Sediment Yield, Spatial Characteristics, and the Long-Term
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Sediment yield, spatial characteristics, and the long-term evolution of active earthfl ows determined from airborne LiDAR and historical aerial photographs, Eel River, California Benjamin H. Mackey†, and Joshua J. Roering Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA ABSTRACT period, equating to a regional yield of 1100 by sliding along transient shear surfaces with t km–2 yr–1 (~0.45 mm/yr) if distributed across a degree of internal deformation or fl ow. They In mountainous landscapes with weak, the study area. As such, a small fraction of span a range of landslide failure styles and have fi ne-grained rocks, earthfl ows can dominate the landscape can account for half of the re- been variably described as landslide complexes erosion and landscape evolution by supply- gional denudation rate estimated from sus- (Iverson, 1986a), landslides (Schulz et al., ing sediment to channels and controlling pended sediment records (2200 t km–2 yr–1 or 2009b), earth slides (Cruden and Varnes, 1996), hillslope morphology. To estimate the con- ~0.9 mm/yr). We propose a conceptual model and mudslides (Chandler and Brunsden, 1995; tribution of earthfl ows to regional sediment for long-term earthfl ow evolution wherein Glastonbury and Fell, 2008). Hereafter, we budgets and identify patterns of landslide ac- earthfl ows experience intermittent activity use the term earthfl ow as advocated by Hungr tivity, earthfl ow movement needs to be quan- and long periods of dormancy when limited et al. (2001) to describe large, slow-moving tifi ed over signifi cant spatial and temporal by the availability of readily mobilized sedi- landslides with macroscale, fl ow-like morphol- scales. Presently, there is a paucity of data ment on upper slopes. Ultimately, high-order ogy. These features morphologically resemble that can be used to predict earthfl ow behav- river channels and ephemeral gully networks “earth glaciers.” They are generally slow mov- ior beyond the seasonal scale or over spatially may serve to destabilize hillslopes, control- ing (<4 m/yr), large (>500 m long), deep seated extensive study areas. Across 226 km2 of rap- ling the evolution of earthfl ow-prone terrain. (>5 m thick), and mechanically dominated by idly eroding Franciscan Complex rocks of the fi ne-grained material, and they behave in a plas- Eel River catchment, northern California, INTRODUCTION tic or visco-plastic manner (e.g., Putnam and we used a combination of LiDAR (light de- Sharp, 1940; Kelsey, 1978; Iverson and Major, tection and ranging) and orthorectifi ed his- Earthfl ows 1987; Zhang et al., 1991b; Bovis and Jones, torical aerial photographs to objectively map 1992; Baum et al., 1993; Chandler and Brunsden, earthfl ow movement between 1944 and 2006. In mountainous landscapes without active 1995; Malet et al., 2002; Coe et al., 2003; By tracking the displacement of trees grow- glaciers, sediment production and hillslope Mackey et al., 2009). Earthfl ows classically ing on earthfl ow surfaces, we fi nd that 7.3% form are primarily controlled by mass-wasting have an hourglass planform shape, with an of the study area experienced movement over processes, including slow-moving, deep-seated amphitheater-like source zone, an elongate nar- this 62 yr interval, preferentially in sheared landslides known as earthfl ows. These large row transport zone, and a lobate compressional argillaceous lithology. This movement is dis- slope failures can establish or modify drainage toe or depositional area (Keefer and Johnson, tributed across 122 earthfl ow features that patterns, alter hillslope morphology, and impart 1983). Topographically, earthfl ows are com- have intricate, elongate planform shapes, a large perturbations on sediment budgets by reg- monly found on hillslopes with low planform preferred south-southwesterly aspect, and ulating the timing, magnitude, frequency, spatial curvature (Ohlmacher, 2007). a mean longitudinal slope of 31%. The dis- distribution, and grain size of sediment entering Active earthfl ows exhibit seasonal movement tribution of mapped earthfl ow areas is well- a channel or river network (Hovius et al., 1998; patterns primarily governed by precipitation and approximated by a lognormal distribution Korup, 2005b, 2006). This has direct implica- groundwater levels, and they can require several with a median size of 36,500 m2. Approxi- tions for fl ooding, channel network evolution, weeks of cumulative rainfall before the onset of mately 6% of the study area is composed sediment transport, aquatic habitat, and infra- movement (Kelsey, 1978; Iverson and Major , of earthfl ows that connect to major chan- structure. While the importance of landsliding 1987). Earthfl ows can potentially dominate nels; these fl ows generated an average sedi- in mountainous landscapes has been long recog- sediment delivery to channels in erosive land- ment yield of 19,000 t km–2 yr–1 (rock erosion nized, we are still challenged to understand and scapes (Putnam and Sharp, 1940; Swanson and rate of ~7.6 mm/yr) over the 62 yr study quantify the ways in which landslides, and in Swanston, 1977; Kelsey, 1978), and yet they particular earthfl ows, modulate the rate and lo- seldom fail catastrophically (Iverson, 2005). We cations of sediment fl ux, and control landscape specifi cally distinguish earthfl ows from large- † Present address: Division of Geological and form, over geomorphically signifi cant time displacement, catastrophic single-event failures, Planetary Sciences, California Institute of Technol- ogy, 1200 E. California Blvd., Pasadena, California scales (Dietrich et al., 2003; Korup et al., 2010). such as rockslides, translational bedrock slides, 91125, USA. Slow-moving earthfl ows are a class of land- debris fl ows, and rotational slumps (Cruden and E-mail: [email protected] slide (Cruden and Varnes, 1996) characterized Varnes, 1996). GSA Bulletin; July/August 2011; v. 123; no. 7/8; p. 1560–1576; doi: 10.1130/B30306.1; 14 fi gures; 1 table; Data Repository item 2011099. 1560 For permission to copy, contact [email protected] © 2011 Geological Society of America Earthfl ow activity and erosion, Eel River, California Extensive work has been undertaken on the 1978; Keefer and Johnson, 1983; Bovis and we lack conceptual models describing the behavior of individual earthfl ows at the daily and Jones, 1992; Ohlmacher, 2007). Accurate inven- long-term evolution of an earthfl ow-prone seasonal scales (e.g., Iverson and Major, 1987; tories of regional earthfl ow activity will enable landscape, and even the long-term evolu- Malet et al., 2002; Coe et al., 2003; Schulz et al., us to quantify rates of earthfl ow-generated sedi- tion of an individual earthfl ow. For clay-rich, 2009a). Other research foci include specifi c earth- ment yield and identify spatial patterns of slide earthfl ow-prone hillslopes (Fig. 1), there is fl ow mechanics such as shear zone dilatancy and activity that infl uence long-term movement. nothing comparable to the colluvial hollow strengthening (Iverson, 2005; Schulz et al., 2009b) and debris-fl ow framework applicable to soil- and the evolution of earthfl ow material strength Long-Term Earthfl ow Evolution mantled uplands (Dietrich and Dunne, 1978; over time (Maquaire et al., 2003). Many attempts Lehre and Carver, 1985), or the competing have been made to model earthfl ow mechan- When observing an earthfl ow-prone land- soil creep and fl uvial erosion processes ap- ics and rheology (e.g., Brückl and Sehidegger , scape, especially on high-resolution digital plicable to gentle ridge and valley topography 1973; Craig, 1981; Savage and Chleborad, 1982; topography (Fig. 1), a striking feature is that (Perron et al., 2009). Iverson , 1986c; Vulliet and Hutter, 1988; Baum nearly all hillslopes appear to have been af- In an attempt to address this knowledge gap, et al., 1993; Angeli et al., 1996; Savage and fected by mass movement, and the morphology we here propose a largely unexplored alterna- Wasowski , 2006; van Asch et al., 2007), primarily of earthfl ow and landslide activity is widespread tive to theories of long-term external forcing. to describe or replicate the observed behavior of (Putnam and Sharp, 1940). Multiple generations Earthfl ow activity may be dependent on fac- specifi c earthfl ows. of dormant slope failures appear to surround the tors intrinsic to individual earthfl ows, namely, comparatively small fraction of active terrain. the long-term balance of soil and rock entering Earthfl ow Spatial Patterns and This observation has prompted speculation that and leaving the earthfl ow. The rate of material Sediment Yield longer-term external forcing may modulate leaving the earthfl ow via mass translation and earthfl ow activity, such that many of the dormant fl uvial erosion is unsustainably fast over long To assess the contribution of landsliding to slope failures we see in the landscape today are time periods (Mackey et al., 2009), whereas regional erosion rates, we must quantify the an artifact of past periods of activity. Sources of the supply of readily mobilized earthfl ow col- transfer of sediment from slope failures to ac- external forcing include glacial-interglacial or luvium is limited by rates of bedrock weather- tive channels and compare this sediment yield shorter-term variations in climate (e.g., Bovis ing and expansion of the earthfl ow source zone. to catchment-wide erosion data sets, such as and Jones, 1992; Fuller et al., 2009), episodes The imbalance of material entering and exiting suspended sediment records. Complicating of base-level fall (Palmquist and Bible, 1980), the earthfl ow can only be reconciled with brief this task, slope failures vary in timing, loca- earthquakes (Lawson, 1908; Keefer, 1984), or periods of earthfl ow activity, separated by long tion, size, mechanism, postfailure behavior, and land use (Kelsey, 1978). periods of dormancy, to allow time for mass to the effi cacy of sediment delivery to the chan- Although the seasonal behavior of indi- accumulate via weathering and recharge the nels (Benda and Dunne, 1997; Lave and Bur- vidual earthfl ows has been well documented, earthfl ow source zone.