The geometry of arroyos on alluvial fans of the Buckeye, Arizona, area is influenced by an impervious, indurated calcic soil (calcrete) at or near the surface of the fans. Channel width-depth ratio, cross-sectional area, gradient, and location of erosion and deposition in the channel are related to calcrete in the bed and banks of the channels.
Calcrete causes three or four sequential types of channel reach: (1) In the headward part of the channel, the bed is on calcrete and banks are in alluvium. The cross-sectional area is small and the width-depth ratio is high. (2) Bed and banks are developed completely in calcrete in reach 2. Cross-sectional area is high and width-depth ratio is low. Gradual incision into the calcrete results in gradient steepening and nickpoints. Reach 2 is not present if incision through the calcrete occurs as one nick-point, thus resulting in sequential reaches of one, three, and four. (3) In reach 3, bed and lower banks are developed in alluvium, and calcrete caps the banks. Here the cross-sectional area is high and width-depth ratio is moderate to high. (4) Lateral undercutting of the calcrete and deposition of topographically lower inner terrace deposits, or berms, on the slip-off slope mark the beginning of the fourth type of reach. The consequence of calcrete destruction and berm formation in the banks of reach 4 is an increase in width-depth ratio and a marked decrease in channel cross-sectional area, resulting in a change from confined to over-bank flow.