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, , ..,~ OlC{Oq VOL. 22, NO. 3 WATER RESOURCES BULLETIN AMERICAN WATER RESOURCES ASSOCIATION JUNE 1986 ~b ~ DESIGNING FOR DYNAMIC EQUILIBRIUM IN STREAMS1 Burchard H. Heede2 ABSTRACf: Streams are dynamic systems, so steady state does not exist for any appreciable period of time. Streams in dynamic equili- brium respond quickly to change, regaining a new equilibrium. From the response system it follows that there is a causative reason why a stream meanders or degrades or aggrades its bed. These actions repre- sent adjustment processes. If humans interfere with them, other ad- justment processes will be initiated. In contrast, if humans work with the ongoing processes, success will be attainable with less efforts and at a lower cost. Local base level change represents one of the most influential channel changes, especially the lowering of this leveL Loss of base level may cause degradation tluoughout a stream net- work, because the main stem is the base level for all its tributaries. Often, degradation causes bank: instability and lowering of streamside water tables that, in turn, endanger the riparian ecosystem. Judging from check dam systems, a rise of the local base level does not raise the bed tluoughout a stream or network; instead, aggradation stops at a given distance. Preventing local base level changes of a ,stream network, therefore, is a cost-effective measure. Examples are presented of treatments causing new critical situations and measures to correct them. (KEY TERMS: dynamic ,equilibrium; adjustment processes; local base level; dams; stream response.) INTRODUCTION Streams have always played an important role in trans- portation, agriculture and, more recently, in other industrial uses. Quite early, people recognized that problems were created when streams began to change their behavior. As a pargmatic people, Americans thought solutions were available and had to be implemented. Thus, the last century saw the creation of agencies, such as the U.S. Army Corps of Engineers, responsible for the management of the Nation's rivers com- patible with land and water uses. Great successes were at- tained in river training and damming. The transformation of the Great American Desert (as the plains east of the Rocky Mountains were once called) into one of the world's most fertile agricultural areas attest to this. An expanding popula- tion has made greater demands on land and streams, and this impact of growth continues. Man interfered more and more until hardly one river existed that had not received impacts from the human hand. Since the beginning of modern human impacts on streams, more than 150 years have passed, a sufficient time span to show in many, if not most, cases results that were not always expected by the early engineers. The early engineer depended largely on personal experience and intuition in the art of hy- draulics. Scientific hydraulics, in its own right, began in the 1930's and modern geomorphology in the 1940's. Both dis- ciplines still await breakthroughs in the fields of sediment transport and stream channels in dynamic equilibrium, and landform and stream system evolution. Thus, while we still search for the knowledge to maintain channels in dynamic equilibrium, demands for interference with streams still in- crease; however, demands for protection of scenic rivers and their environmen t are also increasing. We no longer face many decisions on whether to interfere or not to interfere, because most streams of North America are no longer in their natural condition. In most of our streams, water withdrawal or temporal flow increases, chan- nelizations, dams, bank training works, gravel removal or waste deposits, or upland watershed activities have had their im- pact and forced the streams to initiate adjustment processes. Stream adjustment processes are slow and, hence, persistent. Pressures are mounting to save the land from being lost to stream erosion, or a city from drowning in flood waters. Additionally, today demands refer not only to rivers but also to the small streams, located in mountain lands, forests, and rangelands. This treatise will deal with principles that must be followed when pursuing stream channel design problems. It should be recognized that small streams adhere to the same principles and laws that govern the large ones, unless the small stream does not' carry perennial flow. This type will not be dis- cussed. This treatise will not deliver blueprints to be rigorously followed, because the basic knowledge is still missing for many situations and each situation must be appraised indi- vidually. Instead, it will emphasize compatibility between treatment measwes and present and future channel adjust- ment processes. The discussion of engineering works in this 1 Paper No. 86004 of the Water Resources Bulletin. Discussions are open until February 1,1987. 2Research Hydrologist, USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Forestry Sciences Laboratory, Arizona State University, Tempe, Arizona 85287. (Headquarters is located in Ft. Collins, Colorado, in cooperation with Colorado State University.) 351 WATER RESOURCES BULLETIN