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A. 01,000,000 Z o () w "' a: w "- Iii 100,000 w u. () a; :> () ~ 10,000 SAND CONVEYANCE OR EROSION w :> ~ ~ Z '" PRE.DAM Ci 44.3% ~ .....\ ", POST-DAM 69.0% / ~~..._...~- ui '" a: '" r () "' o "' :> o w z iO z iO "' ~ -- ---," SAND ACCUMULATION 1,000 100 o 10 20 30 40 50 60 70 80 90 100 PERCENTAGE OF TIME EQUALED OR EXCEEDED EXPLANATION - PRE-DAM (May B-March 12, 1963) ......... POST-DAM (March 14, 1963-September 30, 2000) B. ~ 1,000,000 o () w "' a: w "- >- w w u. () a; :> () ~ 10,000 ui '" a: '" r () "' o "' :> o w z '" >- z '" >- "' ~ SAND CONVEYANCE OR EROSION 1950s-early 1960s ~ 37.6% ;:J. > 1940s ~ 47.5%15 w " 1920s 57.7".4:. I POST.DAM 69.0% --~/ ..~~- """-''''::;;;O''',~ "':-; \\ 1,000 SAND ACCUMULATION 100 o 10 20 30 40 50 60 70 80 90 PERCENTAGE OF TIME EQUALED OR EXCEEDED EXPLANATION 1920$ (May 8, 1921-December 31, 1930) 1930$ (January 1, 1931-December 31, 1940) - - 1940$ (January 1, 1941-December 31,1950) 1950s-early 1960s (January 1, 1951-March 12, 1963) -...- POST.DAM (March 14, 1963-Seplember 30, 2000) Figure 22. Flow-duration curves for the Colorado River at Lees Ferry, The gray shaded region shows the pre-dam discharge range under which sand accumulated in the reach between the Lees Ferry and Grand Canyon gaging stations. in Marble and upper Grand Canyons (Topping and others, 2000), The percentages of time when these discharges were exceeded during each period are indicated in italics. (A) Flow-duration curves for the pre- and post- dam periods. (BJ Flow-duration curves for the four pre-dam decades and the entire post-dam period. ~1 ,000,000 o () w "' a: w "- tu 100,000 w u. () a; :> () ~ 10,000 ui '" a: '" r () "' o (/) 1,000 :> o w z <( >- z iO if) ;;; " s ~ /, .... / , ~ / " .... '...... -'",; .... \ \ / J,....."'- .....,>, ',., ;-: ....-[':..... " ........ /,.....[!.. 100 JAN. FEB. MAR. APR. MAY JUNE JULY AUG SEPT. OCT. NOV. DEC. MONTH EXPLANATION -+- PRE-DAM MONTHLY MEDIAN DISCHARGE ....A.. PRE.DAM MONTHLY MINIMUM DISCHARGE - ~ - PRE.DAM MONTHLY MAXIMUM DISCHARGE Figure 23. Pre-dam monthly median, minimum. and maximum discharges of the Colorado River at Lees Ferry. 100 discharges lower than about 5,000 fi3/s, two orders of magnitude more sand was transported past the Lees Ferry gaging station than past the Grand Canyon gaging station. Therefore, when the discharge of the Colorado River was lower than about 9,000 ft3/s, sand accumulated in the reach in Marble and upper Grand Canyons, At discharges higher than about 9,000 ft3/s, however, sand-transport rates at the Grand Canyon gaging station generally equaled those at the Lees Ferry gaging station, and at discharges higher than about 16,000 ft3/s, sand-transport rates at the Grand Canyon gaging station generally either equaled or exceeded those at the Lees Ferry gaging station. Thus, at discharges greater than about 9,000 ft3/s, sand was either conveyed through or eroded from the reach in Marble and upper Grand Canyons. Of the four pre-dam decades, the 1920s were dominated by discharges that likely conveyed or eroded sand, the 1930s and 1950s were dominated by discharges that likely led to the accumulation of sand, and the 1940s were virtually neutral with respect to either accumulation or erosion of sand in Marble and upper Grand Canyons (fig. 228), Discharges under which sand likely accumulated downstream from Lees Ferry were exceeded .' ,'. 44 Computation and Analysis of the Instantaneous~Discharge Record for the Colorado River at Lees Ferry, Arizona-May 8, 1921, through September 30, 2000