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Geomorphology: The All-Important "Shape of Things"

Geomorphology may be the most diverse topics faced by the natural resource specialist. There are no clear definitions; there are unclear boundaries, and discussions blur and mix structures, processes, long term effects, and abilities to predict events or structures. Whether "a study of", a general characterization of the land, or a name for a set of forces is rarely clear.

We tentatively define geomorphology in two ways, one as a study of of the origins, development, descriptions, and classification of the landforms and landscapes of Earth. The other is more general and is a general expression of the land forms and surfaces of an area. It overlaps with "landscapes." Landforms are expressions of many conditions and processes, some of which can be labeled "geomorphic." We emphasize "the study of", standing with the long-accepted meaning of the " ... ologies." Thus, we believe we must go beyond studying to be able to use the results of such study, the available knowledge about a limited list of structures and processes to achieve developed objectives. We analyze and discuss terrain features and do not limit topics to features formed by natural processes. We are curious about what has formed the features (and attempt historical studies to gain predictive power) but we are most interested in the effects of the features on animals and plants and on the movement of materials and energy over time. We know animals and plants influence the rates of geomorphic processes but we rarely have time or resources to study and precisely describe these influences.

Landscapes are addressed primarily under the section entitled "Landscape Ecology" and usually are synonymous with "terrain." Ecosystems, preferable the resource management space, are very much enabled but also constrained by landforms. Understanding them is critical to soundly predicting effects on land and resource management and estimating natural trends in resource benefits over time. Most geomorphic processes operate on longer time scales than the life spans of the organisms within the area being analyzed. Therefore, it is the shape of the land surface, not the formative processes (typically mechanical transport of organic and inorganic matter) that we address herein.

Terrain molds and is molded by climate, vegetation, and geology. Terrain influences site-specific temperature, precipitation, solar radiation, and winds. Through these largely "climatic" effects, terrain influences the distribution of plants ... their growth and occurrence of key functions such as fruiting. Terrain also influences the distribution, movements, energetics, cover, and food habits of animal species.

Martin (1988) did an in-depth study of the literature in the physical sciences and found 120 descriptors of surface shape potentially able to describe 8 major parameters of land surface shape. The key descriptors were:

1. Area
2. Length
3. Elevation
4. Relief
5. Slope
6. Aspect
7. Roughness and texture
8. Pattern, and nominal shape.

Major problems remain for the resource manager. Given complete and "perfect" knowledge about each of the above, what might be the effects? The difference in the land if managed by someone without such knowledge? Herein we are attempting to formulate principles or highly probable simple relations that may be used in decisions (or built into decision-aiding models).

We will eventually be able to model and draw useful conclusions from:

• Map the soil-receiving areas
• Map the water receptor areas
• Map the likely amounts of both to be received
• Map the most-likely-to-burn areas (high dry areas; up-slope from people; up-slope with no stream barrier)
• Map areas sheltered or protected from winds
• Map valley bottoms that constrain wildfires
• Map ridges that constrain wildfires
• Map ridges and depression that collect snow accumulations (suppressing vegetation; increasing water flow)

Other mapping and analyses will yield relations that we now hold with high confidence, among which are:

• Elevation affects frost and growing season
• Elevation limits certain plants
• Generally plants have higher growth potentials (nutrient leaching as water) down-slope
• Higher precipitation falls at higher elevations
• Terrain produces dynamic shadows
• Terrain influences the solar radiation received; albedo affects reflectance
• Water follows gravitational gradients
• Steep concave hillslopes are the predominant sites of small, rapid landslides
• Water quality varies with the lake's position in a watershed
• Lower lakes have higher solute concentrations (Water conductance is higher in lower landscape positions)
• Seasonal variation in lake water quality is inversely related to lake drainage area
• In semiarid grasslands, cattle predominantly forage where soil moisture and nutrients are abundant (swales)
• Trout standing-stock can be better estimated from geomorphological measures than from habitat measures (Lanka et al. 1987)

We'll progressively compute some of these descriptors and automate their presence in Trevey reports. Available relief, for example, is the vertical distance from the initial upland to the level of adjacent graded valleys (Thornbury 1969:120). The concept suggests the land available for cropland and other conventional development for human use. It is in no way related to ownership, access, or barriers and is merely a geomorphic characteristic (Strahler and Strahler 1973). It can be computed in a GIS, marking the first cell, then proceeding up-elevation to mark all contiguous cells that have a slope of 7-percent or less. (Ridges or rims can defy the concept of available relief.)

Some areas may be very flat. The ratio of the minimum to the maximum elevation (e.g., min/max) in the map area is thought to have some uses. The larger the number, the more flat the area. The proportion of flat land (0-5% slopes) to the total land area may be more useful than available relief. We study and define and map flat areas and do not include them in area slope or aspect analyses.

References:

• Lanka, R.P. , W.A. Hubert, and T.A. Wesche. 1987. Relations of geomorphology to stream habitat and trout standing stock in small Rocky Mountain streams. Trans. Amer. Fisheries Soc. 116: 21-28
• Martin, S.M. 1988. Select geomorphological components of wildlife habitat in the Ridge and Valley Province of Virginia. MS Thesis, Va. Tech, Blacksburg, Va. 203pp.
• McCombs, J. W. 1998. Geographic information system topographic factor maps for wildlife management. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA. 141 pp.
• Swanson, F.J., T.K. Kratz, N. Caine, and R.g. Woodmansee. 1988. Landform effects on ecosystem patterns and processes. Bioscience 38(2): 92-98.
• Strahler, A.N. and A.H. Strahler. 1973. Environmental geoscience: interaction between natural systems and man. Hamilton Pub. Co., Santa Barbara, Calif. ix + 5llp. + appendices.
• Thornbury, W.D. 1969. Principles of geomorphology. 2nd ed. John Wiley and Sons, Inc. New York, N.Y. xi + 594pp.
• Visher, S.S. 1945. Climatic maps of geologic interest. Bull. Geol. Soc. Am. 56: 713-736.

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Last revision July 20, 2001.