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J .'. treatise does not signify that engineering measures take pre. cedence over vegetative approaches. Indeed, generally; the establishment of sound vegetation lining channels and banks is the ultimate goal of control. Situations exist, however, where engineering may be imperative for the reestablishment of vegetation. The writer recognizes that effective designs depend not only on comprehensive knowledge of the physical processes but also on intelligent administrative support. For a physical scientist, this latter aspect is outside of the research area and will, therefore, not be discussed here. I hope that this treatise will be helpful in avoiding miscon- ceptions and mistakes in future projects. DYNAMIC EQUILIBRIUM First, let us consider one of the more important laws that govern a dynamic physical system' such as a stream. ~ conditions of dynamic equilibrium, a stream's energy is at a l~vel that allows sediment loads entering a stream reach to equal those leaving it. The available ene!1D' is not only det~ mined b~ the Eischarge and the _morphology of the ch~ controlling the hydraulic variables, but also by the sediment' J2.!d. Indeed, dischare:e and sediment ioaif are the most im- _portant variables. The sediment load requires energyexp;mrr- tures by the flow; hence, a stream determines its own ultimate load. Loads larger than the ultimate cannot be transported and will be deposited. On the other hand, if more free ener- gies are available than expended by the flow, the principle of. continuity requires changes in some or all hydraulic vari- ables, such as width and depth of flow, or morphologic changes leading to additions of sediment load. Whatever process occurs, it is directed toward attainment of a new equilibrium between available and expended energy. If a stream is in dynamic equilibrium, adjustments re- quired by changes occur relatively fast. Drastic cha!![es may throw the .st~ equilibrium, however, andjQng time periods mlly-p-vnlttL before a new equilibrium will be attaine~ Such a situation also will affect the riparian system: because its health depends on a healthy (equilibrium) stream. As discussed elsewhere (Heede, 1984), strong interdependency exists between different systems. Since landform development is the ultimate global process, i.e., erosion and deposition, the concept of dynamic equili- brium has no place on a geologic time scale. This concept can be applied to relatively short time intervals only. THE lllERARCHY OF ADJUSTMENT PROCESSES Prevalence of process type appears to be dependent on the ease with which equilibrium can be regained. Major adjust- ment processes in progressive order toward increased energy requirements for adjustment are the following: bed form changes, bed armor formations, width, pattern, and longitu- dinal profile changes (Heede, 1980). Bedforms can be changed by establishment of ripples, dunes, orantidunes in sand Heede streams, or bars in gravel streams; bed armor, by sorting of the bed material, resulting in predominantly large particles on the bed surface. Width Is Influenced by flows concen- trating in relatively small arms (braids) or occupying the full channel and floodplain. Pattern adjusts by lateral channel migration (meanders), and longitudinal profile changes through aggradation or degradation. Often conditions do not allow the process with least energy expenditure to prevail. For instance, rock outcrop limits opportunity for bar formations, or hard granite sidewalls of narrow valleys force the stream to retain its present alignment. Although longitudinal proftle adjustments require the most time and energy expenditure - degradation more than aggradation - the stream may be forced to adjust by degradation. This is the case where, rela- tive to the available sediment load, oversteepened channel gradients exist and the stream's environment does not allow other adjustments. Situations may exist that do not allow ready interpreta- tion of dominant stream processes, because of simultaneous internal adjustments of hydraulic and morphologic variables. This may lead to the question of which variable change is predominant. For example, did a base level change induce the formation of a new channel proftle, or did the change of a hydraulic component represent the main cause, with the profile change only a by-product? An excellent discussion of this problem by Haible (1980) demonstrates the complexity of stream response. Schumm (1971a) presented generalized equations that re- late parameters in channel morphology to discharge (Q) and bed-material load (QJ. Plus or minus exponents were used to indicate increase or decrease, respectively, if discharge or load changes. Q+ ~b +, d+, A+, S- Q+ -b+ d- '\+ S+ p- s - , , /\, , Where b is channel width, d equals channel depth, A is wave. length, S represents gradient and p sinuosity. Increasing dis- charge causes increases in channel width, depth and wave- length, but a decrease in gradient. If discharge decreases, the opposite reactions occur on the right side of the function. Increase of bed-material load causes increases in channel width, wavelength and gradient, but decreases in channel depth and sinuosity. Opposite responses take place with decreasing bed-material load. An example of this response model is bed-material load increase due to intensified land use, leading to channel aggrada- tion. In turn, the gradient will increase. Increased energy (gradient) is required for the transport of the increased load. Otherwise, channel choking would occur. On the other hand, if land use includes conservation mea- sures, causing bed-material load reduction, kinetic energy of the flow is freed. As a result, the stream picks up material from the channel. If bed material is excavated, degradation 352 WATER RESOURCES BULLETIN