Relations between rainfall–runoff‐induced erosion and aeolian deposition at archaeological sites in a semi‐arid dam‐controlled river corridor

Abstract

Process dynamics in fluvial-based dryland environments are highly complex with fluvial, aeolian, and alluvial processes all contributing to landscape change. When anthropogenic activities such as dam-building affect fluvial processes, the complexity in local response can be further increased by flood-and sediment-limiting flows. Understanding these complexities is key to predicting landscape behavior in drylands and has important scientific and management implications, including for studies related to paleoclimatology, landscape ecology evolution, and archaeological site context and preservation. Here we use multi-temporal LiDAR surveys, local weather data, and geomorphological observations to identify trends in site change throughout the 446-km-long semi-arid Colorado River corridor in Grand Canyon, Arizona, USA, where archaeological site degradation related to the effects of upstream dam operation is a concern. Using several site case studies, we show the range of landscape responses that might be expected from concomitant occurrence of dam-controlled fluvial sand bar deposition, aeolian sand transport, and rainfall-induced erosion. Empirical rainfall-erosion threshold analyses coupled with a numerical rainfall-runoff-soil erosion model indicate that infiltration-excess overland flow and gullying govern large-scale (centimeter-to decimeter-scale) landscape changes, but that aeolian deposition can in some cases mitigate gully erosion. Whereas threshold analyses identify the normalized rainfall intensity (defined as the ratio of rainfall intensity to hydraulic conductivity) as the primary factor governing hydrologic-driven erosion, assessment of false positives and false negatives in the dataset highlight topographic slope as the next most important parameter governing site response. Analysis of 4+ years of high resolution (four-minute) weather data and 75+ years of low resolution (daily) climate records indicates that dryland erosion is dependent on short-term, storm-driven rainfall intensity rather than cumulative rainfall, and that erosion can occur outside of wet seasons and even wet years. These results can apply to other similar semi-arid landscapes where process complexity may not be fully understood. Published 2015. This article is a U.S. Government work and is in the public domain in the USA

Geomorphic process from topographic form: automating the interpretation of repeat survey data in river valleys

“…In Grand Canyon and many other canyon‐bound river valleys of the world, floods deposit sediment (primarily sand; Topping et al , ) in sandbars in the lee of tributary debris fans (Schmidt, ; Schmidt and Rubin, ). A portion of this flood sediment is returned to the main channel by fluvial bar erosion, sheetwash, and gullying (Sankey and Draut, ; Collins et al , ; East et al , ). Aeolian reworking of bars also transfers fine sediment to upland dunes and hillslopes (Figure B; Draut, ; East et al , ), defined here as surfaces within the valley bottom that are above the stage of regularly‐occurring floods (approximately 1270 m 3 /s; Grams et al , ).…”

“…High‐resolution repeat topographic survey data have become ubiquitous in the earth sciences (Passalacqua et al , ). The advent of rapid survey techniques such as LiDAR (light detection and ranging; Collins et al , ) and structure‐from‐motion photogrammetry (Westoby et al , ; Javernick et al , ; Smith et al , ) have occurred concomitantly with reductions in the cost and complexity of collecting survey data (see for example, Eitel et al , ). As a result, geomorphologists are able to readily characterize landscape change by contrasting repeat topographic surveys, a process termed Geomorphic Change Detection (GCD; Wheaton et al , ).…”

“…GCD provides information about the extent, magnitude, and form of landscape changes occurring during an inter‐survey period, providing vital insight to geomorphologists seeking to understand, for example, the overall sediment budget of an area (Grams et al , ), or the spatial distribution of changes through time (Collins et al , ). At the same time, the analysis of repeat topographic data alone does not attribute changes to specific geomorphic transport processes; that is, the reasons why topographic change occurred are not necessarily revealed.…”

“…Wheaton et al , ) is typically completed via user‐defined attribution of change areas (e.g. Collins et al , ), a process that is both time consuming and subjective.…”

“…The relative contribution of each transport pathway to the total valley‐scale sediment flux varies between river systems (Trimble, ; Belmont et al , ; Kasprak et al , ), and may shift in response to altered water or sediment regimes driven by changes in climate or land use (Draut, ; Sankey and Draut, ). GCD and mechanistic segregation of transport processes in digital elevation models (DEMs) and associated DEMs‐of‐Difference (DoDs) provide a means to quantify the relative contribution of transport processes and their variability in space and time (Wheaton et al , ; Collins et al , ).…”