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PARSONS <br />in one system may require adjustments throughout the entire hierarchy. For example, <br />assume that a long -term cooling in climate (an external force) causes accumulation of ice <br />near the poles, at the expense of water in the world ocean. The resulting decline in sea <br />level causes entrenchment to begin in the lower reaches of major rivers. The downcutting <br />may gradually propagate upstream, causing a similar erosional response in each tributary <br />basin. Slopes are re- graded to new elevations. Groundwater tables are lowered. In <br />regions underlain by limestones, a lowered water table may cause a series of solution <br />features, and eventually the surface may collapse. In short, one external change initiates a <br />chain reaction of adjustments in the interrelated subsystems. <br />Even if external factors could be held constant forever (they cannot), some alteration <br />of the system must occur with time. Because mass, in the form of sediment and dissolved <br />solids, is continually being removed from regional systems, some changes in form are <br />inevitable. However, changes in landforms (like all systems) reflect the net effects of <br />external forces on the defining variables. Each parameter within a system responds to the <br />external controls at a different rate, and in a different way. <br />Any concept proposing equilibrium inherently implies a contrasting state of <br />disequilibrium. If variations in external factors demand a response within the system, <br />there must be a period of readjustment during which process and form are out of <br />equilibrium. Landslides, subsidence, and gully erosion are examples of disequilibrium <br />generated when the variables of process and/or geology are altered so they can no longer <br />maintain a balanced relationship. They represent events that occur as the interrelated <br />systems move to re- establish a new equilibrium condition. Such events can happen <br />suddenly or can proceed toward equilibrium over a long period of time, depending on <br />how great the disequilibrium is, and how much energy is involved. The equilibrium state <br />has limits, called thresholds, at which something tangible happens to the system. <br />Threshold conditions may occur rapidly, or may develop in response to gradual, often <br />imperceptible, changes within the system. <br />The best -known thresholds in hydraulics are described by the Froude and Reynolds <br />numbers, which define the conditions at which now becomes supercritical or turbulent. If <br />a moving liquid is very close to one of the threshold conditions described by the Froude <br />or Reynolds number, only slight changes in flow velocity will cause an exceedance of the <br />threshold, which can lead immediately to spectacular changes in the characteristics of the <br />liquid system. In examples of this type, an external variable changes progressively, <br />thereby triggering abrupt changes within the affected system (Schumm, 1974). <br />Responses of a system to an external influence occur at what are referred to as extrinsic <br />thresholds — that is, the threshold exists within the system, but it will not be crossed and <br />change will not occur without the influence of an external variable. <br />Thresholds also may be exceeded when input is relatively constant — that is, the <br />external variables remain relatively constant, yet a progressive change of the system itself <br />renders it unstable, and some type of more -abrupt change occurs. In many cases of this <br />type, the threshold represents a deterioration of resistance, rather than an increase in <br />driving forces. For example, a region characterized by periodic heavy rains may have <br />stable slopes for a long time, but continuous freeze -thaw cycles or other soil - forming <br />processes may gradually reduce the cohesion of the slope material. Eventually, one <br />storm, no more severe than thousands that have preceded it, triggers slope failure. <br />-5- <br />S:\ES \WP\PRO]ECTS\3- StatesW1 Final Tech Memo.doc <br />