A managed flood on the Colorado River: Background, objectives, design, andimplementation

The Colorado River ecosystem in lower Glen Canyon and throughout Marble and Grand Canyons was greatly altered following closure of Glen Canyon Dam in 1963, as flood control and daily fluctuating releases from the dam caused l arge ecological changes. Ecosystem research was conducted from 1983 through 1990, and intensively from 1990 through 1995 when dam releases were modifi ed both for scientific purposes and protection of the river ecosystem. High flows (e.g., beach/habitat building flows) were included in the Glen Canyo n Dam Environmental Impact Statement (EIS), which identified a preferred st rategy for dam operations and protection of the downstream ecosystem. Use o f high flows partially fulfills recommendations of many Fiver and riparian scientists for return of more natural flows, as part of initial efforts in river restoration. In 1996, a seven-day experimental controlled flood was c onducted at Glen Canyon Dam to closely study the effects of a high flow eve nt equivalent to those proposed for future dam management. It is an example of modification of operations of a large dam to balance economic gains wit h ecological protection. Limited to 1274 m(3)/s, the test flood was lower t han pre-dam spring floods. The experiment was conducted to (1) test the hyp othesis that controlled floods can improve sediment deposition patterns and alter important ecological attributes of the river ecosystem without negat ively affecting other canyon resources and (2) learn more about liver proce sses, both biotic and abiotic; during a flood event. Along with an explanat ion of the planning and background of this flood experiment, this paper sum marizes expected and realized changes in canyon resources studied during th e flood. Responses of specific resources to the flood are synthesized in th e following compendium papers.