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I I I I I I I I I I I I I I I I I I I area covered). Each map shows: (a) a description of the rapid: (b) topographic contours (metric contour intervals of 1 and 0.5 m) of the channel: (c) hydraulic information at 5,000 and 30,000 cfs and, where information is available, at 92,000 cfs: (d) water surface elevations: (e) velocity and streamline data at one or two discharges: and (f) approximately five channel cross sections. Three parts of one map (for House Rock Rapid) are attached as Figures 1, 2, and 3. DISCUSSION Debris flows from tributary canyons constrict the main channel episodically, and then floods of differing sizes on the Colorado River widen the constrictions and move material from the debris fans downstream into secondary features referred to as "rock gardens." The shape of the Colorado River channel in the vicinity of the debris fans depends on the relative frequencies of tributary and mainstem floods, on the nature of the material brought into the channel by the tributary floods, and on the competence of the Colorado River. Prior to closure of Glen Canyon Dam in 1963, natural floods had contoured the river channel through the debris fans into remarkably uniform shapes, for which the ratio of narrowest channel width to average channel width (the "constriction") was about 0.5 (Figure 4). Figure 4 is a histogram showing the ratio of river width in the most tightly constricted part of a rapid to an average unconstricted channel width. Data are for 59 of the largest debris fans in the 250-mile stretch below Lees Ferry. These debris fans are probably 1,000-100,000 years old, except the debris fan at Crystal Rapid, formed in 1966. Data are based on the widths of the surface water in the channel on 1973 aerial photos. Crystal Rapid in 1973 is represented by the highly constricted data point on the left. The surface constriction was 0.33: the average channel cross section constriction was probably about 0.25. 21