment dynamics and evolution of a river upstream of a dam, the knowledge on relationships between river topography and sediment transport in the dam’s backwater zone has been poorly applied in reservoir sedimentation management. Therefore, this study evaluated the benefits that topographic control might have on reservoir sedimentation. To do this, three test scenarios were applied to explore the hydraulic and sediment transport regimes in the Middle Yuba River. A two-dimensional hydrodynamic model was employed to simulate the stream hydraulic response to topographic controls. The results indicate that adding a topographic constrictions and expansion sequence can change a river’s capability of entraining sediment and control the spatial distribution of sediment transport regimes by either accelerating sediment flushing near the dam or ponding the sediment farther upstream from dam. Topographic controls perform best for the low and medium flow.

Hydropeaking, a hydroelectricity generation strategy involving rapid changes to flow releases from dams in response to fluctuations in hourly-adjusted electricity markets, has been widely applied due to its economic efficiency. However, these operational practices produce sub-daily flow fluctuations that pose substantial hazards to riverine ecosystems and human activities. To ascertain the downstream impacts of hydropeaking, features of hydropeaking have been analyzed with respect to ecologically relevant hydrologic variables. However, since studies aiming to characterize hydropeaking regime often require manual feature extraction, they are limited to small temporal and spatial scales. Additionally, riverine ecologists have commonly treated hydropeaking as a broadly similar flow-alteration pattern regardless of the complexities of the electricity market and differences in the natural settings where it is applied. Therefore, this study sought to determine whether significantly different hydropeaking patterns exist on a regional scale, as revealed by the variation in hydropeaking over a long temporal scale (>five years). To fulfill this goal, a new algorithm, the Hydropeaking Event Detection Algorithm (HEDA), was developed in R to automate the characterization of hydropeaking flow regimes. Clustering analyses were conducted to explore the similarities and differences of hydropeaking regimes among 33 sites in numerous hydrologic regions of California. Four distinct classes of hydropeaking flow regimes were identified and distinguished by the duration and frequency of hydropeaking. Meanwhile, rate of change, amplitude and timing of hdyropeaking played less important roles in the classification.

River restoration projects with the goals of habitat enhancement and improved flood protection involve ecological, structural, and socio-economic river design. The current best practice virtually assesses the hydro-geo-climatic river landscape with remote sensing techniques and two-dimensional numerical modelling. The resulting data inform the flood-safe design of ecohydraulically viable river design features. Here, we demonstrate the application of a novel algorithm for programmatic river landscape optimization as a function of seasonal habitat area gained, physical stability during floods, and construction costs. In a case study on California’s Yuba River (USA), we illustrate the design of an artificial side channel for enhancing the habitat for Pacific salmon. We show that the placement of nature-based engineering, such as native vegetation plantings or large wood can be almost completely automated. In contrast, the automated optimization of ecologically valuable terraforms (self-sustaining morphodynamic structures) remains a challenge for future numerical models and design algorithms.

This study applies the functional flows model (FFM) that integrates hydrogeomorphic processes and ecological functions to assess physical habitat. Functional flows are discharge values that serve ecological uses. The model was adjusted to evaluate gravel-bed riffle functionality for fall-run Chinook salmon with respect to river rehabilitation on the Mokelumne River and flood-induced channel change on the Yuba River. The goal was to test if differences in ecological performance were traceable to differences in hydrogeomorphic conditions. Ecological functions studied were bed occupation (spawning, incubation and emergence) and bed preparation (river bed reworking periods)- both reliant on shear stress dynamics. Model outputs included number of days that have functional flows, ranges of functional flows that provide favourable sediment transport stages and the efficiency of a site to produce functional flows. Statistical significance of results was tested using non-parametric tests. Functional flows analyses before and after geomorphic alteration indicate that river rehabilitation on the lower Mokelume River increased the number of days with functional flows, while the Yuba River May 2005 flood increased the functional ranges of flows for the test sites. Reach-scale analyses indicated similar ecological performance at reference sites in both rivers. A comparison between both rivers showed that despite a greater geomorphic potential of the Mokelumne River sites to have functional flows, Yuba River sites actually experienced better ecological performance for fall-run Chinook salmon freshwater life stages due to greater flow availability. The FFM provided an objective tool to assess changes in ecological functionality at hydrogeomorphically dynamic sites. © 2010 John Wiley & Sons, Ltd. Copyright © 2010 John Wiley & Sons, Ltd..