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34 - 2 PITLICK AND CRESS: DOWNSTREAM CHANGES IN CHANNEL GEOMETRY channel is sufficient to maintain a gravel bed throughout the study area. Together, the scale and characteristics of the field setting provide a somewhat unique perspective on the relation between discharge, sediment load, and channel geometry in gravel bed rivers. 2. Study Area [s] The present study focuses on a nearly contiguous alluvial segment of the Colorado River between approxi- mately Rulison, Colorado, and Moab, Utah (Figure 1). In this area the Colorado River flows in a southwesterly direction, bisecting the Roan Mesa and Paradox Basin physiographic provinces of the Colorado Plateau [Liebermann et al,, 1989]. As the river traverses this area, it flows through a series of Jurassic- and Cretaceous-age sedimentary rocks which vary in their resistance to erosion. Where the bedrock consists of shale, the river flows within broad alluvial valleys, and where the bedrock consists of sandstone, the river is more confined. This setting is typical of the large rivers in the region [Andrews, 1986: Allred and Schmidt, 1999; Grams and Schmidt, 19991. The differences in valley form are the basis for subdividing the study area into 10 separate reaches, which we designate as "hilly alluvial" or "quasi-alluvial" (Table 1). Fully alluvial reaches are those where the river is free to migrate laterally, and where a wide (0.2-1.0 km) floodplain is present. Quasi-alluvial reaches are more incised and partially bounded by bedrock, but the presence of floodplain segments along one or both banks indicates the river is free to adjust its width as necessary. [6] Changes in channel properties of adjacent alluvial and quasi-alluvial reaches are not as pronounced in the Colorado River as in some other rivers. Exceptions to this general- ization occur in Westwater Canyon, where the river cuts through crystalline bedrock, and in the reaches between Dewey and Moab, Utah, where the river flows across a series of salt-cored anticlines [Doelling, 1985]. Uplift along these anticlines has been occurring for at least the last 2.5 Myr [Colman, 1983]; effects include a steepening of the profile through Professor Valley, and incision into resistant sandstones through the Big Bend reach. Aside from these two reaches, the channel characteristics of this segment of the Colorado River are not strongly influenced by transi- tions in reach type or junctions with major tributaries (this point is pursued in detail later). [7] The annual hydrograph of the Colorado River is dominated by spring snowmelt with most of the runoff originating in the mountains of central and southwestern Colorado. Streamflows are regulated by a series of storage reservoirs and water diversions upstream. These structures are small in comparison to main stem reservoirs farther downstream (e.g., Lake Powell or Lake Mead), but their operations significantly affect the timing and magnitude of peak snowmelt flows. Since 1950 instantaneous peak dis- charges of the Colorado River and its major tributary, the Gunnison River, have decreased by 29-43% [Van Steeter and Pitlick, 1998]. Annual sediment loads have decreased by similar amounts (30-40%), but the contribution of sediment from erosive basins in western Colorado and eastern Utah remains high. Our analysis of streamflow and sediment data from main stem gauging stations indi- cates that the annual suspended sediment load of the Colorado River increases markedly downstream in compar- ison to the discharge (Table 2). Silt and sand constitute a large part of the annual suspended sediment load; we estimate that at least 95% of the total load is carried in suspension. Gravel is a minor component of the total load, however, this material forms the bed of the channel; thus it has a major influence on channel form. Bank materials grade from predominantly gravel with a thin veneer of overbank fines in upper reaches to gravel capped with 1-2 in of silt and fine sand in lower reaches. Channel adjustments in the lower reaches are perhaps limited by the cohesion of fine- grained bank sediment and riparian vegetation, but almost everywhere the bed of the channel and toe-slopes of the banks consist of gravel which is mobile under normal flood flows. [8] Given the history of flow regulation on the Colorado River, it is important to establish whether the study reach is still adjusting to the altered flow regime. In a previous study based on repeat aerial photography, Van Steeter and Pitlick [1998] found that the width of the Colorado River has decreased by an average of 20 in (15%) since the late 1930s. Evidence for narrowing can sometimes be seen in the field in the form of inset benches which lie about 0.5 in below an older floodplain surface. These benches are very discontin- uous, and it is not clear whether they reflect long-term changes in sediment transport capacity caused by reservoir operations or deposition in response to large floods in 1983 and 1984. To determine if channel adjustments are continu- ing, we examined recent trends in width and bed elevation at the U.S. Geological Survey (USGS) gauging stations at Cameo, Colorado, and Cisco, Utah. Discharge measure- ments are made at each of these gauges almost every month; thus, if channel change is continuing, it should be evident in the records of width and bed elevation, as shown in Figure 2. The data in the upper panel (Figure 2a) indicate that there has been no significant change in width at either gauge since 1984, when the last major flood occurred. The data in the lower panel (Figure 2b) show an irregular pattern of scour and fill at the Cameo gauge, and slight scour at the Cisco gauge. The step-like increases in bed elevation at Cameo occurred during moderate floods in 1993 and 1995; similar changes did not occur at the Cisco gauge, nor at any of our previously measured cross sections [Van Steeter and Pitlick, 1998]. The slight scour at the Cisco gauge could be natural or dam-related, but whatever the cause, the total change (0.1 in in 16 yr) is no more than about 2% of the bank-full depth. Thus, if the Colorado River is continuing to adjust to the effects of flow regulation, it is not apparent in the recent record. 3. Methods [9] Field measurements and data characterizing the geo- morphology of this segment of the Colorado River were obtained at closely spaced intervals from Rulison, Colorado to Moab, Utah (Figure 1) (distances are given here in river kilometers, rkm, measured upstream from the Green River confluence). Limited access to short segments in DeBeque Canyon and Westwater Canyon prevented us from collect- ing data in portions of these two reaches. [io] Cross sections of the main channel were surveyed at 1.6-km intervals from rkm 366 to rkm 77 with the exception of the two segments noted above that were difficult to access and a few sites that could not be surveyed for