Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997


Osterkamp, W. R.
U. S. Geological Survey,
1675 West Anklam Road,
Tucson, AZ, 85745

Friedman, J. M.,
U. S. Geological Survey,
4512 McMurry Avenue,
Fort Collins, CO, 80525

Rarely can landforms be understood well without considering the biologic processes that contributed to shape them, and interactions among processes of fluvial-sediment transport and biota can be especially significant to bottomland features. The term biogeomorphology refers to studies uniting biota with geomorphic form and process. In recent decades, studies within the Geomorphology and Sediment Transport discipline group, National Research Program (NRP) of Water Resources Division (WRD), U. S. Geological Survey (USGS), as well as by the geomorphic community in general, largely have been restricted to physical processes. In contrast, hydrologic studies in NRP involving biology generally have been confined to activities of the Ecology discipline group, and joint investigations by geomorphologists and ecologists of NRP have been infrequent. Ecology is the study of organisms in relation to the physical and biotic environment; thus ecogeomorphology likewise could describe research combining biology and geomorphology. Ecological studies of NRP understandably have emphasized hydrology over landforms, and investigations combining geomorphic and biologic processes have been infrequent. With the National Biological Service now part of the USGS as the Biological Resources Division (BRD), the potential for biogeomorphic research is greatly enhanced. BRD brings considerable ecological expertise as well as on-going projects that explicitly link sediment transport and ecology (Friedman and others, 1996; Milhous, 1996; Madej and Ozaki, 1997). This paper reviews previous investigations of biogeomorphology, suggests topics for collaborative NRP-BRD research, and thereby advocates a strong NRP-BRD alliance to address a range of research objectives.

The effect on fluvial systems of interactions among geomorphic processes and vegetation, especially where riverine environments are modified by human activity, have been recognized for many years. Until recently, however, the complexities linking geomorphology, biota, and hydrology have inhibited many researchers from studying system interactions and have served to restrict considerations to narrowly scoped components of surficial processes. A major shift toward integrated studies occurred with the publication of USGS Professional Paper 347, “Geomorphology and forest ecology of a mountain region in the central Appalachians”, by J. T. Hack and J. C. Goodlett (1960). This unique study set a standard for research design by its parity of geomorphic and ecological considerations and interpretations, and was largely responsible for the introduction of vegetation mapping at the quadrangle scale (1:24,000) to biology.

Both before and after publication of Professional Paper 347, numerous studies combined concepts of hydrology and ecology. Examples include papers relating vegetation patterns to frequency and duration of flooding by Hall and Smith (1955), Wistendahl (1958), Sigafoos (1961), Teversham and Slaymaker (1976), and Yanosky (1982), and papers relating vegetation patterns to elevation above a stream channel by Chambliss and Nixon (1975), Nixon and others (1977), and Swanson and Lienkaemper (1982). Less commonly, studies have related vegetation to geomorphic process, in particular to vegetative response to fluvial erosion, meander migration, or deposition of sediment on bottomland surfaces. A sample of these studies is represented by papers of Hefley (1937), Everitt (1968), Johnson and others (1976), Hupp and Osterkamp (1985), and Osterkamp and Costa (1987). Among recent papers illustrative of biogeomorphic research are several concerning the effect of vegetation on bank accretion and channel narrowing (Hupp and Simon, 1991; Johnson, 1994; Friedman and others, 1996), the influence of fluvial processes on patterns of establishment of riparian-zone forests (Scott and others, 1996), erosion, sediment yield, and plant cover in a southwestern watershed (Lane and others, 1995), the consequences of vegetation change on erosion in a semiarid area (Abrahams and others, 1996), and geomorphic and vegetative change owing to disturbance in subtropical watersheds (Scatena and Lugo, 1996).

There appears to be rich potential for innovative research for variable spatial and temporal scales at the interface between geomorphology and ecology, between scientists of NRP and BRD. At a micro-scale, the influences by different plant species on soil chemistry, hence soil hydrology, pedogenesis, and erosion, are largely unknown. Root biomass is acknowledged by watershed managers as an important determinant of erosivity, but sparse subsurface-biomass data, regardless of climatic region or land use, have precluded a quantitative evaluation of erosion rates resulting from the growth and decay of root matter. The extents to which variable densities of root biomass expedite infiltration of precipitation or promote bioturbation and thus particle detachment and sediment discharge also have been inadequately explored. Another determinant of pedogenesis at a micro-scale are the types and rates of mineral residues produced by plants and incorporated into soils. Opaline and calcareous phytoliths are direct results of the assimilation of O, Si, Ca, C, and other elements and nutrients. Photosynthetic and related metabolic processes of plants concentrating SiO2.nH2O, with impurities, for example, secret the opal from leaves and stems and release it to the soil after death and decay of the plant. The size and shape of the phytoliths often are diagnostic of the plant family or genus that yielded them, and when treated as fossil remains, may be indicative of past climates. Impurities, including stable isotopes of O, H, and C, and cosmogenic radioisotopes of C, Si, Al, Be, and Cl, may serve as indicators of the ecological conditions under which the phytoliths were secreted, as well as providing a means to measure phytolith age and thus present a dating method for climate and landscape change. Because phytoliths are relatively stable under most conditions of soil chemistry for periods represented by late-Quaternary time, they may yield information on global change during that period and suggest concomitant changes in vegetation and pedogenic processes, including rates of erosion and deposition.

At a drainage-basin or watershed scale, surficial disturbances of both soil and vegetation generally increase sediment yields one to three orders of magnitude, but the amount of increase within this range is ordinarily difficult to anticipate. Erosion-prediction technology during the last four decades has provided empirical tools on which decisions regarding soil-conservation planning have been based, but only recently have there been attempts to understand the physical and biological interactions that result in the observed differences in erosion rates. Much of the technology was developed for soil loss from agricultural fields and small catchments governed by rapidly-occurring processes of rainsplash particle detachment, dispersed overland flow, and rill erosion. Hence, vegetative cover in the past often was treated as a constant and the effects of vegetation change were poorly documented. Development of process-oriented knowledge on how sediment discharge varies with seasonal change of plant cover could be of immense economic value to agriculture and would provide an improved ability to anticipate soil loss resulting from any type of soil disturbance. Included in this proposal for future research is consideration of scale differences when integrated investigations of watersheds are conducted. Specifically, vegetation changes seasonally and species composition may show significant change over a relatively small number of years, but landscape change may require decades to centuries to exhibit corresponding change for a similar spatial scale. An integrated study of geomorphic, biotic, and hydrologic processes of a drainage basin, therefore, must consider episoidic and low-frequency events, and should probably incorporate concepts such as variable response, complex response, and thresholds.

A potential for geomorphic-ecological research at the drainage-basin to regional scale is the effect of fire on landscapes. Climatic trends and nearly a century of aggressive fire suppression, with a consequent accumulation of fuel, are manifest in time series of area burned in the western United States and Canada (J. L. Betancourt, USGS, written commun., 1996). In the last 2 decades, area burned has increased in most parts of western North America although regional climates tend to behave in opposite phase from the Southwest to the Pacific Northwest. Not only has there been a large increase in annual area burned, but catastrophic crown fires have replaced low-intensity surface fires that were common in the pre-suppression period. The annual cost of suppressing wildfires is now extreme, not including losses of timber and other biologic resources, but the research community has yet to address the geomorphic, ecological, biogeochemical, and hydrologic consequences of accelerated burning by either natural or prescribed fires. Fire is a biological disturbance and a dominant mechanism by which vegetation change in the western United States occurs; it is often synchronized by drought within hydroclimatic regions. Accordingly, the occurrence of fire has profound effect on erosion, sediment movement, and numerous processes resulting from soil loss. Fire also causes rapid mineralization and mobilization of nutrients that become concentrated in runoff and its sediment load. Hydrologic and geomorphic processes that may be affected by fire involve interception, infiltration, soil-moisture storage, snow accumulation and snowmelt, overland flow, and mass movement. Downstream effects may include increased seasonal peak discharge and total flow, increased storm runoff, and increased baseflow. Increased discharges of water and sediment should accelerate rates of bottomland change and favor populations of bottomland species adapted to disturbance. In some cases increases in sediment transport following fire may be similar to sediment waves documented by BRD and WRD in California streams following clearcut logging and heavy rainfall (Madej and Ozaki, 1997).

Improved understanding of the relation between flow regime and habitat for lotic and riparian-zone organisms may require integrated studies of ecology and sediment transport. Many native fish in western rivers need a narrow range of particle sizes for spawning and specific bottomland features such as open backchannels for rearing of young fish (Milhous, 1996). Many riparian plant species reproduce only on moist, recently deposited sediment, but those same species are subject to removal by flows that mobilize underlying surfaces (Scott and others, 1996). Predicting effects of flow alteration on aquatic and riparian communities, therefore, requires integration of models of ecology and sediment transport at scales ranging from that of a gravel bar to that of an entire river. High flows, for example, are essential for maintaining bottomland features such as sand bars as well as populations of many native lotic and riparian species (Scott and others, 1996). Prescription of high flows for a managed river, however, requires an integrated understanding of the effects of varying the seasonal timing, duration, magnitude, and frequency of the flows. Similarly, understanding the biological effects of contaminated sediment introduced to rivers by mining or other activities will require detailed information on both sediment transport and the patterns of abundance and sensitivity to toxicity of the affected organisms.

Exotic shrubs, especially saltcedar (Tamarix spp.) and Russian-olive (Elaegnus angustifloia) are altering riparian habitat, sediment transport, and bottomland geometry of many streams in semiarid parts of western North America. Although abundance of these exotics varies greatly from one drainage to the next, the factors controlling susceptibility to colonization are poorly understood. Physiological studies indicate that spread of these species has been favored by factors associated with water development including increased salinity, decreased flood magnitudes, and changes in flood timing. An increased understanding of these factors will allow water managers to influence populations of exotic species by adjusting the flow regime. Developing the underlying relations will require examination of comparable hydrologic and ecological data from hundreds of sites in several states. The newly configured USGS is uniquely positioned to address this problem because it combines expertise in fluvial geomorphology and ecology with an extensive stream-gaging network.

These suggestions for interdisciplinary collaboration between personnel of NRP and BRD are merely examples that represent but a small fraction of the possibilities, and the extent to which productive research might be accomplished cannot be fathomed without scientists exploring together what those possibilities may be. It does seem clear, however, that many of the current research problems cannot be constrained by the standardly accepted boundaries of disciplines as generally defined, and that cooperative efforts are needed to contiue to yield significant results.


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