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indicates flows filling the charnel and covering lower terraces up to the level of the "juniper stage" are in the range of 310 to 370 m3/s. At 310 m3/s, a large proportion of the existing bed-material sizes could be reworked at most riffle/pool reaches, and much of the coarse material supplied by tributary canyons could be entrained. The juniper-stage discharge, and probably channel reworking, were frequently occurring events before 1966 (recurrence interval 3.2 years). However, streamflows near 310 m3/s have occurred only twice since 1966 (Figure 2) (recurrence interval approximately 40 years). It is likely that significant bed- material entrainment has become virtually nonexistent, and the bed surface has become armored in many Gunnison Gorge reaches since regulation (Elliott and Parker, 1992). Most of the transportable sediment in the Gunnison Gorge is supplied to the river from steep ephemeral tributaries. Debris-flow and sediment-supply processes were studied at five tributary-confluence rapids to evaluate the effect of these processes on the mainstem of the Gunnison River and to determine the magnitude and frequency of their occurrence. A preliminary evaluation of photographic, dendrochronologic, and carbon-14 data indicates relatively frequent tributary activity; however, magnitude estimates will require additional data. All five of the studied tributaries have had at least one large debris flow since 1970. These large events carried sufficient amounts of material to damage or eradicate trees, severely constrict the river with boulder-sized (larger than 256 mm) material, and temporarily dam the river at Red Canyon in 1989, (Barry Nearing, Colorado Division of Wildlife, personal communication, 1992). Smaller debris-flow and mud-flow events were observed from several tributaries in 1992. Previous large events occurred at Chukar Wash, Long Gulch, and Boulder Garden not long before the earliest aerial photos were taken in 1.950, and at Big Cove around 1930. Evidence of damage preserved in tree rings of a juniper (approximate age 300 years) from Boulder Garden indicate four very large events during the life of the tree. Prior to river regulation, sediment supplied by tributaries was periodically removed by large mainstem floods and a distinctive channel geometry was maintained. River regulation since 1966 has caused some riffle/pool reaches to become armored and has reduced the entrainment frequency of debris-flow sediment from rapids. If the recurrence of large, mainstem flood peaks remains at the post-regulation frequency, both the large volume of tributary sediment and the large size of periodically introduced sediment particles could result in significant geomorphic changes which, in turn, will affect channel and riparian conditions, fisheries, river navigability, and esthetic qualities. Streamflow regulation by upstream reservoirs has caused changes in the biological characteristics of the Gunnison Gorge fluvial and riparian zone. Attenuated flood peaks have resulted in less frequent overbank inundation and less severe bank and flood-plain scour. _Consequently, exotic, non-native plants, such as tamarisk (Tamarix ramosissima), redtop (Agrostis stolonifera), and reed canarygrass (Phalaroides arundinacea), etc., have become established or dominant on low-lying terraces (A. Clements, J. Siscoe, BLM, and G. Kittel, The Nature Conservancy, personal communication, 1993). Native trees such as box elder (Acer negundo) have encroached closer to the river channel, reducing the channel width and causing bank and terrace sediments to become more resistant to scour. Reservoir impoundment and hypolimnal releases have resulted in higher discharges and colder water temperatures in what formerly was the low-flow season. These conditions, along with appropriate substrate characteristics and adequate food supply, have enhanced the cold-water (trout) fishery in the Gunnison Gorge but may have displaced the warm- water fishery downstream. Since the mid-1960's, the downstream boundary for optimal trout production has shifted downstream approximately 80 km to the Duncan Camp/Ute