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The 1937 and 1995 aerial photographs are slightly different in scale (1:32,000 and 1:20,000, respectively), and were taken when the river was flowing at different discharges (55 m3/s in 1937 and 79 m3/s in 1995). The higher discharge in the more recent photographs complicates the analysis slightly because, at higher flows, the channel would appear larger than it would for an equivalent or lower flow. However, the error introduced by different flow levels is very small in this case- probably much less than 5% (Van Steeter, 1996)- and even so, the higher discharge occurs in the more recent set of photographs. This means that changes in planform area since 1937 are probably underestimated by a small amount. Further details of the procedures involved in developing similar GIS data bases are discussed in detail by Pitlick et al. (1999). Sediment Loads Annual suspended sediment loads were estimated for various time periods using streamflow and sediment data from the following USGS gauging stations: Colorado River at Glenwood Springs, CO (station 9072500), Colorado River near Cameo, CO (station 9095500), Colorado River near CO-UT state line (station 9163500), and Colorado River near Cisco, UT (station 9180500). We recognize that Glenwood Springs is well upstream of the study area, but we include this station in the analysis because it illustrates how abruptly the sediment load of the Colorado River increases downstream. The hydrologic data from these four gauging stations are of high quality, with record lengths varying from 45 to 100 years. The streamflow record from the Glenwood Springs gauge is the longest, beginning in 1898, but complicated by the fact that in 1966 the gauge was moved to a point downstream of the Roaring Fork River. The records for these two gauges were combined by subtracting daily discharges of the Roaring Fork River, which is also gauged, from the same-day discharges of the Colorado River. The flow record at the Cameo gauge (at RKM 322) begins in 1934 and is continuous through the present. There is another gauge near the town of DeBeque (station 9093700, at RKM 343); however, since this gauge is only 20 km away from the Cameo gauge, and there are few tributaries in between, we have not used these data. The State Line gauge has been in operation since 1951; this gauge was initially located at RKM 224 in Ruby Canyon, but in 1979 it was moved to its present location at RKM 216. The Cisco gauge was established near the Dewy Bridge (RKM 152) in 1914; the record from this gauge is continuous from 1922 through the present. Suspended sediment samples have been taken at each of these gauging stations, however, reliable data are available only since about 1950, and the amount of data collected at individual stations varies considerably. Approximately 80 suspended sediment samples were taken at the Glenwood Springs gauge from 1951-1965 (Iorns et al., 1964). To our knowledge no additional samples have been taken since then. About 50 suspended sediment samples were taken at the Cameo gauge between 1951 and 1954 (Iorns et al., 1964), and several hundred more samples have been taken since 1983 (USGS Water Supply Papers). Over 100 suspended sediment samples have been taken at the State Line gauge since 1977 (USGS Water Supply Papers). Sediment sampling began at the Cisco gauge in 1930 (Iorns et al., 1964), however, there is some question about the representative- ness of samples taken prior to 1950, when a different type of suspended sediment sampler was in use (Topping et al., 1996). Given this concern, we limited our analysis of the Cisco sediment data to samples taken after 1950; since then, several hundred suspended sediment samples have been taken at the Cisco gauge. Suspended sediment loads were estimated for each of these gauging stations by developing a series of sediment rating curves, or empirical relations between water discharge and suspended sediment load. Such curves allow us to estimate daily suspended sediment loads (which are measured only intermittently) from daily discharges (which are measured continuously). The rating curves were 10