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SOUTHERN ARIZONA 15 TABLE 3.-Channel modification in selected drainage basins, .floods of September 1970-Continued Drainage basin Channel modification Reach Central Arizona-Continued Agua Fria River- Continued Agua Fria River, Rock: Springs area- Continued [Between Rock Springs and Lake Pleasant, the flow was less turbulent, a.nd channel changes were not significant. Downstream from the confluence of the Agua Fria River' and Black Canyon Wash the flood inundatedJhe bottom land and deposited 1 to 3 ft of silty sand on a terrace that is 8 to 10 ft above the riverbed.] Black Canyon Wash Major to moderate channel modification. The sudden breaching of the giant log jams in Turkey Creek probably caused the high peak discharge in Black Canyon Wash. In places the high flows severely eroded the channel and adjacent terraces and removed brush and trees. New River near New River Moderate channel modification. Some scouring and minor lateral erosion occurred, gravel bars were rearranged, and the channel was cleared of vegetation. New River downstream from New River Moderate to minor channel modification. Hassayampa" River Ha~sayampa River and tributaries, Wagoner area Channels and adjoining terraces were severely eroded along the Hassayampa River downstream from the mouth of Milk Creek and along Ash, Blind Indian, Cottonwood, and Spring Creeks and in Crooks Canyon. Small flows that caused only slight channel modification occurred in Fools Canyon, along Minnhaha Creek, and along the Hassayampa River upstream from Milk Creek. An average aggradation of about 2 ft occurred along the 300-ft-wide channel of Milk C~ek. In places the flood in Blind Indian Creek covered an area 700 ft wide and built large bars, some of which extended from the stream channel onto the bordering low terraces. The Hassayampa River deposited a large amount of sandy material in a wide area of bottom land near, Wagoner. Hassayampa River. Box Canyon damsite area Channel and adjoining terraces were severely eroded and modified. Although the amount of downcutting in Box Canyon could not be determined precisely, it probably was between 2 and 3 ft and not more than 5 ft. Downcutting did not take place immediately downstream or upstream from Box Can.yon. Hassayampa River, Wickenburg area Moderate channel modification. Most of the low broad terraces that adjoin the channel were inundated; the terraces are as much as a quarter of a mile wide. The flood deposited large amounts of sand in the chanJ?e1 and 6 FLOODS OF SEPT. 1970 IN ARIZ., UTAH, COLO., AND N. MEX. TABLE 3.-Channel modification in selected.drainage basins, floods of September 1970-ContlDued Drainage basin Reach Channel modification Central Arlzona-ContlJiued Hassayampa River, Wickenburg area- Continued Hassayampa River downstream from Morristown on the low terraces. As much as 4 ft of san? and silt was deposited in the reach between ,3 and 5 ~l dow~stream from Wickenburg. Although.thenewly laid deposits ~ere scoured during the waning s~age of the flood, httle erosion of the pre-flood deposits occurred. Moderate to minor channel modification. Most of the adjoining terraces were inundated. NortheastemArIzooa Puerco River Puerco River Polacca Wash Wepo Wash between Pinon and Polacca Oraibi Wash Oraibi Wash, Black Mesa area Minor channel modification. Some bank cavins. and slumping. The 1970 flood caused additional deepenmg of the channel 2 mi east of Gallup, N. Mex.; between 1901 and 1946 the channel was deepened 14 ft (Leopold and Snyder, 1951, fig. 2), between 1946 and 1965 2 ft, between 1965 lind 1969 1 ft, and between 1969 and 19711 ft. The Puerco River moved large amounts of sand, most .of which was deposited a few miles downstream from Its mouth in the Little Colorado River near Holbrook. Severe channel modification. More than 5 ft of s1ump~g and lateral cutting of the arroyo walls occurred, In places. Most of the erosion was ,along sharp bends. The channel was cleared of vegetatIOn. Erosion was severe where Oraibi Wash is entrenched in a deep narrow arroyo. North and northwest of Pirion large-scale slumping of the arroyo walls and ~s much as 40 ft of lateral cutting occurred. Vegetation was removed or bent over. As much as 3 ft of sandy silt was deposited on terraces in the protected parts of the channel. The effects of the fIoo~ ~ere moderate where the arroyo is wide, su~h as at Oralbl. About I ft of sand and silt was depoSited on a well-de....eloped IOIh~ft-high terrace that supp~rts trees and bru~h; 2 to 3ft of sediment accumulated 10 the channel to. places. Downstream from Oraibi, large-scale lateral cuttlDg and slumping .occurred' in the arroyo walls, and blocks of unconsolidated alluvium more than SO ft long and 20 ft thick slid into the channel. In places the channel was deepened as much as 4 ft. Dinnebito Wash Dinnebito Wash, Black Mesa area in the narrow arroyo of less severe but similar to Channel modification Dinnebito Wash was that in Oraibi Wash.