Review of Riparian Vegetation and Fluvial Geomorphology, edited by Sean J. Bennett and Andrew Simon: Water Science and Application 8, American Geophysical Union, Washington, D.C., ISBN 0-87590-357-6, 282 pages, $70.

Until relatively recently, many engineers turned a blind eye to the effects of riparian vegetation when analyzing natural channel roughness and bank stability, and took steps to exclude it from channels engineered for flood defense, stabilization, or land drainage purposes. Concurrently, fluvial geomorphologists recognized the omnipresence of vegetation in the natural channels they studied and described, but viewed it as “nature’s design flourish”—a late addition to the sedimentary structure and form of rivers that provided habitats and improved aesthetics, but played no more than a cameo role in affecting channel process–form relationships or influencing long-term channel evolution. If anyone still doubts that vegetation occupies a central and often pivotal place in the study of river mechanics and fluvial geomorphology then reading this book should convince them otherwise.

The volume compiled and edited by Sean Bennett and Andrew Simon assembles chapters contributed by leading international academics that provide a useful overview of recent research concerning riparian vegetation and hydromorphological processes in rivers and tidal estuaries. The book’s 19 chapters are organized into six parts. After a short preface by the editors, part 1 introduces the book and contains just a single chapter explaining and justifying the content of the book. Part 2 combines experimental and theoretical studies and includes five chapters. Part 3 is devoted to two chapters on large woody debris. Part 4 contains five chapters on bank erosion and channel stability. Part 5 covers floodplains and watershed processes with three chapters. The book closes with part 6, on numerical methods, which contains three papers. There is no epilogue or concluding chapter and the volume has no subject index. Consequently, the book ends rather abruptly and I was going to bleat about the lack of a concluding, overview chapter, until I remembered that Andrew Simon asked me to write one 18 months ago and I turned him down due to pressure of work at that time on a United Kingdom government research project (www.foresight.gov.uk). My apologies to Andrew and the readership of this volume—a concluding chapter would have provided a neater closure.

The preface reveals that the content of the book stems mainly from a special session held at the fall meeting of the American Geophysical Union in December 2001. This at once explains both the immediacy of the science presented in the chapters and the somewhat eclectic selection of issues addressed in the volume. While the editors sought contributions from additional authors (as well as those that presented at the special session) to bring “perspective and balance” to the volume, topic coverage was basically defined by the scope of papers presented at the fall meeting, over which the editors would have had little control. Viewed in this light, chapter 1 does a nice job of teasing out problems and issues in the analysis and treatment of riparian vegetation in fluvial geomorphology, concerning: flow resistance, flow velocity, and turbulence; bank erosion processes; large woody debris and river restoration; riparian vegetation, sediment yield, and channel evolution; numerical modeling; and research paradigms. Having successfully identified a logical framework for discussion of riparian vegetation and fluvial geomorphology, it is odd that the editors abandoned the themes so neatly identified in Chapter 1 when grouping the contributed chapters in the remainder of the book; especially as the themes identified in Chapter 1 would have provided a more coherent and research-oriented framework than that actually used to divide the volume into its six parts.

Part 2 is a case in point. It mixes three studies centered on flume experiments with a conceptually based investigation and a historical reconstruction. While each chapter is valuable in its own right, it is difficult to see a common thread running through the research. Chapters 2 (Garcia et al.) and 3 (Bennett) report investigations of turbulence and spatially averaged flows performed in fixed-boundary flumes that hardly consider the implications for the fluvial geomorphology of real alluvial streams with mobile boundaries. This definitely cannot be said of Chapter 4 (Tal et al.), which reports an expansive study of impacts of vegetation on mobile-bed, braided channels conducted through coordinated flume, field, and computer model investigations. Chapter 5 (Alonso) presents a conceptual analysis of the transport of “stream-borne logs,” which are large woody debris by another name. This begs the question of why this paper is not included in Part 3, which has only two chapters in it, making it thin in comparison to the other parts. Part 2 concludes with a warning from Jim Dungan Smith that intensive land use along the floodplains of many streams in the semiarid, western U.S. leaves them deficient in shrubby vegetation and vulnerable to “unravelling” during large, overbank flows. While Smith’s point is well made, strictly speaking the topic has more to do with floodplain vegetation than riparian vegetation per se (“riparian” is a feature occurring in the vicinity of a river bank). I would have thought that the place for this chapter is in Part 5, “Floodplain and Watershed Processes,” rather than Part 2.

As noted above, Part 3 contains just two chapters. In Chapter 7, Daniels and Rhoads pick up the theme of vegetation–flow interaction, using field observations at the microscale to study the effects of a LWD jam on the spatial distribution of turbulence kinetic energy (TKE) at a bend. In contrast, a macroscale approach is adopted in the companion chapter by Bunn and Montgomery, which compares the effects on debris slides and sediment dynamics of hillslope conditions and in-channel LWD in old-growth and industrial forests on the Olympic Peninsula in Washington state. Again, the findings here are interesting and significant, but the chapter has rather more to say about watershed processes (Part 5) than it does about LWD.

The chapters in Part 4 provide stimulating contrasts in approach and style. Gray and Barker present a somewhat pedestrian review of root reinforcement that is founded firmly in the 1980s and 1990s, making it appear dated in comparison to Pollen and Simon’s treatment of current theories, observations, and models that deal with the combined effects of riparian vegetation on both mechanical stability and bank hydrology. Rutherfurd and Grove also innovate through introduction of a new term, “root plate abutment,” to describe a root wad and soil pedestal beneath a tree that protrudes into the channel from an eroding bankline, acting as a hard point. This term is, in fact, so new that it has apparently not been recognized by other contributors to the book! I say this because, to my eye, the obstruction studied by Daniels and Rhoads could well be described as a root plate abutment (compare Fig. 2 on p. 89 with Fig. 2 on p. 142, Fig. 3 on p. 143, and Fig. 5 on p. 145) and it seems to act exactly in the manner outlined by Rutherfurd and Grove. Yet there is no cross referencing between these closely related and indeed complementary studies.

I have always benefited from reading anything written by Stan Trimble and his contribution to this book is no exception. His iconoclastic chapter is cheerfully out of step with the message conveyed by the book as a whole, that riparian vegetation acts to reduce the effectiveness of fluvial processes and that the constraining influence of vegetation on channel morphology therefore increases with the weight and density of the vegetation on the banks. Trimble argues to the contrary, concluding that channels with grassy banks have smaller bankfull cross-sectional areas, narrower base-flow widths, and lower width-to-depth ratios than do equivalent tree-lined channels. Trimble’s chapter is particularly valuable in that it questions what is becoming a dangerous orthodoxy in river restoration and rehabilitation throughout the USA—that planting trees along rivers is always appropriate and beneficial in terms of channel stability. As Trimble shows, in the words of the song, it ain’t necessarily so. Ikeda et al. contribute the final chapter in part 4, examining nutrients, leaf litter, and sediment fluxes in a mangrove forest on Ryukyu Island, Japan. Ikeda’s renown as a modeler lends credibility to the finding that it is possible to construct reach-scale sediment budgets for tidally influenced river–floodplains systems, but this outcome seems more germane to Part 5 (“Floodplains and Watershed Processes”) than “Bank Erosion and Channel Stability.”

The coastal theme introduced by Ikeda et al. continues in the first chapter of Part 5, by Ross et al. Monitoring results for selected tributaries to Chesapeake Bay illustrate the profound impacts that vegetation communities, land management, and channelization can have on floodplain sedimentation and ecosystem dynamics. Gurnell et al. practice the geographers’ art by working across scales of time and space to elucidate how vegetation propagules and seeds travel through the fluvial system and how the vegetation stands they spawn interact with fluvial processes to influence channel forms and features. For me, this chapter really addresses the core topic of this volume and it eloquently makes the case for considering vegetation and vegetative life cycles in all studies of engineering geomorphology that seek to describe and understand channel process–response and to support improved river engineering, management, and conservation. Griffin and Smith contribute the final chapter on “Floodplain and Watershed Processes”, presenting a companion treatise to that by Smith in Part 2 of the book. However, their chapter uses a contemporary rather than historical example of the floodplain unravelling phenomenon triggered when deep, fast flowing overbank flow encounters floodplain surfaces that lack sufficient cover of woody or shrubby vegetation.

The three chapters in Part 6 demonstrate both the strengths and limitations of numerical methods in accounting for vegetation effects on open channel flow. Smith and Wu and Wang develop rigid-boundary models and choose to test them using data from flumes with fixed beds and no movable sediment. These decisions are understandable, given the complexity of flow-vegetation interactions, but they must limit the applicability of their findings in the domain of fluvial geomorphology which, by definition, deals with deformable channels formed in mobile bed and bank materials. Van De Weil and Darby are more ambitious, modeling a selected reach of Goodwin Creek, Mississippi, using a numerical model, mRIPA, that accounts for sediment transport, bed scour, bar deposition, and bank erosion. The results usefully demonstrate that relatively subtle changes in riparian vegetation characteristics (density, root structure, rooting depth) can produce complex morphological responses, primarily by affecting bank stability, bank roughness, and sediment fluxes. Clearly, it would not be possible to mimic the true complexity of vegetation effects in a model that had neither erodible banks or a sediment flux and to that extent extrapolation of results from rigid boundary, clear water experiments to field situations or predictions is difficult or perhaps impossible.

Van De Weil and Darby’s findings provide a suitable closing justification for this book and they should prompt further research efforts. In the interim, the volume provides information and insights of interest and relevance to river engineers attempting to account for vegetation in their computational methods, river managers making decisions on vegetation control along managed watercourses, and river restorers designing schemes that involve the introduction (or reintroduction) of riparian species.