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SOUTHERN ARIZONA 17 TABLE 3.-Channe/ modification in selected drainage basins, floods of September 1970-Continued Reach Tributaries in the northern Black Mesa area 18 FLOODS OF SEPT. 1970 IN ARIZ., UTAH, COLO., AND N. MEX. TABLE 3.-Channel modification in selected drainage basins, floods of September 1970-Continued Drainage basin Channel modification Northeastern Arizona_Continued Drainage basin Reach Channel modificati.n Moenkopi Wash Minor channel modification. The area in which coal is being strip mined received only moderate amounts of precipitation. The strip mining has not caused any noticeable effects on erosion and deposition in the area of the mines or in downstream areas. FOUl Cornen Area-Continued Chinle Wash Chinle Wash Minor channel modification. In general the flood was confined to the channel. In the Round Rock-Rock Point area, however, the channel is shallow and is bordered by wide 5.ft-high terraces, and the flood inundated an area more than 600 ft wide and deposited severa] inches of sandy silt and silty sand on the terraces. McElmo Creek between Cortez and Yellowjacket Canyon Minor to moderate channel modification. From I to 3 [t of cutting occurred at the falls at Moenkopi and at the falls 5 mi downstream from Moenkopi. As much as 2 ft of cutting occurred upstream from the falls at Moenkopi. Moenkopi Wash, Tuba City area Four Com~rs Area station on Sabino Creek near Tucson (fig, 6, No, 75), The flood washed out Forest Service roaos and damaged recreational areas along the creek; some of the recreational areas were closed to visitors for several weeks. The drainage basins adjacent to that of Sabino Creek had very little runoff, The flow from Sabino Creek was reduced considerably after it entered the sandy channel of Rillito Creek. McElmo Creek Slight channel modification. Low terraces and parts of a few low fields were inundated. Diversion intakes received minor damage. Yellowjacket Canyon upstream from its confluence with McElmo Creek The channel was widened, deepened, and cleared of brushy vegetation, and the edges of terraces were severely eroded. As much as I ft of silty sand was deposited on a 4- to 5-ft-high terrace. CENTRAL ARIZONA The intense rains in the mountains in central Arizona caused sudden large floods in the drainage basins of Tonto and Sycamore Creeks, East Verde River, Oak and Beaver Creeks, and Agua Fria and Hassayampa Rivers (fig, 7), The flood took the lives of 23 persons and was reported to have caused several million dollars worth of property damage, Peak discharges of several hundred cubic feet per second per square mile were measured at many gaging stations, The peak discharges at several stations probably exceeded the 100-year flood and are among the maximum known in the State (fig, 8), The flooding in the area from Buckeye to Scottsdale along the north side of the Gila and Salt Rivers was mainly sheetflow, High flQws occurred in the upper part of the Cherry Creek basin near the Mogollon Rim and in a small area on the east side ofthe Sierra Ancha, The flood peaks, however, are of magnitudes that occur frequently, At the south edge of the flood area, the peak flow at the gaging station on Tortilla Creek (fig, 7, No. 94) was 5,700 ft'ls (161 m'/s),.slightly less than that which occurred in 1966, The 1966 peak is the highest on this stream since 1941, Two people were killed in the 1970 flood when their vehicle was engulfed by a sharp rise of the creek as they attempted to cross the creek, The areal distribution of high peak discharges was about the same as that of the large amounts of precipitation except in several small watersheds in the Sierra Ancha and Mazatzal Mountains where rainfall was intense but peak discharges were low (A. R, Hibbert, U,S, Forest Service, Rocky Mountain Forest and Range Experiment Station, written commun" 1971). The high peaks in the large streams resulted when the flows from several Montezuma Creek, lower reach The channel was deepened and widened slightly, and many fresh scars are exposed along the banks. Large amounts of sJJty sand were deposjted on 5- to 8-ft-high terraces. At Aneth, the channel was aggraded, and a small delta was built into the San Juan River. From 2 to 3 ft of sand was deposited in the channel at the bridge on State Highway 262 west of Aneth. Montezuma Creek McElmo Creek downstream from its confluence with Yellowjacket Canyon Near Hatch Trading Post the flood was confined largely to the wide channel; however, the terraces and vegetation that border the channel were eroded severely. At the State Highway 262 crossing near Aneth, the flood inundated most of the bottom land. The highway fill served as a dike across the channel and diverted the flow into a cutoff channel. The flow was impounded temporarily before the highway washed out; large sand bars deposited by the flow clogged the channel between the highway and the cutoff channel. One bar was nearly 18 ft high and covered a dense stand of mature saltcedar on a low terrace. The sand bars caused flood peaks that reached a height of 18 ft upstream and 16 ft downstrea.m from the highway.