ABSTRACT
Processes of alluvial fan aggradation in southwestern Arizona were analyzed for 12 drainage basins (1–300 km2) and their associated ephemeral washes. Areas of net deposition of sediments along ephemeral washes may occur locally in incised reaches and extensively in reaches that are not incised below the fan surface. Incised reaches of washes contain topographically low channels floored with coarse grained sediments. These low channels are flanked by berms of fine grained sediment. Deposition in incised reaches occurs by lateral extension of coarse channel sediments over berm sediments producing a local maximum channel width. The width of the channel where this deposition occurs is proportional to the distance from the headwaters divide of the wash (Lc) and the drainage area (Ad) above the maximum channel width. The distance from the head of the wash and drainage area above the maximum channel width are related by the expression, Ad = 2.5 Lc 1.1.
Aggradation outside the low channels occurs as deposition of fine grained berm sediments. These sediments are deposited as water is lost by infiltration into coarse channel sediments. Berm sediments extend laterally and coalesce over low interfluves in distal regions of the fans. The drainage area (Ad) and wash length from the drainage divide (Lb) above this berm aggradation are related by the power function, Ad = 0.138 Lb. The source area of sediment provided for berm aggradation is the interfluves in the upper drainage basin. Berm aggradation is directly related28 to higher values of mean interfluve width of a basin, or larger source areas.
All ephemeral washes and drainage basins in southwestern Arizona which display channel deposition and berm aggradation obey the two mathematical expressions given. Thus, these two power functions may be used for predicting areas of net deposition and associated sediment types on alluvial fans, by relating physiographic characteristics of drainage basins to clastic deposition in southwestern Arizona.
