RESEARCH FOCUS RESEARCH FOCUS: Infrequent, large-magnitude debris flows are important agents of landscape change Scott W McCoy* Department of Geological Sciences and Engineering, University of Nevada, Reno, Reno, Nevada 89557, USA How do surface processes shape the landscapes in which we live? et al., 2013), and (2) millennial-scale denudation rates determined from Is it the every-day flow of rivers that gently, yet persistently, erodes and the concentration of 10Be in sediment (Granger et al., 2013). By differ- transports sediment from highlands to ocean basins, dissecting the land encing the digital elevation models obtained before and after the event, surface into networks of ridges and valleys? Or is it cataclysmic events Anderson et al. were able to quantify the net volume of sediment evacu- of incredible magnitude that, despite their infrequency, conspire to shape ated from the landscape by debris flows, as well as the equivalent basin- Earth’s surface? These questions highlight the debate over the relative average lowering depths. Using recently published long-term erosion rates importance of extreme events in sculpting Earth’s surface, and are as old from the Colorado Front Range, the authors were then able to calculate as the science of geology. Although geologists have gathered data and how many years of hillslope erosion and transport it took to produce the proposed theories supporting both Hutton’s (1795) and Lyell’s (1830) uni- regolith evacuated by the debris flows. formitarianism and Cuvier’s (1818) catastrophism for over 200 years, the The intriguing result is that, in a single event, debris flows transported paper by Anderson et al. (2015, p. 391 in this issue of Geology) shows that hundreds to thousands of years worth of accumulated hillslope material the debate is still active and that, even with new tools, we have much to into the main stem rivers. From this result, they reached the conclusion learn about the degree to which observations of modern sediment trans- that debris-flow erosion and transport do the majority of the geomorphic port processes quantify the full range of formative geomorphic events. work in the studied steep valley networks, despite the low frequency of If infrequent, extreme events dominate the long-term (103–106 yr) their occurrence. rate of erosion and sediment transport, then historic records of sediment The results and conclusion from Anderson et al. compare favorably flux that have not captured an extreme event might grossly underestimate with other studies that have attempted comparisons between net sediment the actual long-term sediment flux from a landscape (Kirchner et al., 2001; transport from large debris-flow events and long-term erosion rates. Eaton Carretier et al., 2013). Misunderstanding such a discrepancy between et al. (2003) compared net sediment transport by debris flows initiated dur- modern and long-term erosion rates can lead to inaccuracies in: predict- ing hurricane strikes in the Appalachian Mountains (eastern United States) ing the life span of reservoirs; determining the impact of changing land to decadal sediment fluxes. They found that, even with thousand-year use; setting attainable water-quality standards; and mitigating sediment- recurrence intervals, as determined from radiocarbon dating of hillslope related hazards, such as rapid mass movements like landslides or debris material in headwater valleys, debris flows still transported a majority of flows, and extreme river channel aggradation. At longer time scales, accu- the sediment out of the steep valley networks. Similarly long recurrence rate portrayal of the magnitudes and spatial-temporal patterns of sediment intervals and dominance of sediment transport by debris flows were found fluxes is critical for understanding how landscapes evolve, how sediment for small basins in the Coast Ranges of the western United States (Dietrich fluxes might change with a changing climate, and what flux of sediment and Dunne, 1978; Benda, 1990; Reneau and Dietrich, 1991; Lancaster and and nutrients is required to maintain healthy ecosystems. Casebeer, 2007). Debris flows and related rapid mass movements are perfect arche- Taken together, a clear picture is emerging. In steep valley networks, types of extreme events, as too clearly demonstrated by the recent mass debris flows can erode and transport the majority of weathering products movement disaster in Oso Washington, USA (Iverson et al., 2015). Debris produced by the landscape. What is less clear from these studies of sedi- flows are gravity-driven mixtures of mud, rock, water, and other incidental ment budgets in steep lands is whether the episodic passage of debris debris that flow down slope at speeds that can exceed tens of meters per flows contributes directly to incision of bedrock-floored valleys by impact second (Iverson, 1997). They commonly initiate in response to intense wear versus simply accelerating bedrock weathering in valley bottoms by or prolonged precipitation events as either shallow landslides that liquefy removing thick regolith cover. As noted by Anderson et al., most debris- upon initiation of motion (Iverson et al., 1997) or from intense surface- flow paths in the Colorado flood area were scoured clean to bedrock. Simi- water runoff that rapidly mobilizes large quantities of sediment (Cannon lar qualitative observations of apparent bedrock scour by debris flows are et al., 2001; Kean et al., 2013). But quantifying the importance of rare, commonly reported in valleys with slopes steeper than ~5° (Stock and extreme events including large debris-flows is complicated by the obvious Dietrich, 2003, 2006; Hsu, 2015). In one four-year monitoring study, factors of infrequent, sporadic occurrence, and by the hazardous nature of measurements of bedrock incision made in a steep bedrock-floored valley trying to observe them directly. demonstrated that debris-flow scour caused cm-scale bedrock lowering Critical to resolving the importance of infrequent large events in (McCoy et al., 2013). In many steep lands, valleys with gradients steeper setting the pace of landscape evolution are accurate measurements of than 5° can make up the dominant portion of the valley network relief and both net sediment transport occurring during an extreme event, and of network length (Fig. 1; Stock and Dietrich, 2003). This implies the inci- the long-term erosion rate. Anderson et al. managed to obtain both these sion of ridge-valley topography in unglaciated mountain ranges might not crucial pieces of information for an extreme precipitation event that trig- be derived purely from rivers, but also by debris flows. gered over 1100 landslides and debris flows in the Colorado Front Range Significant consideration has been given to bedrock incision by riv- (Colorado, USA; Coe et al., 2014). Their strategy called for the use of ers because it is thought to set the relief of mountain ranges, as well as tools that simply didn’t exist 20 years ago: (1) multi-temporal meter-scale the pace of landscape evolution, by transmitting tectonic and climactic resolution digital elevation models obtained from airborne lidar (Glennie changes throughout the landscape (Whipple and Tucker, 1999). But there is no agreed-upon mechanistic framework to describe the controls of bed- *E-mail: [email protected] rock incision by debris flows, unlike rivers, which in turn raises questions GEOLOGY, May 2015; v. 43; no. 5; p. 463–464 | doi:10.1130/focus052015.1 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 5 For| www.gsapubs.org permission to copy, contact [email protected]. 463 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/43/5/463/3548990/463.pdf by guest on 29 September 2021 Eaton, L.S., Morgan, B.A., Kochel, R.C., and Howard, A.D., 2003, Role of debris flows in long-term landscape denudation in the central Appalachians of Vir- ginia: Geology, v. 31, p. 339–342, doi:10.1130/0091-7613(2003) 031 < 0339 :RODFIL>2.0.CO;2. George, D.L., and Iverson, R.M., 2014. A depth-averaged debris-flow model that includes the effects of evolving dilatancy. II. 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