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statistical comparisons. Thus for each period prior to and after 1950 (or 1966) the mean annual flood (or average peak discharge, Q,) and the mean annual flow (average annual discharge, Qj were calculated, and the significance of differences between pre- and post-development periods was evaluated using a T-test. Average annual hydrographs for pre- and post-development periods were also constructed by averaging daily values. These hydrographs illustrate differences in the timing and volume of runoff before and after the dams were constructed. 80 R G CL 70 R d 60 o? N? 50 c ?c oa 40 ?o do 30 Cd >a 20 io R E? 10 U Figure 4. Cumulative reservoir storage capacity as a percentage of annual streamflow for the Colorado River near Cameo, and the Gunnison River near Grand Junction. Data from Liebermann et al. (1989) and USGS Water Supply Papers. Sediment Loads Sediment measurements have been made routinely on the upper Colorado River at only a few locations, and then only in the last two decades. The U.S. Bureau of Reclamation (USBR) measured suspended sediment at the Cameo and Gunnison River gauges intermittently in the 1950s (Iorns et al., 1964). The USGS continued measuring suspended sediment at the Gunnison River gauge through 1965. To our knowledge, no further sediment measurements were made at these gauges until the late 1970s, when the USGS again began collecting sediment and water quality data regularly at these sites, and also at the State Line gauge. Nearly all of the sediment measurements that have been made on the Colorado River and Gunnison River are of the suspended load. In 1984, a few bed load samples were taken at a site near the town of DeBeque, CO, approximately 10 km upstream from the Cameo gauge (Butler, 1986). Most of the bed load was finer than 16 mm, which is approximately the median grain size of the subsurface bed material. On the basis of these measurements, Butler (1986) concluded that suspended sediment accounted for more than 98% of the total sediment load of the Colorado River, a point that we support later in this report. Average Bed Elevations Changes in average bed elevation were determined by compiling information from the archived field notes of USGS discharge measurements at the Cameo, Gunnison River, and State Line gauges. Among the many hundreds of discharge measurements that are available for these gauges, we selected three measurements in each year corresponding to pre-peak, peak, and post-peak time periods. The average bed elevation for these measurements was then calculated by taking the 1910 1920 1930 1940 1950 1960 1970 1980 1990