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'. . TREATHENTS CAUSING NEW CRITICAL LOCATIONS: EXAMPLES The thought that control measures may create new critical locations or situations should be familiar to us at the waning of this century. The consequences of impairments of the en- vironment have proven to be serious and costly. Chemical insect control, coyote decimation, or river training, while well-meant in many instances, have backfired on us, producing unexpected serious consequences. It i" imperative, therefore, that we consider in treatment plans the possible formation of new critical locations. Examples from large streams will be quoted mainly, be- cause we know more about their history than that of small streams. The latter received attention only during the last few decades. As stated in an earlier section, we should keep in mind that process response is the same in large and small streams, as long as we deal with perennial waterflows. It is obvious that potential and available energies are much larger in large than small streams and with this also the magnitude of response. Let us consider some important stream control measures and the possibility that they may cause new critical loca- tions. The building of river dams for flood control and/or additional water supplies provides a classic example. Dams hold back not only water but also sediment. Generally, all of the bedload and most of the suspended load will be stored in the reservoir. Because of flow velocity decrease, the large sediment particles are deposited at the delta formed by the reservoir, and the fines, at the dam. Sluicing this sediment through gateways in the dam, where conditions would allow this approach, proved to be unsuccessful in the past (Ferrell and Barr, 1963). With time, sediment deposits at the reservoir delta will cause channel changes, because the deposits raise the stream's local base level, represented by the location of the junction between stream and reservoir. The raise will cause aggrada- tion for some distance upstream that may lead to occasional floodings in the aggradated stream reach. Both processes may be enhanced by vegetation occupying the very favorable en- vironment for growth provided by well-watered sediments (Maddock, 1966). The lltrellm relll'h-A6wnslleitlll from the dam receivp~ evrn !!trongpr impads ~ because of sediment withdrawal. Unless channel bed and banks are protected by bedrock or rock of sufficient hardness and size, ~radation and the subseQuent j.owering of local base levels for trihntaries located dOVin- stream from the dam may commenr.e and r.ontinlle for a loug time. Degradation is strongest near the dam and diminishes -downstream (Begin, et aL, 1981; Williams and Wolman, 1984). In large rivers, several hundred kilometers may be affected (Hammad, 1972). Investigations have shown that degradation proceeds at relatively hi~ rates during the first 10 to '-Q 'years after dam closure and recedes thereafter (Williams and Wolman, 1984). ~imlltell on the Color"l)o ~ll('r indi- cate time spans of ] 80 tn 200 Y8arE may 0(' u{'el)prl hef~ Lnew equilibrium is established.' Meanwhile, the so-called Heede beaches of the river continue being diminished by erosion. The beaches, which essentially are bars, consist of easily reo movable material and therefore represent an easy source for increasing the sediment load of the "hungry" Colorado River. The impact by visitors may be insignificant relative to the modifications by adjustment processes (Dolan, et aL, 1977). If time periods for adjustment require several hundred years, the possibility increases that drastic external geomor- phologic changes will occur before the new equilibrium is attained. For instance, in tectonically unstable terrain, uplift or earthquakes could create a new set of conditions that pre. vent the continuation of the present-day adjustment processes. Schumm (1971 b) discussed historical cases of river changes caused by upwarping and tilting of the earth's crust. Estimates on stream developments, covering long time spans, must there. fore be tested with time and should not be taken at face value, especially since, in the first place, they were given only as orders of magnitude. Degradation not only causes local base level changes within a stream but also in the tributaries. Once the bed of the master stream has been lowered, tributaries form waterfalls at the location of junction. These waterfalls are headcuts on the bed and as such advance upstream. By this mechanism tributaries adjust their bed to that of the main stream. The process of adjustment proceeds throughout the stream system, until the smallest arteries are adjusted also. Long time periods may evolve, however, before the total system finds its new equilibrium. I have inspected stream systems that in the past apparently were upset by changes in watershed condition from land use; for example, Cottonwood Wash, Sitgreaves National Forest, and BIue River, Apache National Forest, both located in the Arizona White Mountains. In both systems, the master streams had acquired a new dynamic equilibrium after ex- periencing a period of degradation in the headwaters and aggradation in the downstream reaches. In the Cottonwood Wash system, adjustment had been attained, excepting the small upland arteries. There, numerous headcuts existed on the bed and headward extension of the small channels was in progress. At Blue River, however, all major tributaries were still actively adjusting, as indicated by their waterfalls at the junction with the master stream. The waterfalls were located in alluvium and hence not bedrock-controlled. Another example, but of much larger magnitude than the two Arizona stream systems mentioned, is the Missouri River. The upper Missouri has long reaches with headcuts on the bed of several meters depth, demonstrating that degrada- tion is still in progress. Degradation has reached the fertile agricultural uplands where irrigation ditches degrade and gullies develop. More than one-half of a century evolved before the main river degradation affected the uplands. As discussed earlier, stream energy reduction can be more easily attained by processes other than longitudinal prof1le changes, such as meander formations that lengthen the stream course and hence decrease gradient and energies. Due to relatively soft materials in farmlands bordering streams, the 354 WATER RESOURCES BULLETIN