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520 TOPPING Ef AL: COLORADO RIVER SEDIMENT TRANSPORT, 1 Grand Canyons was more sensitive to annual changes in the sediment supply than was bed elevation in the predam river in Glen Canyon. To determine the potential differences in the degree of sediment supply limitation in Marble and Grand Canyons versus Glen Canyon, we examined: (1) the differences in coupled hysteresis in suspended-sediment concentration, grain size, and bed elevation at the Grand Canyon and Lees Ferry gages and (2) the differences in vertical trends in grain size in predam flood deposits in Glen, Marble, and Grand Canyons. 5.1. Differences in Annual Hysteresis in Sediment Concentration and Grain Size Because the groundwater and surface-water input to the Colorado River between the Lees Ferry and Grand Canyon gages is typically small, Ihe downstream increase in flow be- tween these gages is minimal. Indeed, the long-term average increase in discharge between the Lees Ferry and Grand Can- yon gages based on data from water years 1923-1962 was only 13 m3/s (i.e., an increase of only 3% over the mean-daily discharge at Lees Ferry during this period). Because the down- stream increase in discharge is slight, suspended-sediment con- centration as a function of discharge at the two gages can be compared without correcting for downstream changes in flow. In the predam era the magnitude of annual hysteresis in suspended-silt and clay concentration was comparable at each gage (Figure 4a), but the magnitude of annual hysteresis in suspended-sand concentration was much greater at the Grand Canyon gage (Figure 4b).J\tthe Grand Canyon gage, suspend- ed-sand concentrations were much lower during the period from June 1 through July 20 (i.e., the last portion of the rising limb and most of the receding limb of the typical snowmelt flood) than those measured in similar flows during the rest of the year. This systematic annual variation in suspended-sand concentration at this site arose because the first portion of the snowmelt flood reduced the supply of the finer (i.e., 0.0625- 0.25 nun) sand (Figures 40 and 4d). Thus the annual hysteresis in suspended-sand concentration at the Grand Canyon gage was coupled to an annual hysteresis in grain size, with the suspended sand sampled from June I through July 20 typically being coarser than that sampled during the rest of the year. In contrast, at the Lees Ferry gage, relatively little annual hyster- esis in concentration or grain size existed for either the finer or the coarser sand (Figures 4b, 40, and 4d). In addition to the difference between the magnitudes of the annual hysteresis in suspended-sand concentration a flow- dependent difference also existed between the sand-transport rates at the Grand Canyon and Lees Feny gages. On average, in the predam river, more silt and clay were in suspension than sand, but flow-dependent differences in sand-transport rates were lar.ge enough to dominate the total fine-sediment (i,e., sand, silt, and clay) data [see Howard and Dolon, 1981, Figure 12]. Sand-transport rates were generally higher at the Lees Ferry gage than at the Grand Canyon gage during low flows, whereas the opposite was true during high flows depending on season. In flows below about 200-300 m3ts, suspended-sand concentrations were substantially higher at the Lees Ferry gage than at the Grand Canyon gage (Figures 4b, 4c, and 4d). In contrast, in flows in excess of about 400-500 m3ts the opposite was generally true but also depending on both grain size and season (Figures 4c and 4d). At these higher flows and occur- ring independently of season, the concentration of suspended coarser (i.e., >0.25 mm) sand was typically higher at the Grand Canyon gage than at (he Lees Ferry gage, Fur finer sand, however, sand-transpon rates at the two gages generally dif- fered only during the initial portion of the annual snowmelt flood (i.e., when flows first exceeded about 400-500 m'ts). During this initial period of higher flows, concentrations of suspended finer (0.0625-0,25 nun) sand were substantially higher at the Grand Canyon gage. Then, in similar high flows during the latter part of tbe snowmelt flood (i.e., after about June 1), concentrations of suspended finer sand at the Grand Canyon gage decreased to become comparable to those at the Lees Ferry gage. 5.2. Discussion of Differences in Annual Hysteresis in Sediment Concentration and Grain Size The difference between the magnitudes of coupled annual hysteresis in suspended~sand concentration and grain size be- tween the Grand Canyon and Lees Feny gages suggests that the predam Colorado River in Grand Canyon was annually supply-limited with respect to sand to a far greater degree than it was in Glen Canyon. Though annual hysteresis existed in the concentration of silt and clay at both gages, it was of the same magnitude, suggesting that the degree of annual supply limi- tation with respect to silt and clay was similar in both Grand and Glen Canyons. Because substantial annual hysteresis in suspended-sand concentration existed in the finer (0.0625-0.25 DlID) sand at the Grand Canyon gage, the river in Grand Canyon was probably supply-limited with respect to this size class of sand on an annual timescale. This represented an annual supply limitation with respect to 80-90% of the sizes of sand in transport (based on the data presented in Figure 4). Because little annual hysteresis existed in the concentration of coarser (>0.25 mm) sand at the Grand Canyon gage, the river in Grand Canyon was either not annually supply-limited with respect to this size class of sand, or it was supply limited with respect to this size class of sand on a timescale that was longer than either (1) the period between sediment-resupplying events to Marble Canyon and upper Grand Canyon or (2) the length of time required to transport sand coarser than 0.25 mm out of Marble Canyon and upper Grand Canyon. The difference in sand-transport rates between the Grand Figure 4, (Opposite) Predam sediment concentrations as a function of water discharge at the Lees Feny and Grand Can- yon gages shown at the same scale. Cross-hatched region over- laying the Grand Canyon gage data in Figure 4 indicates the region in coneentration-discharge space occupied by the Lees Ferry data. Silt and clay concentrations were generally uniform at both gages, indicating no demonstrable storage of silt and clay in Marble Canyon and the upper Grand Canyon. At low flows, sand concentrations at the Lees Ferry gage were higher than at Grand Canyon gage, indicating sand accumulation in Marble Canyon and upper Grand Canyon. At high flows the stored sand was eroded from this reach, as reflected by the initially higher sand concentrations at the Grand Canyon gage. Sources of data are as follows: at the Grand Canyon gage, the 620 suspended-sediment samples collected (with modern sam- pling equipment) from June 6, 1944, through December 19, 1962, and analyzed for grain size, and at the Lees Feny gage, the 504 suspended-sediment samples collected from July 21, 1949, through December 19, 1962, and analyzed for grain size. (a) Suspended-silt and clay concentrations at the two gages. (b) Suspended.sand concentrations at the [wo gages. (c) Coneen. trations of suspended finer (0.0625-0.25 mm) sand at the two gages. (d) Concentrations of suspended coarser (>0.25 mm) sand at the two gages,