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I NTRODUCT ION The demand for electrical power, irrigation and flood control in the Pacific Northwest has resulted in the construction of dams on most major river systems. These developments provide the necessary water storage and produce major changes in the river downstream of the project. This report covers changes which have occurred in the aquatic environment, periphyton, benthos, and fish populations of the Kootenai River following impoundment by Libby Dam. The Kootenai River (spelled Kootenay in Canada) is t;le second largest tributary of the Columbia River. Its drainage basin has an area of about 50,000 square kilometers (19,300 square miles) and includes parts of south- eastern Br it ish Col umbia, northern Ida ho, and northwestern Montana. The river originates in Kootenay National Park, British Columbia, flows south into Montana, then northwest through Montana and Idaho and into Kootenay Lake in Canada; it then flows southwest from Kootenay Lake and joins the Columbia River at Castlegar, British Columbia (Figure A). The Kootenai River is approximately 780km (485 miles) in length, of which 266 km (165 miles) is in the states of Montana and Idaho. The basin ranges in elevation from about 418 m (1,370 feet) above mean sea level at Castlegar, British Columbia, to the 3,618 m (11,870 feet) peak of Mt. Assinibone on the Continental Divide in the northeastern part of the basin. The section of the river in the United States ranqes from an elevation of about 704 m (2,310 feet) to 533 m (1,750 feet) above mean sea level. The gradient of the river in this section is 0.6 m/km (3.4 ft/mile). Construction was begun on Libby Dam in 1966. In March, 1972, the river was impounded and its reservoir, Lake Koocanusa, was formed. Approxi- mately 80 kilometers (48 miles) of the reservoir is in Montana with the remaining 70 km (42 miles) in British Columbia. Regulation has altered the flow regime, temperature patterns, sediment loads and water quality of the Kootenai River. These environmental changes have had profound effect on the biological communities living in the Kootenai River downstream from Libby Dam. Libby Dam reversed the natural flow regime. Historically, the highest flows occurred from April through July, with the median peak flows being about 60,000 cfs during May and June, with low discharges of about 2,000 cfs occurring during the winter and early spring. The average annual discharge at Libby, Montana is 12,000 cfs. Since impoundment, low flows normally occur from April through July. During the remainder of the year, flows generally range from an operational minimum of 4,000 cfs to a maximum of 23,000 cfs. Maximum discharge prior to impoundment was 121,000 cfs as compared to 40,000 cfs following impoundment. An International Joint Commission order for Kootenai Lake requires that Lake Koocanusa be drawn down to elevation 2,412 feet msl to accommodate 2,000,000 acre-feet of storage by January 1 each year, which results in maximum power production in October through December each year. iv