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the Lees Ferry gaging station (Glen Canyon reach) and a separate injection at the Lees Ferry gage and sampling at locations down stream from Lees Ferry (Grand Canyon reach), In each case, the injection at Lees Ferry'was made two days hefore the correspond- ing injection at the dam to avoid commingling of the injections, Established techniques for estimation of dye dosage, sampling, and laboratory analysis of dye sam- ples were used for this study (Wilson et ai" 1986; Kil- patrick and Wilson, 1989), For the Glen Canyon reach, a dosage of 21.5 kg of dye was used for the 1989 measurement and 9,1 kg of dye was used for both 1991 measurements, For the Grand Canyon reach, a dosage of 127 kg of dye (635 kg of 20 percent stock solution) was injected in the first measurement, Very low dispersion rates kept peak concentrations higher than estimated during that measurement; therefore, half the amount of dye - 63,5 kg - was injected for the second measurement, Dye was injected as a line source over a period of a few minutes, For the 1989 measurement in the Glen Canyon reach, dye was divided into four equal parts and poured into the river simultaneously from the transfonner deck of the dam, For the 1991 measure- ment in the Glen Canyon reaches, dye was poured from a boat as the boat moved across the center part of a cross section of the channel just downstream from the dam, For the Grand Canyon reach, dye was poured from a raft as the raft passed through the cen- ter two-thirds of the flow in the cableway section at Lees Ferry, Tracer Sampling and Sample Analysis. Most samples were collected by dipping a sample bottle just under the surface near the stream bank or tossing a bottle in a sample holder into the flow a short dis- tance from the bank, Samples were collected where velocity was judged to be evenly distributed across the channel, An automatic sampler (Kilpatrick, 1972) was used to collect samples over much of the dye cloud at Pumpkin Springs, For measurements in the Glen Canyon reach, samples were collected from the center of flow from cableways at gaging stations Colorado River below Glen Canyon Dam (09379910) and Col- orado River at Lees Ferry (09380000) (Figure 1). For the May 1991 measurement, near-surface dip samples were coUected from a boat with a hand sampler from three points across the channel. Dye was sampled at seven sites in the Grand Canyon reach during the steady-flow measurement (Figure 1) and at six sites during the unsteady-flow measurement, The leading edge of the dye cloud was not sampled at the three downstream sites during the unsteady-flow measure- ment, WATER RESOURCES BULLETIN Graf Filter fluorometers were used to measure dye con- centration in the field to pennit adjustment of sam- pling interval and to ensure that sampling continued until dye was past the site. Samples were collected in glass vials and transferred to the U,S, Geological Sur- vey laboratory in Tucson, Samples were remeasured under constant temperature conditions in the labora- tory, An equivalent background concentration of from 0,01 to 0,14 11g/l, resulting from suspended material rather than dye fluorescence, was established at most sites from measurements of samples collected before arrival of the dye cloud, The background concentra- tion was subtracted from concentrations measured in the laboratory to give the dye concentrations used in the analysis, Data Analysis. The weight of injected dye recov- ered at each sampling site during steady flow was computed by multiplying the area under the time- concentration curve by the discharge during sampling and a factor to correct the units, The computation showed that more than 90 percent of dye injected was recovered at sampling sites, Rhodamine WT dye decomposes in sunlight and adheres to sediment, Losses in the range of 30-50 percent have been mea- sured in other rivers (Graf, 1986), Initial estimates at two sites sampled during the unsteady flow measure- ment indicate that dye Joss was greater during unsteady flow, Greater loss may be attributed to stranding of dye in eddy zones when stage dropped, Because recovery of dye was very high, concentrations presented in this report have not been adjusted to account for dye losses, Curves of dye concentration as a function of time at a sampling site were plotted, and the first three moments of the distributions were computed using numerical integration, The first moment is dye-cloud centroid - the rate of travel of the centroid gives velocity through the measured reach, The second and third moments - variance and skewness - are mea- sures of the dispersion or spreading of the dye cloud, A quantity called unit-peak concentration was defined in previous studies (Kilpatrick and Taylor, 1986; Hubbard et ai" 1982) to compare results from different measurements, Unit-peak concentration is a peak concentration that has been normalized for dis- charge, amount of dye injected, and dye loss during measurement, For this study, unit-peak concentration was computed by dividing the measured peak dye concentration by the weight of dye injected and multi- plying by the discharge, Dye loss was insignificant during each measurement, and no adjustment of con- centration for loss was required, Discharges at each gaging station were detennined from recorded stage and a stage-discharge relation - 270