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TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, I - SEDIMENT-YEARS 1040-1962 (JUL Y-MARCH) - SEDIMENT-YEARS 1949-1962 (APRIL-JUNE) - - - - - SEDIMENT-YEARS 1966-1970 - - - SEDIMENT-YEARS 1966-1996 I :-- ~ \. .....- . - "'. '- --. - ..--..-""":..---':..- - 5000 2000 ~ 1 UJ <!l 1000 a: <( J: U (f) o 500 a: UJ .... <( 300 ~ ~ 200 o UJ Z <( .... Z 100 ~ (f) ~ 27.0% 50 20 537 SAND CONVEYANCE OR EROSION SAND ACCUMULATION 68.1% 73.6% o 10 ~ ~ ~ ~ ~ ro ~ 00 100 % OF TIME EQUALED OR EXCEEDED Figure 11. How-duration curves constructed from the instantaneous discharge record of the Colorado River at the Lees Fel'I)' gage for the predam season of sand accumulation and storage (July-March), the predam season of sand erosion (April-June), sediment years 1966-1970, and sediment years 1966-1998. The cross- hatched region indicates the predam. 200-300 m3fs transition from sand accumulation to sand conveyance or erosion; also shown are the percentages of time at which flows of 250 m'ls were equaled or exceeded in each period. The predam curves were oonstructed from data from sediment ye"" 1949-1955, 1958, and 1960 -1962. Because the raw stage and discharge data from October 1977 to September 1979 and October 1980 to September 1981 oould noI be found, the curve for sediment ye"" 1966-1998 was oonstructed exclusive of these periods of missing data. replaced by steady 30 m'ls flows (Figure 9b). Closure of the dam in 1963 occurred in March; thus the dam was closed after the predam season of sand accumulation and storage (July- March), when storage of sand in Marble Canyon and the upper Grand Canyon was probably at an annual maximum. In addition to greatly diminishing the sediment supply, Glen Canyon dam has transformed a reach in which accumulation or storage of sand dominated over most of a year to a reach in which conveyance or erosion of sand dominates over most of a year. In the average predam year, newly input sand was re- tained in storage in Marble Canyon and upper Grand Canyon for 9 months (given the assigned uncenainties). In contrast, in the average postdam year, newly input sand may be retained in storage for only 2 months. Our interpretation of the postdam sediment budget agrees more closely with the early interpretation made by Laursen et al. [1976] than with three of the more recent interpretations. Laursen e/ al. [1976] ooncluded that under normal power plant releases, the capacity of the river to transport fine sediment exceeded the supply. In contrast, Howard and Dolan [1981], Andrews [1990, 1991], and U.S. Department of Ihe Interior [1995J all predicted multiyear fine-sediment accumulation in Marble Canyon and upper Grand Canyon during normal power plant fluctuating flows. Our analyses do not preclude long-term fine-sediment accumulation under nonnal power plant flows in the postdam river but rather suggest that one cannot conclude this, given the uncertainties in the sediment budgeL Howard and Dolan [1981] predicted that fine sediment should accumulate at a system-wide average rate of about 1 m per decade under normal power plant flows. They based their oonclusion on (1) a high annual sediment yield for the ungaged tributaries of 780 l/1an' and, more importantly, (2) the assump- tion that the uncertainties in the measured sediment loads were zero. Andrews [1990, 1991] based his conclusions on an estimated annual sediment export past the Grand Canyon gage of 9-11 million t, not on the measured mean-annual 1966- 1972 export of 14 million I. The U.S. Department of the Inferior [1995] based their conclusion on the assumption of stable re- lationships between the discharge of water and the sand. transport rate (Le., stable sand rating curves). Topping et al. [this issue) have shown that because the grain size of sand on the bed of the Colorado River changes substantially over time, relationships between the discharge of water and sand- transport rale change significantly over time. During and im- mediately following a tributary flood, the sand on the bed of the Colorado River fines as the sand supply becomes en- hanced. This causes the sand-transport rales in the Colorado River 1O increasl' indepelld~ntly of the discharge of water.