Richard A. Marston

Channel metamorphosis, floodplain disturbance, and vegetation development : Ain River, France

Abstract The purpose of this paper is to describe and explain channel metamorphosis of the Ain River in east-central France and the effects of this metamorphosis on floodplain disturbance and vegetation development. The Ain River is a 195 km long stream originating in the Jura Mountains which flows into the Rhone River between Lyon, France, and Geneva, Switzerland. The lower 40 km of the Ain River, beyond the mountain front, are situated in a valley of outwash deposits where the floodplain is 0.2 to 1.2 km wide. A complex mosaic of floodplain landscape units has developed. Maps dating back to 1766 and six sets of aerial photographs dated between 1945 and 1991 were used to document changes in channel pattern. Aerial photos and field surveys were used to compile maps of landscape units based on dominant vegetation life-forms, species, and substrate. Six maps dated between 1945 and 1991 were digitized in ARC/INFO and an overlay was generated to determine the changes in landscape units as related to channel disturbance. Change from a braided to a single-thread meandering channel probably took place in the period 1930–1950. The process of river entrenchment has occurred throughout the Holocene but has accelerated in the present century due to shortening of the river course, construction of lateral embankments, and vegetation encroachment following reservoir construction and cessation of wood-cutting and grazing. The increase in horizontal channel stability coupled with channel entrenchment have decreased floodplain disturbance and lowered the water table by approximately one meter. Pioneer and disturbance-dependent landscape units have experienced a more terrestrial-like succession to an alluvial forest. Abandoned channels have also been replaced by alluvial forests. On poorly drained soils, shrub-swamp communities of willow and hydrophytic herbaceous plants have been replaced by mixed forests of ash, alder, black poplar, and oak. On well drained alluvial soils, ash and oak dominated hardwood forests have declined in favor of mesophytic stands of black poplar. All types of vegetation, but particularly dry grasslands-shrublands, have been cleared for mines, campgrounds, agriculture, and other types of development. Using several measures, landscape diversity decreased between 1945 and 1991.

Mapping the spatial and temporal distributions of woody debris in streams of the Greater Yellowstone Ecosystem, USA

The objectives of this study were: (1) to document spatial and temporal distributions of large woody debris (LWD) at watershed scales and investigate some of the controlling processes; and (2) to judge the potential for mapping LWD accumulations with airborne multispectral imagery. Field surveys were conducted on the Snake River, Soda Butte Creek, and Cache Creek in the Greater Yellowstone Ecosystem, USA. The amount of woody debris per kilometer is highest in 2nd order streams, widely variable in 3rd and 4th order streams, and relatively low in the 6th order system. Floods led to increases in woody debris in 2nd order streams. Floods redistributed the wood in 3rd and 4th order streams, removing it from the channel and stranding it on bars, but appeared to generate little change in the total amount of wood throughout the channel system. The movement of woody debris suggests a system that is the reverse of most sediment transport systems in mountains. In 1st and 2nd order tributaries, the wood is too large to be moved and the system is transport-limited, with floods introducing new material through undercutting, but not removing wood through downstream transport. In the intermediate 3rd and 4th order channels, the system displays characteristics of dynamic equilibrium, where the channel is able remove the debris at approximately the same rate that it is introduced. The spatial distribution and quantity of wood in 3rd and 4th order reaches varies widely, however, as wood is alternatively stranded on gravel bars or moved downstream during periods of bar mobilization. In the 6th order and larger channels, the system becomes supply-limited, where almost all material in the main stream can be transported out of the central channel by normal stream flows and deposition occurs primarily on banks or in eddy pool environments. Attempts to map woody debris with 1-m resolution digital four-band imagery were generally unsuccessful, primarily because the imagery could not distinguish the narrow logs within a pixel from the surrounding sand and gravel background and due to problems in precisely coregistering imagery and field maps.

Recent advances quantifying the large wood dynamics in river basins: New methods and remaining challenges

Large wood is an important physical component of woodland rivers and significantly influences river morphology. It is also a key component of stream ecosystems. However, large wood is also a source of risk for human activities as it may damage infrastructure, block river channels, and induce flooding. Therefore, the analysis and quantification of large wood and its mobility are crucial for understanding and managing wood in rivers. As the amount of large-wood-related studies by researchers, river managers, and stakeholders increases, documentation of commonly used and newly available techniques and their effectiveness has also become increasingly relevant as well. Important data and knowledge have been obtained from the application of very different approaches and have generated a significant body of valuable information representative of different environments. This review brings a comprehensive qualitative and quantitative summary of recent advances regarding the different processes involved in large wood dynamics in fluvial systems including wood budgeting and wood mechanics. First, some key definitions and concepts are introduced. Second, advances in quantifying large wood dynamics are reviewed; in particular, how measurements and modeling can be combined to integrate our understanding of how large wood moves through and is retained within river systems. Throughout, we present a quantitative and integrated meta-analysis compiled from different studies and geographical regions. Finally, we conclude by highlighting areas of particular research importance and their likely future trajectories, and we consider a particularly underresearched area so as to stress the future challenges for large wood research.

Impacts of reforestation and gravel mining on the Malnant River, Haute-Savoie, French Alps

The Malnant River is a rapidly incising river with a French name that translates as ‘‘bad creek,’’ reflecting local opinion of the hazards from dramatic channel changes that have occurred in the last few centuries. Downcutting in the last three decades has created severe problems for farmers in this small watershed (16 km 2 ) as bridges are undermined, streamside roads are threatened, and irrigation diversion structures are rendered unusable. The purpose of our study was to determine the extent and causes of downcutting. A detailed landcover map dated 1732 revealed that forest cover had been reduced by that time to 10% of the present-day cover. The Malnant was strongly affected by floods and debris torrents during the 18th and 19th centuries that delivered massive amounts of sediment. During the 20th century, reforestation reduced the sediment delivery from hillslopes. In addition, gravel extraction in the Malnant and in the Fier River (of which the Malnant is a tributary) has lowered the base level for the river. This initiated a knickpoint that moved upstream. Weirs placed in the Malnant in 1968 were used to measure rates of bed incision in the field. With a mean width of 4.0 m and degradation up to 36 cubic meters per meter channel length, the lower 4.5 km of the Malnant has experienced a net loss of approximately 163,000 m 3 of bed material. Above the 4.5-km point on the

Land, Life, and Environmental Change in Mountains

One of the greatest challenges facing mountain scholars is to separate environmental change caused by human activities from change that would have occurred without human interference. Linking cause and effect is especially difficult in mountain regions where physical processes can operate at ferocious rates and ecosystems are sensitive to rapid degradation by climate change and resource development. In addition, highland inhabitants are more vulnerable to natural hazards and political-economic marginalization than populations elsewhere. This address focuses on the Nanga Parbat massif in the Himalaya Range of Pakistan, Garhwal Himalaya of northwest India, and Manaslu-Ganesh Himals of central Nepal. I have highlighted three special insights that geographers offer to increase understanding of human impacts on the stability of mountain landscapes. First, the mixed methods and theories we employ—quantitative and qualitative, postpositivist science and social theory, muddy-boots fieldwork linked with GIScience—together position geographers to resolve the debate over human-triggered changes in the physical landscape in mountains and explain the frequent disconnect between mountain science, policymaking, and resource management. Second, academic scholars and policymakers have come to realize that most problems require training, experience, and expertise in understanding physical and human systems. Third, modern techniques of measuring rates of geomorphic change help place the human factor in perspective and explain spatial variability of natural hazards. Forecasting environmental change remains elusive in “the perfect landscape” of mountains.

DISTRIBUTION OF LARGE WOODY DEBRIS ALONG THE OUTER BEND OF MEANDERS IN THE AIN RIVER, FRANCE

The distribution of large woody debris (LWD) was studied along the concave outer bend of three meanders in the Ain River, a 195-km-long tributary of the upper Rhone River. The Ain River is a sixth-order channel dominated by a gravel-cobble bed substrate that meanders through a floodplain covered largely by riparian forest vegetation. The mass of LWD was measured in a 15-m-wide forest band along the three meander bends, with total loads calculated to be 56.1 t ha-1 at the Mollon study site, compared with 22.9 and 21.5 t ha-1 at the study sites of Bublane and Blyes, respectively. The distribution of LWD within any one meander concavity was dependent on three main variables: (1) the position of the concavity in relation to the main flow axis, (2) the height of the bank, and (3) the presence and position of overbank flow channels in the concavity. The type of vegetation community along the channel margin is nondiscriminating, favoring the conclusion that the LWD comes mainly from upstream of the bends rather ...

Biogeography, ecoregions, and geomorphology affect fish species composition in streams of eastern Oklahoma, USA

Stream fish assemblages are structured by biogeographical, physical and biological factors acting on different spatial scales. We determined how physical factors, geomorphology and stream habitat, influenced fish species composition (presence–absence) in eastern Oklahoma, USA relative to the ecoregion and biogeographic effects previously reported. We sampled fish assemblages and surveyed geomorphology and habitat at 107 stream sites in the Boston Mountains, Ouachita Mountains, and Ozark Highlands ecoregions in eastern Oklahoma. Partial canonical correspondence analyses (pCCAs) and variance partitioning showed that patterns of endemism related to drainage basins and ecoregions explained important variation in fish species composition in all streams, but stream size and local channel morphology explained more variation overall. Stream size effects were most important in explaining variability in fish species composition in both northeastern and southeastern Oklahoma streams. Local channel morphology and substrate characteristics were secondarily important. Variables typically considered important as fish habitat (aquatic vegetation, etc.) had little effect on fish species composition.

RECENT GLACIER CHANGES IN THE WIND RIVER RANGE, WYOMING

Parallax measurements on matching aerial photograph stereopairs from 1958 and 1983 were used to calculate the ice lost from Dinwoody Glacier in the Wind River Range of Wyoming. The ice remaining in Dinwoody Glacier was measured using a portable radio echo-sounder. Isopach maps of lost ice thickness and remaining ice thickness in the glacier were constructed from these point measurements. Calculations of lost and remaining ice volumes, converted to water-equivalent values, were derived from planimetric measurements from these isopach maps. The water equivalent remaining in Dinwoody Glacier is approximately equal to that lost between 1958 and 1983. Should this rate of downwasting and retreat continue, Dinwoody Glacier will disappear in 27 years, with significant adverse impacts on late summer and early fall water supplies for downstream irrigators and instream flow needs. [Key words: glaciers, glacier runoff, radio echo-sounding, Wind River Range, Wyoming.]

Process–form linkages in meander morphodynamics Bridging theoretical modeling and real world complexity

Meandering rivers are one of the most dynamic earth-surface systems. They play an important role in terrestrial-sediment fluxes, landscape evolution, and the dynamics of riverine ecosystems. Meandering rivers have been of fundamental interest to researchers across a wide range of disciplines, from fluvial geomorphology to fluid mechanics, from river engineering to landscape ecology, owing to the intriguing complexity of meander morphodynamics. This interest also comes from the socio-economic concerns due to the river hazards caused by bank erosion, channel change, and flooding, as well as the adverse responses of meandering rivers to human- and climate-induced changes in the environmental conditions. An in-depth, process-based understanding of the dynamics of meandering river–floodplain systems is critical in order to investigate the responses of these systems to the changes in environmental conditions. Over the last few decades, there have been significant advances in river meandering research, with contributions from both theoretical modeling and experimental and field-based research. This paper presents a detailed overview of river meandering research, particularly focusing on the advances in the process-based understanding of meander morphodynamics. It also discusses the standing challenges in addressing the dynamics of real meandering rivers and their floodplain patterns and processes, and potential future directions in river meandering research. The paper advocates the crucial need for bridging theoretical modeling with field- and laboratory-based research in order to inform accurate assessments of river-hazard risks and facilitate ecologically sound river-management and restoration practices with the aim of supporting healthy ecosystems.

Geoecology and mass movement in the Manaslu-Ganesh and Langtang-Jugal Himals, Nepal

This study describes and explains the spatial distribution of mass movement in the central Nepal Himalaya. Judgments were formulated on the origin and rates of mass movement using field evidence, topographic maps, geologic maps, and SPOT imagery. Mass movement scars were mapped in the field during a 240-km traverse of the Langtang-Jugal Himal and a 300-km traverse of the Manaslu-Ganesh Himal. Chi-square analyses revealed that the frequency of slope failures varies with slope aspect, and position above/below the Main Central Thrust (MCT). Human disturbance did not account for a statistically significant increase in mass movement, except in sites occupied by mid-slope roads and where excessively steep slopes, marginal for agriculture or grazing, have been deforested.