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identify the physical and biol-ogical characteristics most sensitive-to changes in streamflow regime. Geomorphic, hydrologic, and biologic research has provided new information that can be incorporated in resource-management decisions necessitated by changes in consumptive and instream water-use demands, available streamflow, recreational needs, and climate variability. The purpose of this paper is to illustrate how various streamflow characteristics affect fluvial and riparian conditions, visitor use, and resource-management activities in the Gunnison Gorge. EFFECTS OF STREAMFLOW REGULATION SINCE 1966 The fluvial and riparian environments of the Gunnison Gorge are heavily influenced by two major processes: tributary sediment supply and mainstem sediment transport. Most of the transportable sediment in the Gunnison Gorge is supplied to the river from steep ephemeral tributaries during flash floods and debris flows that follow intense convective summer storms near the Gunnison Gorge. Once in the river's mainstem, larger gravels, cobbles, and boulders are transported only by high discharges resulting from spring snowmelt in the mountains. These high flows created and maintained the channel characteristics that were typical of the Gunnison River in the study area before 1966. Discharge data for the study area are available from 1911 to the present from the USGS gaging station 09128000 Gunnison River below Gunnison Tunnel (Figure 1). Upstream reservoirs have altered the timing and magnitude of flood peaks since 1966 (Figure 2). Monthly mean discharge during the snowmelt season (April-July) has decreased 63% during the three-decade period since reservoir regulation, whereas monthly mean discharge for the remainder of the water year (August-March) has increased 170% (Elliott and Parker, 1992). The magnitude of specific flood frequencies also has decreased substantially. The 10-year flood decreased from about 420 to 200 m3/s, and the 5 year flood decreased from about 370 to 160 m3/s (recurrence intervals determined using methods prescribed by the U.S. Interagency Advisory Committee on Water Data, 1982). The contemporary bankfull discharge, with a recurrence interval of 1.3 years, is 62 m3/s. The pre-regulation discharge of comparable recurrence interval was 170 m3/s. Changes in the flood magnitude/frequency relation have had an effect on channel geometry, streambed condition, and riparian vegetation in the Gunnison Gorge. A concern in the Gunnison Gorge is the degree to which entrainment of coarse, tributary-supplied bed material has been decreased by diminished mainstem flood flows. Periodic entrainment and transport of coarse bed material (cobbles and boulders) is important for maintaining channel characteristics and biologic habitats in riffle/pool reaches and for maintaining river navigability in rapid reaches. In order to assess the potential for entrainment, it was necessary to determine the size of bed material that could be entrained by a specific discharge in a given river reach and then to estimate the frequency of discharge sufficient to entrain a relatively large portion of the particle sizes present on the bed. An assumption was made for the purpose of this study that relatively frequent entrainment of bed material at least as large as the median particle size, or 50th-percentile W50), is indicative of significant bed-material mobility. Channel geometry and bed-material size distributions were measured at several riffle/pool and rapid reaches in the Gunnison Gorge. Discharges for several significant stages in the study reaches were determined from