CHARACTERIZING ARID-REGION ALLUVIAL FANS WITH AIRBORNE LASER SWATH MAPPING DIGITAL TOPOGRAPHIC DATA

Range-front alluvial fan deposition in arid environments is episodic and results in multiple generations of piedmont landforms. These distinct depositional units are often defined by qualitative descriptions of their surface morphology, desert varnish accumulation, clast rubification, desert pavement formation, soil development, and stratigraphy. Although quantifying the differences between alluvial fan units has proven difficult in the past, high-resolution airborne laser swath mapping (ALSM) digital topographic data are now providing researchers with an opportunity to study landforms in unprecedented detail. ALSM data, which is generally on the order of ≤ 1 m in the horizontal and 10 to 20 cm vertically, allows the construction of highly detailed digital elevation models, from which a number of topographic indices can be extracted. Surface roughness, often the most obvious characteristic distinguishing fans of different age, has traditionally been one of the hardest metrics to quantify. Here we use ALSM data to calculate surface roughness on two alluvial fans in northern Death Valley, California. We define surface roughness as the standard deviation of slope in a three-meter by three-meter moving window. Nearly 73,000 individual surface roughness values were randomly extracted from eight distinct lithostratigraphic units. Comparison of surface roughness values between mapped fan surfaces shows that each unit is statistically unique at the 99% confidence level. Furthermore, there is an obvious smoothing trend from the presently active channel to a deposit with a cosmogenic ¹⁰Be surface exposure age of ~70 ka. Beyond 70 ky, alluvial landforms become progressively rougher with age. These data suggest that alluvial fans in arid regions smooth out quasi-exponentially with time until a threshold is crossed where steep channel walls begin to diffuse into convex hillslopes, and surfaces become rougher again with increasing age.