Evaluating process domains in small arid granitic watersheds: Case study of Pima Wash, South Mountains, Sonoran Desert, USA

This paper provides support for the concept of geomorphic process domains developed by Montgomery (1999) by linking geomorphic processes to ecological variations seen in the Pima arid granitic watershed of the Sonoran Desert, Phoenix, Arizona. Closer joint spacing shows a statistically significant correlation with lower percentages of mineral grain attachment as measured by digital image processing of backscattered electron microscope imagery. Lower mineral grain attachment leads to more frequent spalling of rock surfaces, as measured by varnish microlamination (VML) ages of the last spalling event. In contrast, more distant joint spacing leads to in situ ¹⁰Be erosion rates of 3.4-8.5mm/ka and the emergence of low domes and kopje granitic landforms; these low domes also serve as knickpoints along ephemeral washes. Distant jointing thus plays a key role in generating the bare bedrock surfaces that funnel limited precipitation to bedrock margins - enhancing the canopy cover of perennial plants next to the bare bedrock. Joint-influenced geomorphic processes at Pima Wash generate four distinct process domains: (PD1) armored drainage divides; (PD2) slopes with different granite landforms; (PD3) mid- and upper basin channels that mix knickzones, strath floodplains, and sandy alluvial sections; and (PD4) the main ephemeral channel transitioning to the piedmont. Distant jointing promotes bedrock exposure and rock armoring along drainage divides in PD1 that then concentrates runoff and promotes perennial plant growth. More distant joint spacing on slopes in PD2 promotes exposure of granitic bedrock forms that shed overland flow to their margin and promotes flora and fauna growths along the margins of low granitic domes and kopjes. Similarly, wider joint spacing along ephemeral washes in PD3 leads to knickpoints, which in turn act to concentrate moisture immediately downstream. The stream terraces in PD4 influence the ecology through xeric desert pavements on terrace treads and roofs for coyotes (Canis latrans) and gray fox (Urocyon Cinereoargenteus) dens on terrace scarps via stage 3 pedogenic carbonate. These four process domains occur in six other randomly selected granitic watersheds with drainage areas <5km² in the Mojave and Sonoran Deserts. Results on rates of geomorphic processes in the Pima Wash watershed provide new insight in the desert geomorphology of small granitic watersheds. Catchment-wide denudation rates (CWDRs) recorded by ¹⁰Be sampled along the main ephemeral wash vary between 15 and 23mm/ka and do not appear to be influenced by knickpoint or knickzone occurrence; instead slightly lower CWDRs appear to be associated with sediment contributions by subbasins with more abundance of bare bedrock forms. Resampling for CWDR after a 500-year flood event from hurricane moisture at two sites along the main ephemeral channel revealed no detectable changes; this finding confirms the average representativeness of CWDR as a long-term denudation proxy and also means that sediment transport on these arid granitic hillslopes must be incremental and without rapid crest to wash transport. The first reported measurements of incision rates into a small granitic Sonoran Desert watershed, using ¹⁰Be and VML, reveal rates on the order of 70-180mm/ka in the lower quarter of Pima Wash for the last 60ka - producing a narrow and deep trench. As this base-level fall propagates upstream, erosion focuses on weaker material with higher joint densities; this facilitates the emergence of domes and kopje landforms with more widely spaced jointing.