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metrical, and the average slope of the river is about 0.3%. The lower 20 mi of the can- yon (RM 0-20) are bounded by poorly con- solidated Weber Sandstone. The lower can- yon is narrow, deeply incised, and very sinuous (sinuosity = 2.6). The river has an average slope of about 0.1%. Unlike allu- vial sections of the Yampa River, where channel morphology appears to be adjust- ed to relatively frequently occurring flows (Andrews 1986), the canyon-bounded reaches are dominated by relatively infre- quent flows. Coarse sediment deposits that form the bars occur where stream power drops below critical thresholds necessary to maintain transport of cobbles and boul- ders, during the infrequent high magni- tude events (Graf 1979; Webb et a!. 1988). The stream power minima occur in Local reaches that are located immediateLy up- stream of canyon bends and constrictions and immediately downstream of canyon expansions. Therefore, canyon morpholo- gy controls the distribution of bars within the canyon (Lisle 1986; O'Connor et a!. 1986). The spawning bars at RM 16.5 and RM 18.5 are located upstream of sharp bends in the canyon. FIELD DATA COLLECTION AND ANALYSIS Field data were collected at RM 16.5 and RM 18.5 when the discharge of the Yampa River was about 1,200 ds. Eight cross sec- tions originally surveyed in 1983 by the USFWS for PHABsIM modeling purposes were relocated and resurveyed at RM 16.5. Discharge and velocity measurements were conducted at various locations around the RM 16.5 spawning bar to determine the totaL discharge in the river and flow dis- tributions in the branch channels for hy- draulic model calibration purposes. A de- tailed geomorphic map of the reach was constructed in the field (Figure 3). Surface sediment gradations were measured at 15 locations by the Wolman pebble count method (Wolman 1954; see Kondolf and Li 1992 for a detailed description), and 5 subsurface sediment samples were ob- tained for subsequent laboratory analysis of sediment gradations. Stationary and drifted trammel nets determined the pres- ence of squaw fish at various locations around the bar. Similar data were also col- lected at RM 18.5 (Alternate spawning bar), and some sediment sampling was also per- formed at Mathers Hole (RM 17). Bar Morphology The bar at RM 16.5 is formed and main- tained by backwater conditions during high discharges, when the critical dis- charge for entrainment of coarse sedi- ments is exceeded in upstream reaches that are unaffected by backwater. Observations of high-water marks and subsequent hy- draulic modeling indicated that the bar was I~ 118 almost totally submerged at a discharge of about 10,000 ds. At lower discharges, the Yampa River bifurcates into two main branch channels (left and right looking downstream) that convey almost equiva- lent flows around the bar. A minor chute channel that heads on the left branch chan- neL delivers flow into the right branch channel about two-thirds of the way down the bar (Figure 3). The subaerially exposed portion of the bar at the time of the field visit was sub- divided into two geomorphic mapping units on the basis of elevation. The highest surface was mapped as the primary bar and further subdivided into subunits on the basis of field evidence of either submer- gence or subaerial exposure during the 1991 peak discharge (Figure 3). The higher por- tion of the primary bar is mantled by flu- vially deposited cross-bedded sands that are partially vegetated. Sand deposits on top of underlying gravels and cobbles pro- vided field evidence for reduced hydraulic energy, the result of increasing backwater, as the bar became submerged during high- er discharges. The lower portion of the pri- mary bar is unvegetated, and the surface cobbles are cemented in place by a sun- baked mud drape that made removal of the surface cobbles difficult. The lower elevation surface that is mar- ginaL to the primary bar was mapped as the secondary bar (Figure 3). It was formed by erosion and dissection of the primary bar at a lower range of discharges that are less affected by backwater from the bend downstream. The intercobble spaces on the Rivers. Volume 4, Number 2 April 1993