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SOUTHERN ARIZONA . Drainage haJin TABLE 3.-Channel modification in selected drainage basins. floods of September 1970 Channel modlfication 'Reach Soatbem Arizona Los Robles Wash Sabino Creek Altar Wash Brawley Wash downstream from Three Points Sabino Creek Moderate channel modification. In the upper reaches the channel was deepened 1 to 2 ft, and some lateral cutting and bank caving occurred. Sand and gravel bars as much as 4 it high were formed in places, and logs were deposited in the channel. Considerable sand and gravel aC,cumulated in the channel near the bridge on State Hlghwa.r 286. which was washed out during the flood. A bar haVIng a maximum relief of 6 ft was deposited across most of the channel a quarter of a mile upstream from the bridge; here the sediment blockage was sufficient to cause the water to overtop the 9~, to to-ft-high terrace that borders the ~hannel. The terrace was formed prior to 1920, and this probably is the first time that it has been flooded. In the reach half a mile upstream from the bar 2 to 3 ft of sediment was deposited. In the reach from the bridge to Three Points the flood was confined in the deep narrow channel; only small amounts of sand and gravel accumulated in the channel, which was deepened about 2 ft in at least one place. Gen7rally, the, channel was aggraded slightly, and only minor scounng oCCUrred. Although only a small amount of gravel was transported into the area, a few inches of silt to silty sand was deposited on the flood plain. Minor to moderate channel modification' some scouring and movement of gravel. ' Centnl ArlzoNl Sycamore Creek Sycamore Creek upstream from Sunflower Major channel modification. The flow from the large tributaries deposited gravelly detritus in Sycamore Creek. This deposition and the removal of gravel resulted in alternating aggraded and degraded reaches. Downcutting was limited to about 3 ft because bedrock is at or near the surface of the channel in most o~ the reach. In places the shallow channel was almost filled by at least 4 ft of sediment. In general; lateral cutting was not severe except half a mile north of Sunflower, where about 50 ft of the right bank was removed. Sycamore Creek downstream from Sunflower Severe to moderate channel modification. Low terraces were eroded and v:egetation was removed from the channel. About 2 ft of fill was deposited at the washed-out bridge 5 mi south of Sunflower on State Highway 87. . East Verde River East Verde River Moderate channel modification. The flood cleared the channel of brush and loose rock and toppled many trees that were growing in the channel. Minor lateral cutting occurred in many places. 1 13 14 FLOODS OF SEPT. 1970 IN ARIZ., UTAH, COLO., AND N. MEX. TABLE 3.-Channel modification in selected drainage basins. floods of September 1970-Conrinued Drainage basin Reach Cbannel modification Central Arizona-Continued Weber Creek at Camp Geronimo Minor channel modification. Flow was not sufficient to move large boulders in the channel. Slight scouring occurred in a few places. Oak Creek Oak Creek Minor channel modification. A few debris slides occurred in Oak Creek Canyon. Beaver Creek Wet Beaver and Dry Beaver Creeks Most of the channel modification along Wet Beaver and Dry Beaver Creeks occurred where the streams debouch from the canyons. Scour holes several feet deep and as much as 300 ft long formed in places. In general the channels were deepened; however, in places cobble and boulder bars were deposited. Only a few of the pre-flood bars and scours were recognizable after the flood. In places lateral cutting was as much as 25 ft; where present, the roots of large juniper, pinyon, and ponderosa pine trees protected the banks from more severe erosion. The flood was less turbulent downstream and caused much less channel modification. Upstream from McGuireville, Dry Beaver Creek overflowed onto the nearby terraces and deposited a layer of reddish-brown sand about Ift thick. Downstream from McGuireville, as much as 3 ft of silty sand was deposited along the slopes and banks of the c~annel; many trees growing in the channel were uprooted and carried downstream. Agua Fria River Moderate to minor channel modification. Agua Ftia River upstream from Rock, Springs area Agua Fria River. Rock Springs area Major to moderate channel modification. The most severe channel modification occurred between the point where the river leaves its canyon and Rock Springs. Most of the sand and gravel transported through the canyon has been deposited in this reach and has formed a series of large bars and cut and fill terraces. The flood easily moved cobbly and bouldery gravel and severely eroded the bars and low terraces. The large gravel bars deposited upstream from Interstate Highway 17 caused the floodwater to spread over parts ofthe adjoining terraces.