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Table 2. Dates and observations of the Colorado River being frozen at Lee's Ferry, Arizona, or other points along the river. Date Observations January 1866 January 15, 1878 December 31, 1879 January 13, 1898 late December, 1898 late December, 1911 January 1-2, 1925 1928 1933 River frozen at Crossing of the Fathers in Glen Canyon, allowing Navajo traders to cross on the ice (Webb, 1996, p. 97). River frozen for two weeks. Mormon missionaries crossed on the ice pulling wagons across (Webb, 1996, p. 97 and note 54; Reilly, 1999, p. 75). River frozen for one day, long enough for Mormon colonists to move animals and wagons across the river (Webb, 1996, p. 97). River frozen, R.B. Stanton rowed downstream and had to chop through the ice to reach Lee's Ferry (Reilly, 1999, p. 159-160). Animals were crossed over the river on the ice (Reilly, 1999, p. 170). Men crossed the river, frozen for several days, on foot (Reilly, 1999, p. 245). Navajo traders drove pack mules across the river (Webb, 1996, p. 97). The river was frozen for nearly a month (Reilly, 1999, p. 170). Completion of Navajo Bridge eliminates observations of the river freezing except in gaging station records (Webb, 1996, p. 225). Ice affected the gaging record at Lee's Ferry for 64 days, from December 1932 to February 1933. choppier and seemed to have extra force when hitting the boats and passengers. Many historical river trips experienced sandstorms, which are uncommon now. On his trips in 1937 and 1938, Buzz Holmstrom noted several of these, commenting that "windy all night - sand in everything this morning" (1937) and "sky so full of sand sun blotted out" (1938). Erosion of sandbars, as well as the increased size of sand in the ones that remain, minimize the potential for significant sandstorms along the river corridor at present. Nevills Staveley was impressed with the high degree of cooperation among guides now, instead of the competitive atmosphere that prevailed on her last trip. Rigg and Nevills Staveley remarked that the deterioration of air quality as a major change; Rigg stated that the sky was much haz~er now than when he ran the river and that it significantly degraded the experience. He also lamented the loss of the Bass cableway, which once spanned the river downstream of Bass Rapid (mile 108). Water Temperature The cold water released from the bottom of Lake Powell was a very noticeable change to most of the Old Timers, who generally experienced the warm water of summer on their river trips. Generally, the water temperature of releases at the dam are about 490F year round, meaning that the river is cold in summer and warm in winter. Historical observations of the river freezing over in winter (Table 2), are important to understanding changes in aquatic habitat as a result of the presence of Glen Canyon Dam as well as potentially understanding the effects of climate on the river. As discussed in Webb (1996), the river froze frequently in the late 19th century, occasionally with enough ice to allow the passage of wagons over its surface at Lee's Ferry (Table 2). Geomorphology The Colorado River in Grand Canyon has numerous debris fans, debris bars, and eddies that influence its longitudinal profile. The debris fans generally occur at the mouths of tributaries and create constrictions and rapids. Cobbles and boulders are reworked from the debris fan and transported downstream; these particles accumulate in debris bars that create secondary constrictions and rapids. The primary and secondary rapids are separated by a pool that typically has an eddy on one or both channel margins. Eddies form between the debris fan and debris bar or other downstream obstructions, such as bedrock walls. Sand bars are deposited on both the upstream and downstream sides of the debris fan, as well as on the downstream side of the eddy (Schmidt and Graf, 1990). All of these geomorphic features have changed historically (Webb, 1996), and were mentioned by the Old Timers. Debris Flows and Floods Two types of tributary floods occur in Grand Canyon (Melis, 1997; Melis and others, 1994). Debris flows are slurries of water and poorly sorted sediment that transport large boulders into the Colorado River, creating rapids. The sediment concentration of debris 8 OBSERVATIONS OF ENVIRONMENTAL CHANGE IN GRAND CANYON