Abstract
A landscape’s sediment grain size distribution is the product of, and an important influence on, earth surface processes and landscape evolution. 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. Boulders affect landscape evolution; their dynamics and effects on landscape form have been the focus of substantial recent community effort. We review progress on 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 eroding hillslopes by landsliding, rockfall, and/or 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 sediment 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 eroding rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, river channels adjust their geometry to accommodate the boulders supplied from adjacent hillslopes such that rivers can erode at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels, paired with the channel response to boulder delivery, drives channel-hillslope feedbacks that affect the transient evolution and steady-state form of boulder-influenced landscapes. At the event scale, boulder dynamics in eroding landscapes represent a component of geomorphic hazards that can be mitigated with an improved understanding of the rates and processes associated with boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in landscape boundary conditions (tectonics, climate, and lithology) with the goal of understanding boulder dynamics as one component of landscape self-organization.
Highlights
Illuminating Earth’s history—and predicting its future—requires a thorough understanding of the processes that shape landscapes and deliver sediment from source to sink
We focus on synthesizing work that treats boulder production, transport, and influence over landscape evolution on hillslopes and in river channels, as well as the hazards that stem from the presence of boulders
This review focuses on recent progress on the following guiding questions, each of which corresponds to a section of the paper: 1. What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution?
Summary
Illuminating Earth’s history—and predicting its future—requires a thorough understanding of the processes that shape landscapes and deliver sediment from source to sink. When the largest grains in the grain size distribution (GSD) measure meters to tens of meters across, trans port is often infrequent enough that the largest grains remain immobile over human observation timescales—and potentially much longer—in all but the most energetic environments (Fig. 2). These largest grains, which we call “boulders” without implying a specific size or shape, have long intrigued geomorphologists (e.g., Gilbert, 1877; Hack, 1965; Judd and Peterson, 1969; Rodine and Johnson, 1976; Beaty, 1989) who speculated about the production, transport, and degradation of large, Fig. 2. In this paper we review recent work with the goal of synthesizing observations and models and identifying fruitful avenues for future research
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