THE SPATIAL AND TEMPORAL DISTRIBUTION OF LARGE ROCK BLOCKS AND CONTROL ON LANDSCAPE EVOLUTION IN THE OZARKS

The Upper Buffalo River watershed, located in the northern Arkansas Ozarks, has slopes peppered with massive blocks detached from relatively flat-lying sandstone capping many of the ridgelines. These blocks are generally 5 m across, however, along slopes and in bounding channels below, blocks are as long as 30 m and can exceed 500 m³. The energy required to move blocks of this size down gentle slope gradients of ~ 5-10° is hard to conceptualize without invoking landslides or topple and/or tumble as dominant block transport mechanisms. By applying recent theories on block transport, we investigate the processes moving these massive blocks and test if local conveyor belt-like transport can indeed move blocks of such enormity. We hypothesize that most blocks travel via destabilizing landslides, with aspect being a potential control on temperature and soil moisture impacting cracking rates and landslide potential. To test transport hypotheses we combine field measurements, GIS analysis, and process-form model testing. As of July 2022, we collected data on 111 blocks in two watersheds on both north and south-facing slopes. Preliminary results show block distribution, strength (using R-values from a Schmidt hammer as a proxy for strength), and size vary with aspect and landslide presence. Blocks extend from ridgeline to channel only on south-facing slopes with the widest spatial distribution on a prominent landslide slope. On the north-facing slope, blocks are restricted to < 200 m just below the source caprock. Block size increases with distance on north-facing slopes, decreases with distance on south-facing slopes, while landslide blocks are more uniform in size (IQR 2.28 m). We find no correlation between block size distribution and hillslope form (e.g., convex, concave, or linear) on surveyed slopes, suggesting a re-evaluation of our current understanding of block control on hillslope form when blocks exceed 1 m in height and are unlikely to be overtopped by sediment. By better quantifying the mechanisms of large block production and transport, a crucial missing piece will be added to modeling hillslope-channel coupling and its control on river channel incision and thus landscape evolution of the southeastern Ozark Mountains.