7032 - Laserfiche WebLink

Home Browse Search

Orand Canyon of the Colorado River Wyoming Carf~ia "/ .~ \ ( New Mexico (, \ ( .,:,\ : {"" , , -:::--' ------ '" . , '" "" \ ( ~ ........ " ) I '':; . ~ - -- -7 - - -' Colorado River watershed Figure 2. The Colorado River watershed in the vicinity of the Grand Canyon. (USGS base map.) called beaches) were thus periodi- cally eroded and replen~shed with sediment. same time, elimination of high- water discharges has resulted ill the rapid development of dense flood- plain vegetation in areas which were formerly inundated (Fig. 3), and wind deflation is removing large quantities of fluvial sediment above the current high-water levels. With the present controlled flow, the higher terraces are no longer flooded, and the lower terraces and bars are eroding (see Fig. 1). At the Table 1. Pre- and post-dam statistics for the Colorado River Lees Ferry Gauging Station Grand Canyon Gauging Station Post- Post- Pre-dam dam Pre-dam dam Median discharge (efs) 7,400 12,200 8,200 12,800 Mean annual flood (cfs) 86,000 27,000 86,000 28,000 10-year recurrence interval flood (cfs) 123,000 30,000 122,000 40,000 Discharge equaled or exceeded 95% of time, based on average daily flows (cfs) 3,600 5,500 4,000 5,900 Median sediment concentration (ppm) 1,500 7 1,250 350 Sediment concentration equaled or exceeded 1 % of time (ppm) 21,000 700 28,000 15,000 Data based on U.S. Geological Survey records. 394 American ScieJitist, Volume 62 The environmental changes pres- ently occurring along the Colorado channel might elicit only limited interest if man's use of the river were to continue as it was during the. pre-dam era; however, the number of people taking Colorado River boat trips has increased dra- matically in recent years. The 200th person to make the river-run did so in the early 1950s; since then, more than 100,000 people have made the trip. Because of this increase in human traffic, conserva- tion groups and the National Park Service came to the conclusion that the river's "carrying capacity" might have been reached or per- haps exceeded, and in 1971 Grand Canyon National Park started lim- iting boat trips to approximately 10,000 persons a year (2). , Thus the two questions of impor- tance for management of this unique landscape are (1) In what manner and how rapidly is the Grand Canyon adjusting to the new river regime? and (2) Is the in- creased use of the river by man in- fluencing these adjustments? Pre-dam hydrology and fluvial morphology From the Glen Canyon Dam to Lake Mead, a distance of 280 miles, the Colorado River falls from 3,000 to 850 feet above sea level. The av- erage gradient is over 7 feet a mile, or about 25 times that of the Mis- sissippi (3). The 161 rapids of the Colorado, among the river's major visitor attractions, account for a significant amount of the decrease in elevation along its length. The twenty largest rapids, with drops of up to 40 feet, account for approxi- mately 20 percent of the fall be- tween Lees Ferry and Lake Mead. The depth of the river averages about 50 feet, and widths of 200 to 300 feet are common (4). The river is less than 80 feet wide at its nar- rowest point. Prior to construction of the Glen Canyon Dam, the Colorado River's mean annual flood height was about ten times the present median discharge (Table 1). Floods exceed- ing 100,000 cubic feet per second occurred every few years. The two highest floods of record were ap- proximately 300,000 cfs, in 1884,