Anne Chin

Bank Erosion as a Desirable Attribute of Rivers

ABSTRACT Bank erosion is integral to the functioning of river ecosystems. It is a geomorphic process that promotes riparian vegetation succession and creates dynamic habitats crucial for aquatic and riparian plants and animals. River managers and policymakers, however, generally regard bank erosion as a process to be halted or minimized in order to create landscape and economic stability. Here, we recognize bank erosion as a desirable attribute of rivers. Recent advances in our understanding of bank erosion processes and of associated ecological functions, as well as of the effects and failure of channel bank infrastructure for erosion control, suggest that alternatives to current management approaches are greatly needed. In this article, we develop a conceptual framework for alternatives that address bank erosion issues. The alternatives conserve riparian linkages at appropriate temporal and spatial scales, consider integral relationships between physical bank processes and ecological functions, and avoid secondary and cumulative effects that lead to the progressive channelization of rivers. By linking geomorphologic processes with ecological functions, we address the significance of channel bank erosion in sustainable river and watershed management.

The morphologic structure of step–pools in mountain streams

Abstract Step–pool sequences in streams in the Santa Monica Mountains of southern California impart a characteristic morphologic structure that varies with slope. But despite the strong association with slope that suggests a direct process control, analysis of 464 step–pools in 13 study reaches reveals the relationship to reflect correlation, but not necessarily causality. The direct control on step height is particle size, whereas discharge is the probable causal process determining step wavelength. Because both particle size and discharge generally change with channel slope downstream, a slope variation with step wavelength and height is the apparent indirect geometric result. A conceptual model of process-form linkages is proposed based on these results, whereby the step–pool morphology is produced by positive relationships between wavelength and discharge, and between height and particle size. The model offers new insights for explaining the step–pool morphology, and it is potentially useful in channeling future efforts toward the appropriate scales and variables.

Urbanization and Adjustment of Ephemeral Stream Channels

Comparatively few studies of stream channel adjustment following urbanization have been undertaken in dryland environments. In the new master planned community of Fountain Hills, a residential area near Phoenix, Arizona developed since the 1970s, surveys in 1987 and 2001 of ephemeral wash channels show that they are larger than comparable channels in humid areas, reflecting the effects of rare but substantial floods. Morphological adjustment is spatially varied and is influenced by wide road crossings that are responsible for fragmentation of the adjusting channels into segments. By 2001, these segments are characterized by scour immediately downstream of a crossing and a relatively high width-depth ratio farther downstream before the next road crossing. Such spatially distributed responses have caused management problems unique to arid environments, so that, although road drainage was originally allowed to flow into the washes at the crossings, the stormwater network has now been augmented to improve drainage and to inhibit scour at the crossings. In maintaining such washes, consideration of channel adjustments as a result of urbanization could form the basis for an approach comparable to restoration methods in more humid areas.

On the Stability of Step‐Pool Mountain Streams

Although step‐pools are generally considered to be stable bedforms, stability is not absolute, but depends on size, scale, and perspective. Hydraulic analysis of the stability of step‐pool sequences in the Santa Monica Mountains, California, indicates that they are active channel features that are generally restructured within 5 to 100 years. The degree of mobility depends on step particle size. Steps are stable within small temporal and spatial scales, where they function as independent variables that dissipate stream energy and regulate channel hydraulics, but stability decreases at larger scales, where step pools become dependent variables that respond to discharge and sediment load. Hence the role of step‐pools changes from energy dissipation to one of channel adjustment. These results underscore the need to consider larger spatial and temporal scales in order to reveal the complete function and significance of step‐pools in mountain fluvial systems.

The geomorphic significance of step–pools in mountain streams

Abstract This paper develops a conceptual model to define the changing role and significance of step–pools as energy dissipators in steep mountain channels. Although energy dissipation is a significant function of step–pools, this role changes over time with variations in discharge. Steps are effective in reducing stream energy at low flows, but their effectiveness diminishes with increasing stage. Accordingly, the manner in which energy dissipation occurs also varies. With increasing flows, spill resistance gives way to a dominance in form and grain resistance. The conceptual model for the changing role of step–pools is illustrated with data from hydraulic analysis and modeling of step–pools in the Santa Monica Mountains of California. The model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics. The model offers a new articulation for the geomorphic significance of step–pools in mountain streams, and it serves as a useful template for a more complete understanding of step–pools over a longer time scale.

Linking Theory and Practice for Restoration of Step-Pool Streams

Step-pools sequences are increasingly used to restore stream channels. This increase corresponds to significant advances in theory for step-pools in recent years. The need for step-pools in stream restoration arises as urban development encroaches into steep terrain in response to population pressures, as stream channels in lower-gradient areas require stabilization due to hydrological alterations associated with land-use changes, and as step-pools are recognized for their potential to enhance stream habitats. Despite an increasingly voluminous literature and great demand for restoration using step-pool sequences, however, the link between theory and practice is limited. In this article, we present four unique cases of stream restoration using step-pools, including the evolution of the approaches, the project designs, and adjustments in the system following restoration. Baxter Creek in El Cerrito, California demonstrates an early application of artificial step-pools in which natural adjustments occurred toward geomorphic stability and ecological improvement. Restoration of East Alamo Creek in a large residential development near San Ramon, California illustrates an example of step-pools increasingly used in locations where such a channel form would not naturally occur. Construction of a step-pool channel in Karnowsky Creek within the Siuslaw National Forest, Oregon overcame constraints posed by access and the type and availability of materials; the placement of logs allowed natural scouring below steps. Dry Canyon Creek on the property of the Mountains Restoration Trust in Calabasas, California afforded a somewhat experimental approach to designing step-pools, allowing observation and learning in the future. These cases demonstrate how theories and relationships developed for step-pool sequences over the past two decades have been applied in real-world settings. The lessons from these examples enable us to develop considerations useful for deriving an appropriate course of design, approval, and construction of artificial step-pool systems. They also raise additional fundamental questions concerning appropriate strategies for restoration of step-pool streams. Outstanding challenges are highlighted as opportunities for continuing theoretical work.

Perceptions of Wood in Rivers and Challenges for Stream Restoration in the United States

This article reports a study of the public perception of large wood in rivers and streams in the United States. Large wood is an element of freshwater aquatic ecosystems that has attracted much scientific interest in recent years because of its value in biological and geomorphological processes. At the heart of the issue is the nature of the relationship between scientific recognition of the ecological and geomorphological benefits of wood in rivers, management practices utilizing wood for river remediation progress, and public perceptions of in-channel wood. Surveys of students’ perceptions of riverscapes with and without large wood in the states of Colorado, Connecticut, Georgia, Illinois, Iowa, Missouri, Oregon, and Texas suggest that many individuals in the United States adhere to traditionally negative views of wood. Except for students in Oregon, most respondents considered photographs of riverscapes with wood to be less aesthetically pleasing and needing more improvement than rivers without wood. Analysis of reasons given for improvement needs suggest that Oregon students are concerned with improving channels without wood for fauna habitat, whereas respondents elsewhere focused on the need for cleaning wood-rich channels for flood risk management. These results underscore the importance of public education to increase awareness of the geomorphological and ecological significance of wood in stream systems. This awareness should foster more positive attitudes toward wood. An integrated program of research, education, and policy is advocated to bridge the gap between scientific knowledge and public perception for effective management and restoration of river systems with wood.

On the origin of step‐pool sequences in mountain streams

This paper reports a field test of the prevailing antidune theory to explain the mechanisms of step-pool formation in steep mountain streams. The theory is supported by laboratory data but it has not received extensive field testing that would enable wide applicability to natural conditions. Data from the Santa Monica Mountains of California are used to test the antidune model where step wavelength and reconstructed hydraulic conditions are compared with the formation domain predicted by experimental and theoretical studies. Results show general agreement between field and laboratory data. Step wavelength and hydraulic requirements are consistent with those of the antidune process of formation, but results also suggest that true antidunes may be difficult to achieve in the steep headwaters where channels may be dominated by large roughness elements. These findings strengthen the link between theory and observation in clarifying how step-pools form; they contribute to the basis for elucidating why step-pools form in mountain streams.

Understanding Human–Landscape Interactions in the “Anthropocene”

This article summarizes the primary outcomes of an interdisciplinary workshop in 2010, sponsored by the U.S. National Science Foundation, focused on developing key questions and integrative themes for advancing the science of human–landscape systems. The workshop was a response to a grand challenge identified recently by the U.S. National Research Council (2010a)—“How will Earth’s surface evolve in the “Anthropocene?”—suggesting that new theories and methodological approaches are needed to tackle increasingly complex human–landscape interactions in the new era. A new science of human–landscape systems recognizes the interdependence of hydro-geomorphological, ecological, and human processes and functions. Advances within a range of disciplines spanning the physical, biological, and social sciences are therefore needed to contribute toward interdisciplinary research that lies at the heart of the science. Four integrative research themes were identified—thresholds/tipping points, time scales and time lags, spatial scales and boundaries, and feedback loops—serving as potential focal points around which theory can be built for human–landscape systems. Implementing the integrative themes requires that the research communities: (1) establish common metrics to describe and quantify human, biological, and geomorphological systems; (2) develop new ways to integrate diverse data and methods; and (3) focus on synthesis, generalization, and meta-analyses, as individual case studies continue to accumulate. Challenges to meeting these needs center on effective communication and collaboration across diverse disciplines spanning the natural and social scientific divide. Creating venues and mechanisms for sustained focused interdisciplinary collaborations, such as synthesis centers, becomes extraordinarily important for advancing the science.

Urban stream channel hazards

In managing urban stream channels there are pressures to use soft engineering techniques to restore channels wherever possible, to undertake management within a drainage basin context, to produce sustainable solutions and to consider community views. However, specific methods for characterizing the channel network in terms of possible management options have not been developed explicitly for urban areas. A method of characterizing the entire drainage network of urban areas, based upon segmentation of the stream channel network according to the incidence of road crossings and stormwater outfalls, is proposed together with consideration of ways in which the segments can be characterized, including stream channel hazards as a means of providing one basis for urban channel management.