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I ?F7 National Water Summary 1987-Water Supply and Use: INSTREAM WATER USE 109 U INSTREAM WATER USE IN THE UNITED STATES-WATER LAWS AND METHODS FOR DETERMINING FLOW REQUIREMENTS By Berton L. Lamb' and Harvey R. Doerksen2 INTRODUCTION Water use generally is divided into two primary classes-offstream use and instream use. In offstream use, sometimes called out-of-stream or diversionary use, water is withdrawn (diverted) from a stream or aquifer and transported to the place of use. Examples are irrigated agriculture, municipal water supply, and industrial use. Each of these offstream uses, which decreases the volume of water available downstream from the point of diversion, is discussed in previous articles in this volume. Instrea n use, which generally does not diminish the flow downstream from its point of use, and its importance are described in this article. One of the earliest instream uses of water in the United States was to turn the water wheels that powered much of the Nation's industry in the 18th and 19th centuries. Although a small volume of water might have been diverted to a mill near streamside, that water usually was returned to the stream near the point of diversion and, thus, the flow was not diminished downstream from the mill. Over time, the generation of hydroelectric power replaced mill wheels as a means of converting water flow into energy. Since the 1920's, the generation of hydroelectric power increasingly has become a major instream use of water. By 1985, more than 3 billion acre-feet of water (3,050,000 million gallons per day) was used annually for hydropower generation (Solley and others, 1988, p. 45)-enough water to cover the State of Colorado to a depth of 51 feet. Navigation is another instream use with a long history. The Lewis and Clark expedition journals and many of Mark Twain's novels illustrate the extent to which the Nation originally depended on adequate streamflows for basic transportation. Navigation in the 1980's is still considered to be an instream use; however, it often is based upon a stream system that has been modified greatly through channelization, diking, and construction of dams and locks. The present (1987) inland water navigation system in the conterminous United States consists of about 12,000 miles of maintained waterways, over which about 500 million tons of cargo is carried each year (U.S. Army Corps of Engineers, 1988, p. 16). Although not so widely practiced in recent years, streams have been used to dispose of raw waste products from homes, communities, and factories. This use has been discouraged by law and public policy because of public health concerns and the damage it causes to the environment. Beginning in the mid-1960's, other instream uses gained new prominence in the water-resources arena-the assertion of a legal right to a free-flowing stream for biological, recreational, and esthetic purposes. These uses themselves, however, are not new. Riverine habitat always has produced fish, and the beauty of flowing water always has evoked a strong sense of esthetic appreciation. What is new is the emerging legitimacy and awareness of these non- economic uses under State and Federal laws and regulations. In the past, environmental uses of flow- ing water were ignored, for the most part, under a long-standing legal tradition that favored offstream uses and certain instream uses that had a strong economic basis. The history of the instream-flow policy debate really concerns these recently recognized types of instream uses. Although the more traditional water uses have been protected by law, the recognition of other instream uses has resulted in substantial changes in State water laws. Although methods for determin- ing the volume of water needed for most traditional water uses are relatively straight-forward and well- established, methods for determining water require- ments for the instream uses have been developed only recently and are continuing to evolve. Water laws that have favored the more traditional water uses, the inherent nature of conflict between instream and offstream water uses, and the special kinds of technological and philosophical problems posed by the "newer" types of instream uses are described below. Water laws that have been passed to accommodate the more recently recognized instream uses are summarized. WATER-LAW CHANGES-THE WEST Water is a finite but renewable resource. In times and places of plentiful supply or small demand, major conflict over the available supply is not common. In the Western States, however, because of the chronic scarcity of water, it is not surprising that "water wars" are common. It also is not surprising that the water laws in this and region evolved to pro- tect those who "got there first." In the and West, two early and major water uses were hydraulic mining and irrigated agriculture. These uses, which often required water to be transported for long distances from the stream to the point of use, also often consumed a large part of the diverted water (Gould, 1977, p. 4-5). To recognize this offstream nature of water use and to protect the earliest users, a body of water law, known as the appropriation doctrine, evolved. This law has two primary principles-first in time is first in right, and benefi- cial use of water is the basis of the right. First in time means that the earliest water-right holder has a right to all the water needed to fulfill the right, and then the second, the third, and so forth, can claim their rights. Each water right depends upon supplies avail- able after all prior rights have been satisfied (Gould, 1977, p. 5). Thus, as the supply decreases, lower priority water right holders must stop using water until the more senior users can be satisfied. RECEIVED AU G 2 01990 "I 'U.S. Fish and Wildlife Service. 2U.S. Department of the Interior, Office of Program Analysis.