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PARSONS <br />Through many years of debate regarding conditions on the Platte, no comprehensive <br />geomorphic assessment of the short- or long -term stability of the braided form has been <br />completed, nor has the possibility been examined that geomorphic thresholds are being <br />induced or naturally crossed for segments of the river. The current investigation task, <br />Task Al — Qualitative Assessment of Natural Geomorphic Thresholds of the Platte River, <br />consisted of an evaluation of past and current morphologic conditions of the Platte River, <br />and identification of possible threshold conditions affecting the river, together with an <br />assessment of whether a braided river is ever anything but a transitional form. <br />GEOMORPHIC CONCEPTS, GEOMORPHIC PROCESSES, AND TIME <br />Landforms represent the results of some interaction between driving forces and <br />resisting forces (this discussion is condensed from Ritter, 1978). Driving forces in <br />geomorphology include climate, gravity, and other forces generated inside the earth. <br />Resistance to driving forces is provided by the geologic framework. Driving forces and <br />resisting forces interact via process mechanisms, which are the methods by which one <br />thing is produced from something else, or are the vehicles by which a quantity of one <br />system is transferred into, and participates in, the mechanics of another system (e.g., <br />erosion and movement of sediment in a fluvial system). <br />All natural fluvial systems exist in a state of dynamic equilibrium — that is, all <br />landforms within a fluvial system (such as a drainage basin) are mutually adjusted to <br />reflect an equilibrium condition between the framework (geology, soils) and the <br />prevailing processes. The equilibrium landforms will last as long as the controlling <br />factors are not changed, because all elements of the surface will downwaste at the same <br />rate. Thus, in the ideal case, landforms become independent of time. Changes do occur, <br />but only in response to altered process or geology. Because a new equilibrium form will <br />be established rapidly (in the sense of geologic time) whenever changes occur, most <br />topography should be adjusted to present conditions. However, geomorphic responses to <br />altered conditions do not always proceed at the same rate. <br />Landforms may be considered as part of an open system, in which energy and mass are <br />constantly supplied and removed. Losses and gains of energy or mass are kept in a steady <br />state by continuous adjustment of forms within the system. Landforms serve as <br />regulatory agents to balance gains and losses. For example, a drainage basin is a system <br />composed of many parts (slopes, valleys, floodplains, soils, rivers, etc.), each of which <br />can logically be considered as a separate subsystem. The subsystems may contain even <br />smaller parts (soil profiles, stream channel cross - sections), which themselves function as <br />identifiable systems. The Earth's surface thus consists of a hierarchy of systems, each in <br />instantaneous equilibrium. <br />Each system or subsystem can be defined by measurable variables or parameters <br />(velocity, slope angle, grain -size distribution) which, taken together, indicate the <br />character of the system at the time of measurement. Under equilibrium conditions, these <br />variables are totally adjusted to each other and to the external forces that provide or <br />remove energy and mass. Realistically, exact equilibrium may never be attained in the <br />steady state because each system responds to continuously changing external variables <br />(variables outside the system boundaries), and most systems are interdependent. That is, <br />changes in external variables cause reactions within systems, and a change of parameters <br />-4- <br />SAES \WP\PR0JECTS\3- States\A1 Final Tech Memo.doc <br />