Strath development in small arid watersheds: Case study of South Mountain, Sonoran Desert, Arizona

Analyses of ephemeral granitic drainages of <5 km² at South Mountain metamorphic core complex, central Arizona, reveal a previously undocumented process of bedrock strath formation in this setting. Granitic channel banks experience a higher degree of mineral decay than that of granitic channel floors. Electron microscope observations show that grussification along the granitic channel banks occurs through abiotic processes of biotite oxidation and biotic processes associated with mycorrhizal fungi and roots of plants preferentially growing along channel banks. Digital image processing of backscattered electron microscope (BSE) images measured: (a) an enhancement of porosity along channel banks 2x to 5x greater than mid-channel positions; and (b) the gradual separation of grains over a 13-year period caused by the roots of Paloverde (Parkinsonia microphylla) trees. Ongoing mineral decay along banks facilitates differential erosion similar to Montgomery's (2004) hypothesis. Ephemeral washes migrate laterally into the decayed granite of their banks and erode the distal end of bounding pediments, expanding beveled bedrock straths. Direct observations of strath widening in six drainages during three distinct flash floods reveal a range from 4 to 23 millimeters of lateral bank erosion and <1 mm of channel bed abrasion. The widening of straths is likely limited by long-term rates of in situ physical separation of granitic minerals.