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100.000 """ .s 10,000 ~ , "0 > ~ u: ~ '0 ~ 1.000 . - -- - .' . . 100 0.1 . .....'- - -- . . . I . . . . . . Debris-Flow Volumes -V =11,808 A"",R'=0.8 mu - -.V :::5728 A0278.R2=O.47 ... Drainage Area (km2) 10 100 1000 Figure 13. The relation of tributary drainage area to debris-flow volume for 30 historical debris flows in Grand Canyon, with linear regressions of both maximum and average debris-flow volumes. large sample sizes are required. Representative samples of Grand Canyon debris-flow deposits for laboratory sieving cannot be easily collected because of a prohibitively large sample weight. Therefore, we used several methods in combination with sample collection to estimate the particle-size distributions of Grand Canyon debris flows. Point counts Point counts (Wolman, 1954; Rice and Church, 1996) were used in conjunction with sieve analysis to describe the particle-size distributions of most debris-flow deposits (Melis and others, 1994). We stretched tape measures over the surfaces of the debris-flow deposits to form a sampling grid. The length of a transect ranged from 50 to 100 m, and the spacing between tape measures was 0.5 to 2.0 m. At preselected intervals, which ranged from 0.5 to 2.0 m, depending on the size of the largest particles on the surface, we measured the intermediate (b-axis) diameter of the particle directly beneath the tape. Particles were not double- counted; if the same par1icle was measured twice, the second measurement was discarded. Measured intermediate (b-axis) diameters were aggregated into single 4> categories (e.g., -I to -24> (2 to 4 mm)). One hundred to four hundred particles were measured during each point count. Use of 100 particles in each point count theoretically results in standard errors of estimate of less than :1:20 percent (Rice and Church, 1996). Pit excavations At "Crash Canyon" (mile 62.6-R), Tanner Canyon (mile 68.5-L), and Prospect Canyon (mile 179.3-L), we directly measured par1icle size in pit excavations. We excavated a I m3 volume into recent debris-flow deposits. All par1icles >64 mm were either weighed in the field or the particle was assumed to be an elliptical solid and its weight was calculated, assuming a density of 2,650 kg/m3 for limestone and sandstone and 2,700 Kg/m3 for basalt. We retained at least I kg of the remaining particles for laboratory sieve analysis. Dry,sieve analysis We collected large, representative samples of debris-flow deposits for sieve analysis in conjunction with point counts or pit excavations. The amount and size fraction of the sample depended on the extent of the deposit and the logistics of transporting the sample. To complement the point counts, we typically 30 Sediment Delivery by Ungaged Trlbutarla. of the Colorado River In Grand Canyon