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3 c: 2 0 .~ '3- -~ ~ "- OJ :J c: .i al 0 N 'E '" " c: '" /i5 -1 -2 o o '" o '" '" ~ o '" '" o ~ '" ~ o ... '" o CD '" o t- '" ~ o <Xl '" ~ o '" '" o o o '" o "' '" Figure 3. Standardized annual precipitation for the Grand Canyon region from 1904 through 1998 based on data from 14 weather stations (table 3). and match estimates of suspended load and total load. We assume that in most of the drainages concerned, the difference between the two loads, bed load, is quite small, and certainly well within the range of our final estimates, A plot of regional data of mixed type (fig. 6; suspended load from gaging stations and total load from reservoirs) supports this view. Regional Sediment-Yield Data Other than at gaging stations on the Colorado River and its major tributaries, few sediment- transport data have been collected in Grand Canyon (Garrett and others, 1993; Rote and others, 1997). Therefore, streamflow-sediment yield must be estimated empirically_To develop a regression equation of sediment yield versus drainage area and to determine which other sediment-yield estimation techniques might be appropriate, we assembled sediment-yield data for the region, Sediment loads at gaging stations on the pre- dam Colorado River, its major tributaries, and small drainages Sugrst a regional sediment yield of 105- 820 Mg km- yr-I (table 4), These yields assume minimal long-term change in storage (Graf, 1987). The sediment yield of the Paria River is 820 Mg km-2yr-l, which is high for the Grand Canyon region, The sediment yield of 134 Mg km-2yr-1 for the Little Colorado River, measured at a gaging station upstream from most of the typical bedrock units exposed in Grand Canyon, is possibly low, Given some similarities in Mesozoic hedrock lithology, the sediment yield of 155 Mg km-2yr-1 for Moenkopi Wash near Tuba City (table 4) may be appropriate for estimating sediment yield in upper Marble Canyon (upstream from mile 17), but the record length for this station is only 3 years, Andrews' (1991) estimate (table 5), based on the difference in gaged sediment load in the Colorado River between Lee's Ferry and river mile 87 (Grand Canron gage), is considerably higher at 2,130 Mg km- yr-I, Andrews (1991) assumed that the additional yield is sand eroded from the bed of the Colorado River. On the basis of a range in drainage area most comparable with that of Grand Canyon tributaries (fig. 2a), the most appropriate data are sedimentation data from small reservoirs in northeastern Arizona (Fort Defiance region of the Navajo Indian Reservation; Hains and others, 1952) 10 Sediment Delivery by Ungeged Trlbuterles of the Colorado River In Grand Canyon