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TOPPING ET AL: COLORADO RIVER SEDIMENT TRANSPORT, 2 548 300 ilj- 200 ~l c~ z'l' <~ lOll <II. -' ~ <z ZO g~ 0 .. ,,0: w< "Ie <IIZ .100 l3~ W "'0 '" ".... < zw " -> .200 '" ~i= w 0. z:5 0. <w :> ,,0: 0 -300 ~ ..,. N z 0< z 0 Z;o 0 ;:: Qw ~ ;:: 0 ....~ z 0 w Om w "' w< 0 <II X "10 t x x w '" X 0: ::> o o <II ___ 3--27..g& (DAY 1) ! ~ 3-28-96 (DAY 2) ______ 3-29-96 (DAY 31 --e- 3-30-96 (DAY 4) ____3-31.96 (DA.Y 5) ---+- 4-1-96 (DAY 6) -+-4-2-68 (OAY7) -400 .20 0 20 40 60 BO 100 120 140 180 180 200 220 STREAMWISE DISTANCE FROM UPPER GAGE (m) Figure 5, Measured changes in cross-sectional sand area on each of the 7 days of high discharge relative to the bed topog- raphy surveyed on the day before the 1996 flood experiment. During the 1996 flood experiment, aggradation (fill) of the bed in the downstream portion of the Grand Canyon gage reach OCCUlTed simulIaneouslywith degradation (scour) of the bed in the upstream portion of the reach. (Figure 6a). During the 3 weeks following the flood, this bar began to rebuild (as shown by Ihe increase in sand area in cross sections 3 and 4), and tbe volume of sand in the reach recov- ered slightly (Figure 6b). 3. Observations During tbe 1997 Test Flow 3,1. Background August through September 1997 was a period of substantial sediment transport in the Paria River. During these 2 months, approximately 2.0 :!: 0.4 million t (t indicates metric ton) of sand and 2.4 :!: 1.2 million t of silt and clay were delivered to the Colorado River by a series of large floods in the Paria River [after Topping, 1997J. Most of this sediment input oc- culTed during four large floods: a 115-m'/s flood on August 10, a 72-m'/s flood on September 7, a flood with two peaks of 85 m.l/s and 110 mJ/s on September 15, and a 95-m'/s flood on September 26; three of these floods were greater than the 90 m'/s bank-full discharge (Topping, 1997J of the Paria River. Because of these floods, calendar year 1997 ranked among the top 20% in terms of sand input and among the top 12% in terms of silt and clay input during the 75 yeaTS of gage record on the Paria River. During the months of August and Septem- ber the little Colorado River also supplied sediment to the Colorado River but not in quantities nearly as large as the. sediment inputs from the Paria River. In an attempt to prolong the residence time of this new sediment in Marble and Grand Canyons, a test flow on the Colorado River was designed by the Grand Canyon Monitoring and Research Center to trans- fer some of this sediment from the channel boltom to higher environments on the channel-margin sand bars. This test flow consisted of a steady 877-m'/s (31,000 cfs) flow released from Glen Canyon Oam for 48 huu,s during November 3-5. 1997. i 3.2_ Metbods The USGS Ariwna District oonducted a program of sus- pended- and bed~sediment measurements at the four majn- stem gages depicted in Figure 1: thc lees Ferry gage, the lower Marble Canyon gage, the Grand Canyon gage, and the Above Diamond Creek gage. At each of these gages, cross- sectionally averaged suspended-sediment samples were col- lected up to several times daily using 0-77 bag samplers. Bed- sediment samples were collected using BM-54 samplers at the lower Marble Canyon and Above Diamond Creek gages. On November 11, an additional sample of the bed was collected at the Lower Marble Canyon gage using a pipe dredge. 3.3. Results At all four gages during the 2 days of steady high discharge, as during the 1996 llood experiment, suspended-sand and silt and clay concentrations decreased as the suspended sand ooarsened (Figures 7a-7c). Though the degree of suspended- sand coarsening was similar at all gages, the relative decrease in suspended-sand ooncenlration was greater at the Lower Marble Canyon and Grand Canyon gages than it was at the lees Ferry and Above Diamond Creek gages. Importantly, unlike during the 1996 flood experiment, during the 2 days of the 1997 test flow the sand concentrations measured at the Lower Marble Canyon gage equaled those measured at the Grand Canyon gage. This change occurred by a doubling of the sand-transport rates at the Lower Marble Canyon gage be- tween April 1996 and November 1997, not by a decrease in the sand-transport rates at the Grand Canyon gage. Thus the large sand inputs from the Paria River in 1997 had the effect of doubling the sand export rate from Marble Canyon. This sug- gests that during August-October 1997 (given the upstream flow and sediment boundary conditions that existed), the avail- able environments for storing sand in Marble Canyon were small relative to tbe magnitude of the sand supplied by the Paria River. At both the Lower Marble Canyon and Above Diamond Creek gages, coarsening of the suspended sand was accompa- nied by winnowing of the finer sediment from the bed, though this winnowing occurred in different manneTS at the two gages (Figures 7d and 7e). In evaluating changes in the grain-size distribution of the bed, it is sometimes useful to track changes in both the median grain size 01 the fine sediment (i.e., sand and finer material) and also the fraction of the fine sediment on the bed composed of sand finer than 0.125 mm. The frac- tion of the fine sediment on the bed oomposed of 0.0625- to O.l25-mm sand is a useful indicator of the state of the sand supply because half of the sand input by the Paria and uttle Colorado RiveTS is between 0.0625 and 0.125 mm (D. J. Top- ping, unpublisbed data, 1997). At the lower Marble Canyon gage, winnowing of the finer sediment from the bed caused an increase in the median size of the fine sediment on the bed (from 0.38 to 0.42 mm) and a decrease in the fraction of the fine sediment on the bed composed of 0.0625- to O.I25-mm sand (from 3.1 10 \.2%). The bed at this site continued to coarsen during the 5 days of moderately high flows (ranging from 447 m'/s (15,800 cfs) to 63\ m'/s (22,300 cfs)) following the 2-day lest flow. By November 11 the median size of the fine sediment on the bed had incrcased from 0.42 to 0.47 mm, and the fraction of the fine sediment on the bed composed of 0.0625- 10 O.l25-mm sand had decreased from \.2 to 0.67%. At the farthest downstream gage, the Above Diamond Creck gage, the bed sedimenl was also winnowed during the test flow -' ~ u: