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~ 0 17S 350 SCAlf N FEIT V '/ " ~ SAND-OOMlNA TED PRIMARY BAR -f ",'\C,,' ~ FA~:POSED AT 10,000 CFS ~ ~ SUlMERGEO AT 10,000 CFS '~' ~ " \~.;" 4.0 ~ SECONDARY BAR ~ TERTIARY BAR ~ WEBER SANDSTONE OUTCROP Qa ALlUVIUM Qc COLLUVIUM l H RIFflE 624 DlSQY.RGE N CFS @ CROSS SECTION NO. FLOW FIGURE 3. Detailed geomorphological map of the spawning bar at RM 16.5, Yampa River. Primary, secondary, and tertiary bar surfaces are identified as well as cross-section locations for the hydraulic modeling. Discharges in the individual channels on 12 and 13luly 1991 are also shown. surface of the secondary bar were covered by a soft mud drape that had been depos- ited during the recessional flow as the bar surface was becoming subaerially exposed. The depositional process was observed at the secondary bar margin where wash-load caliber sediments were settling from sus- pension in locally ponded areas following a recent small rise in discharge due to thunderstorms. The branch and chute channels had eroded and dissected the secondary bar. 0.001 100[~--,,-- PARTla.E SIZE (;n) ,~~~_____.~_. _~r~.l '. -' 9Oi--------- -- ----.-<----------- I ,- OOt -------.---!-lr+--~B'R~Ni----- 70:------ =:= ==== ~-- I :E~~~~i =:= =~7~ ~ )Q~,.- - .-:-. ~~----;--------...---- , . I ':[ ____ / . _____L 0,01 Z .~~o~- i s 1 2--~--S~---2"~- PARTlOE SIZE (mn) FIGURE 4. Grain size distribution curves for var- ious bar surface and subsurface sediments on the spawning bar at RM 16.5, Yampa River (25.4 mm = 1 in.). Subaqueous riffles were located within the channels. On the basis of field observation of the mobility of the sediments in the rif- fles, the riffles were further subdivided into either riffles or tertiary bars. If the cobble- gravel size sediments were at, or near, a condition of incipient motion the riffles were mapped as tertiary bars (Figure 3). Bar Sedimentology Surficial sediment size distributions were determined on the lower surface of the pri- mary bar (four counts), the secondary bar (five counts), and the riffles or tertiary bars (six counts). Subsurface samples were ob- tained from the lower primary bar (two counts) and the secondary bar (two counts). The soft mud drape between the surface cobbles on the secondary bar surface was also sampled. Average gradations for all of the surficial and subsurface samples for the various mapping units were determined (Figure 4). At the primary bar-forming discharges, backwater conditions caused deposition of a heterogeneous mixture of very poorly sorted sediments that ranged in size from I M. D. Harvey et a1. 119 II~