RONALD I. DORN Impact of consecutive extreme rainstorm events on particle transport: Case study in a Sonoran Desert range, western USA A KATHY LIPSHULTZ FULL-LENGTH FEATURE PRESENTATION

Purpose Analyze how particles (“grus”) detach from granitic crests during typical and extreme precipitation events Broaden understanding of processes and rates of soil erosion, specifically in arid environments

Locality Sonoran Desert range, western USA

Field Area Specifics Weather  Average annual precipitation = 20.8 cm  Aug., Sep = and 500-year precipitation events Vegetation  Desert trees, scrub bushes, and succulents (cactus) Geology  Granitic lithologies  Irregularly-shaped steep bedrock slopes and smooth grus- covered landforms

Methods Strontium isotopes  Assess nature of suspended load by determining where sediments came from using 87 Sr/ 86 Sr ratios (is this externally- sourced dust?) Pits  Assess the state of granitic decay by digging pits into bedrock and observing weathering Sediment traps (28, 7 per ridge crest)  Record amount of bedload grus detached from rain  Traps set prior to every predicted precipitation event for 3 yrs

Sediment traps

Results Strontium isotopes  Silicate in summer dust and suspended sediment from same source  Leachate from grus grains = mix of granitic grus and dust silicate Pits  Biotite hydration and oxidation causes cracking  Cracks allow water to enter, increasing mineral porosity Sediment traps  Rains of 2mm or less = no transport  Rainfall rates of 36 mm/h -> significant increase in detachment

Conclusions Suspended load from hill crest to trap is mostly eolian dust, NOT granitic bedrock Subsurface decay (dissolution, chemical alteration) responsible for grus detachment in pits Higher amounts of exposed soil, larger drainage areas, and steeper slopes increase grus detachment and transport Detachment increases non-linearly with precipitation intensity