Progress in boulder-mantled landscapes: A Sisyphean challenge

Sediment grain size distribution is the product of, and an important influence on, surface processes. Grains can be large enough that the motion of a single grain, infrequently mobile in size-selective transport systems, constitutes or triggers significant geomorphic change. We define these grains as boulders, and review the literature to address five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution? 2) How do boulders influence hillslope processes and long-term hillslope evolution? 3) How do boulders influence fluvial processes and river channel shape? 4) How do boulder-mantled channels and hillslopes interact to set the long-term form and evolution of boulder-influenced landscapes? 5) How do boulders contribute to geomorphic hazards, and how might improved understanding of boulder dynamics be used for geohazard mitigation?

Boulders are produced on hillslopes by landsliding, rockfall, and exhumation through the critical zone. On hillslopes dominated by local sediment transport, boulders affect hillslope soil production and transport processes such that the downslope boulder size distribution sets the form of steady-state hillslopes. Hillslopes dominated by nonlocal transport are less likely to exhibit boulder controls on hillslope morphology as boulders are rapidly transported to the hillslope toe. Downslope transport delivers boulders to rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, rivers adjust their shape to enable erosion at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels drives channel-hillslope feedbacks that affect landscape evolution and steady-state form. Boulders cause geomorphic hazards that can be mitigated with an improved understanding of boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in tectonics, climate, and lithology with the goal of understanding boulder dynamics as one component of landscape self-organization.