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<br />c <br />5 10000 <br />o <br />w <br />Ul <br />II: <br />w <br />0- <br />Ul <br />II: <br />w <br />W <br />::; <br />o <br />iii <br />;:J <br />o <br />:!!: <br />uj <br /> <br />~ <br />o <br />Ul <br />is <br />Ul <br />;:J <br />o <br />w <br />z <br />~ <br />~ <br />en <br />:!!: <br /> <br />00:2343 <br /> <br />ptO-O&m peood <br />(May 8. 1921 . _ 12. 1983) <br /> <br />1000 <br /> <br /> <br />po_ period <br />(M<WCIl14. 1~' SopIori>er 30. 2<100 <br /> <br />/ <br /> <br />-....-._i! <br /> <br />-..-............... <br /> <br />100 <br /> <br />10 <br />o <br /> <br />20 40 60 80 <br />PERCENTAGE OF TIME EOUALED OR EXCEEDED <br /> <br />Figure 3. Graph showing flow duration curves of the <br />Colorado River at the Lees Ferry gage for the pre-dam <br />and post-dam periods (Topping et aI., 2003). <br /> <br />10' <br /> <br />100 <br /> <br />*' <br /> <br />~",",") <br /> <br />:g 10.1 <br />Z <br />o <br />~ 10" <br />a: <br />I- <br />z <br />~ 10-0 <br />z <br />o <br />o <br />10-' <br /> <br />Hr' <br /> <br />'* <br />:g 10-1 <br />6 <br />~ 10-' <br />a: <br />1z <br />~ 10.0 <br />8 <br /> <br />. JVLY21-MAY31MEASVREMENTS <br />o JUNE l-JVLY20MEASUREMENTS <br /> <br />TOTAL SAND (0.0625-2.0 mm) <br /> <br />concentration at any given discharge was 2 <br />orders of magnitude at the Grand Canyon <br />gage, whereas this variation was about half an <br />order of magnitude at Lees Ferry. <br />The relative concentration of fine sedi- <br />ment at the two gages differed between the <br />low-flow and flood-flow seasons. When flows <br />were less than about 250 m3/s, the concentra- <br />tion of sand in suspension was greater at the <br />Lees Ferry gage than at the Grand Canyon <br />gage (Fig. 4). At flows less than about 150 m3/ <br />s, the concentration of suspended sand at Lees <br />Ferry was about 2 orders of magnitude more <br />100 than at the Grand Canyon gage. When flows <br />exceeded about 500 m3/s during the rising limb <br />of the annu.al spring flood, the concentration of <br />suspended sand at the Grand Canyon gage <br />exceeded that at the Lees Ferry gage. The <br />concentration of sand in suspension at the <br />Grand Canyon gage subsequently decreased to <br />approximately equal the concentration of sand <br />in suspension at the Lees Ferry gage. <br />These seasonal differences in concentra- <br />tion imply that there was a 9-mth period <br />between July and the following March when <br />sand accumulated, because more sand was <br />delivered into Marble and upper Grand Can- <br />yons than was exported downstream (Topping <br />et at, 2000b). This was the period when <br />discharge was mostly less than 250 m3/s. <br />During the spring snowmelt flood between <br />April and June, the amount of sand exported <br />past the Grand Canyon gage was approxi- <br />mately equ.al to the amount transported past the <br />Lees Ferry gage plus the amount of sand that <br />had accumulated in the reach since the previ- <br />ous July. Decrease in the concentration of <br />sand in transport past the Grand Canyon gage <br />during the spring flood resulted from depletion <br />of the supply that had accumulated during the <br />preceding low-flow season. Thus, fme-sedi- <br />ment deposits between the gages were eroded <br />and exported downstream during the annual <br />spring snowmelt flood. Presumably, the total <br />amount of sand on the bed and along the banks <br />in Marble and upper Grand Canyons was least <br />immediately upon recession of the snowmelt <br /> <br />Figure 4. Graphs showing pre-dam sand concentrations <br />as a function of water discharge for the Lees Ferry and <br />Grand Canyon gages. Cross-hatched region overlaying <br />the Grand Canyon gage data indicates the region in <br />concentration-discharge space occupied by the Lees <br />Ferry data. (Topping et aI., 2000b, Fig. 48). <br /> <br />... <br /> <br />. . <br />. ;'. . .......+ .... .,1.# ~.o....~_ <br />\~.~ # ~~~3~~~~~~ <br /> <br />. ., ~. . ... 00.0,- 0 d' <br />::..,.~+. .... <br />. . <br />. . . <br /> <br />o <br /> <br />o <br /> <br />LEES FERRY GAGE <br /> <br />100 1000 <br />DISCHARGE, IN CUBIC METERS PER SECOND <br /> <br />10' <br /> <br />TOTAL SAND (11.0625 - 2.0 mm) <br /> <br />100 <br /> <br /> <br />10" <br /> <br />GRAND CANYON GAGE <br /> <br />Hr' <br /> <br />100 1000 <br />OISC:HARGE. IN C:UBIC METERS PER SECOND <br /> <br />2.0 Water and Fine-Sediment Fluxes 5 <br />