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August 1994 FLOW REGIME AND RIPARIAN VEGET A nON 300 ~ fication in the Santee and Cooper river system in South Carolina. Species-specific establishment or mainte- nance criteria have also been used to explain patterns of riparian vegetation and to assess impacts of hydro- logic alterations. For example, Rood and Mahoney (1990) explained the decline of western plains cotton- wood forests downstream of dams by showing that the altered flows less frequently meet establishment re- quirements of cottonwood. Franz and Bazzaz (1977) related the occurrence of riparian trees to inundation duration and then pre- dicted how backwater from a proposed downstream reservoir would alter tree distribution. Harris et al. (1985) used hydraulic simulation models of the In- stream Flow Incremental Methodology (Bovee 1982) to describe plant species distributions along belt tran- sects orthogonal to the stream channel. We extend the approach of Franz and Bazzaz (1977), using hydraulic models similar to those of Harris et al. (1985), to pre- dict the response of riparian vegetation to changes in discharge. Our method uses a direct gradient analysis to describe the present distribution of vegetation rel- ative to inundation duration, as determined from a flow-duration curve for an appropriate period of rec- ord. A proposed dam or diversion has a predictable effect on the flow duration curve. We use the new flow duration curve in combination with our gradient anal- ysis to predict the change in riparian vegetation for a reach of the Gunnison River in south-central Colorado. STUDY AREA Black Canyon of the Gunnison National Monument is located along the Gunnison River on the western slope of the Rocky Mountains in Montrose County, Colorado, at longitude 107045' west and latitude 38034'30" north. The Monument was established in 1933 in recognition of the scenic quality of the steep- walled canyon cut into pre-Cambrian gneiss. In 1934 there was little vegetation in the canyon bottom (War- ner and Walker 1972), probably because ofthe com- bined effects of alternating scouring floods and very low flows (Zimmerman 1969). Between 1936 and 1976, four dams were constructed on the Gunnison and Tay- lor rivers upstream of the Monument for power gen- eration and irrigation (United States Department of the Interior 1990). Operation of the dams has reduced peak flows and raised low flows, allowing development of vegetation on the canyon bottom. This vegetation could be affected by future changes in water manage- ment. The National Park Service chose a 450-m reach for intensive study. The width of the canyon bottom with- in this reach varies from 40 to 90 m, the gradient is 0.0128 m/m, and the elevation is ::::: 1707 m. The wa- tershed area is 10000 km2 (Hansen 1987), and average annual precipitation is 370 mm (Colorado Climate Center 1984). Because of the steep canyon walls, the study reach is inaccessible to livestock and has prob- 545 ~ 200 '" .S- Q) OJ CB ~ u C/) 100 (5 1975 1980 1990 1985 Time FIG. 1. Weekly average discharge of Gunnison River be- low Gunnison Tunnel. ably never been grazed. Discharge is measured at East Portal, ::::: 14.5 river kilometres (distance along the riv- er's path) upstream ofthe study reach at United States Geological Survey Gage 09128000 (Ugland et al. 1989). There are no dams, diversions, or important tributaries between the gage and the study reach. Because the closest dam upstream of the Monument is a reregu- lation dam, the Black Canyon is not subject to the large daily fluctuations in discharge associated with power- peaking operation of some hydroelectric dams. Peak discharge results from snowmelt and generally occurs in June (Fig. 1). The physicochemistry of this portion of the river is described by Stanford and Ward (1985). Ours is the first detailed description of the riparian vegetation of the Monument. METHODS Definitions Five hydrologic terms are used frequently in this paper. Discharge is the flow of a stream in cubic metres per second. We used discharge data reported as daily averages. The stage-discharge relation is a curve re- lating discharge to the elevation of the water surface at a cross-section or point. The flow regime is the pat- tern of variation in discharge over time. Flow duration is the fraction of time a given discharge is equalled or exceeded. Inundation duration is the fraction of time a point on the bottomland is inundated. Flow duration and inundation duration are unitless quantities cal- culated using the hydrologic record for an appropriate period. The inundation duration of a point inundated once a year for a day is the same as that of a point inundated once every 7 yr for a week (0.00274). Model overview Our model is based on direct gradient analysis (Whit- taker 1956, 1967, Bedinger 1979, Jongman et al. 1987), which describes the position of vegetation along en- vironmental gradients. We related vegetation to the