9331 - Laserfiche WebLink

Home Browse Search

Figure 11. Hydraulic geometry relations for the present cable- way for three time periods. Dot. ted lines show the rdation for the first period of chani1e1 narrowing between 1930 and 1938. Dashed lines show the relation for the pe- ri od when width was stable be- tween 1939 and 1957. Solid lines show the relation for the period between 1963 and 1993 when tlIere was a slow rate of channel narrowing. Note the change in bank angle implied by the change in slope of the width relation fol- lowing 1938. Error envelopes for width data rep~ent the 95 % confidence limits. substantial rewidening of th~ bankfull channel at any time since 1911. The processes of floodplain stripping and rewidening by floods, however, cannot be discounted if the Green River were to return to a hydrologic regime similar to the pe- riod between 1895 and 1930. The history of channel change and the calcu- lated shifts in effective discharge demonstrate that any correlation between effective discharge and the discharge that inundates alluvial deposits in vertically-accreting systems is fortuitous, since the elevation of overbank deposits is continually increasing over time. At the present cableway, the discharge that inundates the 'banks has increased during the past 30 years. However, the effective discharge has not similarly responded. Proposed Mechanism for btverse Stratification of Floodplain Deposits Severql researchers have noted the presence of inversely-gr;ided river deposits (Rubin et aI., ~::: ~~ ~ :;::;.-" :::-- ~.~>,--- ~;~-:;-:--. ~9/ WIDTH(m) -",,~~~;?,-;:,::.. ~~~9- --~::;;.- ......-;-::;~ ~:;;-- ~5;.:.- ~"~- 4- Z ALLRED AND SCHMIDT 120 110 '.. .... 100 90 80 5 MEAN DEPTH (m) 1 0.5 MEAN VELOCITY (m/$) 0.1 10 50 100 DISCHARGE (m3ts) 500 1000 1998; Iseya, 1989). Rubin et al. (1998) found in- versely-graded fluvial deposits created by the 1996 experimental flow released from Glen Canyon Dam. They concluded that fine sands were selectively winnowed from the bed during the flood, leaving behind higher concentrations of medium and coarse sand that could still be sus- pended. Iseya (1989) also identified inverse grad- ing of overbank deposits, and proposed a mecha- nism for their emplacement. She suggested that silts and clays are deposited during the early stages of flooding when flow over the floodplain is shallow and velocities are low. As flood mag- nitude increases, sand-sized particles can be sus- pended in the channel but cannot be suspended over the flood]:>Iain. Thus, these larger particles fall from suspension onto the floodplain, leading to inverse grading. Our data describe another mechanism for the disruption of th,e typical fining-upward se- quences commonly found in fluvial deposits; upward-coarsening may be an inevitable result of vertical accretion. Rubin et al. (1998) and Iseya (1989) described mechanisms whereby de- posits coarsen during a single flood event. Green River deposits cOarsen because floods of higher magnitude are necessary to inundate a vertically- aggrading floodplain. We distinguish 3 concep- tually different zones in the inset floodplain de- posit (Fig. 13). Lower Zone-Low-Elevation Part of De- posit. During the early formation of the flood- plain, the elevation of the deposit is very low. Bedload, composed of medium and coarse sand, moves across the bar. Climbing bedforms accrete vertically and form the lower part of what even- tually becomes the floodplain. Intennediate Zone-Mode~te Elevations. As aggradation continues, floods of moderate magnitude can overtop the surface. In these mod- erate floods, flow depth l:ind velocity are suffi- ciently low that only fine particles are suspended near the surface, and these fine particles are trans- ported onto the floodplain. Silts and clays are TABLE 4. TWENTIETH-CENTURY CHANGES IN CHANNEL AREA FROM AERIAl- PHOTOGRAPHY Active channel Mean channel Percent 011938 active Secon<:Iary Percent of 1938 area width channel area channel area $econdary channel area Year (km2) (m) ('Yo) (km2) ('Yo) 1938 4.20 159 100 0.49 100 1952 4.00 152 95 0.35 71 1962 4.05 153 96 0.36 73 1985 3.58 136 85 0.27 55 1993 3.46 131 82 0.24 49 1768 Geological Society of America Bulletin, December 1999