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Physical Characteristics of Study Reaches In the study area the Colorado River flows in a southwest 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 Permian- to Tertiary-age bedrock formations that vary in their resistance to erosion. Where the bedrock consists of shale, such as the Mancos or Morrison Formations, the river flows within very broad alluvial valleys (e.g. near Parachute and Grand Junction, CO, or between Cisco landing and Dewey, UU); where the bedrock consists of sandstones, such as the Wingate, Navaho or Entrada Formations, the river is more confined (e.g. Ruby-Horsethief Canyon, Dewey Canyon, and Big Bend). These differences in bedrock lithology and valley morphology are the basis for subdividing the 300-km study area into 11 separate strata or subreaches that we designate as "fully alluvial" and "quasi-alluvial" reaches. Fully alluvial reaches are those where the river is generally free to migrate laterally, and where a wide floodplain is present; examples include the Rifle-DeBeque reach near Parachute, the 15- and 18-mile reaches near Grand Junction, and the Cisco-Fish Ford Reach near Cisco, UT. Quasi-alluvial reaches are more confined by bedrock, yet still have a floodplain; examples include DeBeque Canyon, Ruby- Horsethief Canyon and Dewey Canyon. Although there are some clear visual and morphologic differences between fully alluvial and quasi- alluvial reaches, the Colorado River generally does not exhibit abrupt changes in channel gradient, bed material size or bankfull channel width as it flows through these different reaches; exceptions to this generalization are the Westwater Canyon and Big Bend reaches, which are both bounded by resistant bedrock, and the reaches through and below Moab UT, which are sand-bed. Specific attributes of the 11 study subreaches (not including Westwater Canyon) are discussed below, and reach-average characteristics are summarized in Table 1. Table 1. Average slope, bankfull width (w), depth (h), and median surface grain size (D50) of the Colorado River in specific subreaches. Subreach Name and Location Reach Type' slope w (m) h (m) Dso (mm) - -------------------------------------------- Rulison-DeBeque (RKM 365-328) ------------------- A ----------------- 0.00196 ------------- 114 ------------ 2.45 ------ --- 57 DeBeque Cyn. (RKM 327-316) QA 0.00150 77 3.12 52 15-mile reach (RKM 298-275) A 0.00175 134 2.54 58 18-mile reach (RKM 274-246) A 0.00130 175 3.01 52 Ruby-Horsethief Cyn. (RKM 245-206) QA 0.00100 129 3.64 47 Cisco-Fish Ford (RKM 180-153) A 0.00066 147 4.49 38 Dewey (RKM 151-140) QA 0.00047 132 5.14 34 Professor Valley (RKM 138-126) A 0.00149 203 4.61 70 Big Bend (RKM 124-113) QA 0.00098 106 6.43 63 Moab (RKM 111-105) A 0.00034 151 5.13 28 Potash (RKM 103-77) A 0.00028 197 4.51 0.025 1) A indicates alluvial reach; QA indicates quasi-alluvial reach. 6