Controls on alluvial fan long-profi les Jonathan D. Stock* Kevin M. Schmidt David M. Miller U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA ABSTRACT these hypotheses for alluvial fan long-profi les Calculations that match observed fan long- using detailed hydraulic and particle-size profi les require an exponential decline in Water and debris fl ows exiting confi ned data in sediment transport models. On four gravel transport rate, so that on some fans valleys have a tendency to deposit sediment alluvial fans in the western U.S., we fi nd that approximately half of the load must be depos- on steep fans. On alluvial fans where water channel hydraulic radiiare largely 0.5–0.9 m ited off channel every ~0.20–1.4 km downfan. transport of gravel predominates, chan- at fan heads, decreasing to 0.1–0.2 m at distal This leads us to hypothesize that some allu- nel slopes tend to decrease downfan from margins. We fi nd that median gravel diam- vial fan long-profi les are statements about ~0.10–0.04 to ~0.01 across wide ranges of eter does not change systematically along the the rate of overbank deposition of coarse climate and tectonism. Some have argued upper 60%–80% of active fan channels as particles downfan, a process for which there that this pattern refl ects grain-size fi ning slope declines, so downstream gravel fi ning is currently no mechanistic theory. downfan such that higher threshold slopes cannot explain most of the observed channel are required just to entrain coarser parti- slope reduction. However, as slope declines, Keywords: alluvial fan, long-profi le, Mojave cles in the waters of the upper fan, whereas channel-bed sand cover increases systemati- Desert, sediment transport. lower slopes are required to entrain fi ner cally downfan from areal fractions of <20% grains downfan (threshold hypothesis). An above fan heads to distal fan values in excess INTRODUCTION older hypothesis is that slope is adjusted to of 70%. As a result, entrainment thresholds transport the supplied sediment load, which for bed material might decrease systemati- Sediment exiting confi ned valleys in debris decreases downfan as deposition occurs cally downfan, leading to lower slopes. How- fl ows or entrained in fl oods commonly spreads (transport hypothesis). We have begun to test ever, current models of this effect alone tend laterally, forming fans (e.g., Fig. 1). Where the to underpredict downfan slope changes. This proportion of sediment transported by water trac- *E-mail: [email protected] is likely due to off-channel gravel deposition. tion is substantial, these fans are commonly called A Globe, Providence Mts., CA Hanaupah, Death Valley, CA B Figure 1. Photographs of alluvial fans whose modern channels transport gravel by water fl ow over a similar range of slopes. (A) Globe fan drains a low-relief (~500-m) source terrain of the Providence Mountains, eastern Mojave Desert. There are no active faults mapped in this area, and channel gradients decrease from ~0.06 at the active fan-head (high- albedo) channel to ~0.01, where the fan joins a sand-bedded axial wash. (B) Hanaupah fan drains a high-relief (~3000-m) source terrain of the Panamint Mountains, Death Valley, California. An active fault occurs in the lower right (see also Fig. 4), and fan gradients decrease from ~0.07 to less than 0.01, where the fan joins Badwater Playa in the foreground. GSA Bulletin; May/June 2007; 120; no. 5/6; p. 619–640; doi: 10.1130/B26208.1; 23 fi gures; 4 tables. For permission to copy, contact [email protected] 619 © 2007 Geological Society of America Stock et al. alluvial, after Drew (1873). For alluvial fans with slope may vary down different azimuths (e.g., part, a statement about the rate of downfan bed- steep source catchments and traction-transported Hooke and Rohrer, 1977), but long-profi les are material deposition. Later fl ume experiments gravel, channel slopes commonly decrease from commonly concave up (e.g., Denny, 1965). (e.g., Ikeda and Iseya, 1988) have shown that ~0.04–0.10 at fan heads to 0.01 or less at fan bases Drew (1873) proposed the fi rst explanation for Drew’s hypothesis can be demonstrated in the (e.g., Fig. 2). It is widely observed that deposits this concavity. He hypothesized that deposition laboratory, where fl ume channel slope decreased become increasingly fi ne grained downfan, from reduced sediment supply progressively downfan. as gravel supply rate was reduced for a con- gravel-dominated proximal deposits, to sand- Consequently, the slope required to transport the stant discharge. But this hypothesis was never dominated distal deposits (e.g., Lawson, 1913; supply of sediment decreased downfan (e.g., tested on alluvial fans because subsequent work Eckis, 1928; Chawner, 1935). Average surface Fig. 3A). Concave-up long-profi les were, in focused on the control of slope by grain size 3000 Globe 2500 Sheep Creek Hanaupah Lucy Gray 2000 1500 elevation (m) 1000 Figure 2. (A) Channel long- profi les down most active axial thread of four intensively stud- ied fans and their source areas— 500 Globe, Sheep Creek, Hanaupah, and Lucy Gray. Data extracted CATCHMENT FAN from U.S. Geological Survey 0 A 7.5′ topographic maps, starting at valley head (see Stock and –10,000 0 10,000 20,000 Dietrich, 2003). Ticks indicate meters from fan head cross sections where we collected hydraulic geometry and grain 1 size; endpoints approximate fan Globe terminuses. (B) Slopes for each Sheep Creek long-profi le using every contour ′ Hanaupah on U.S. Geological Survey 7.5 topographic maps. We attempt 0.1 Lucy Gray to test whether this slope change is a statement about threshold sediment transport associated with downfan bed-material fi n- ing or sediment transport rates 0.01 that decline downfan. 7.5’ slope 0.001 CATCHMENT FAN B 0.0001 –10,000 0 10,000 20,000 meters from fan head 620 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-profi les alone. Workers proposed that local slope is set by A the coarsest grain fraction that can be entrained 6 at a given reach (Fig. 3B); as this threshold grain size decreases downfan, fan slopes decline (e.g., Blissenbach, 1952, 1954; Bluck, 1964; Hooke, 4 1968; Kesel, 1985; Kesel and Lowe, 1987). In Slope (%) Ikeda and Iseya, 1988 one of the most commonly cited examples, Blis- senbach (1951, 1952, 1954) measured maximum ~ clast size at cut banks down dissected fans of the 2 qsed Santa Catalina Mountains, Arizona. He reported 0 0.5 1 a strong covariance between maximum clast size Bedload transport (g/cm/s) and local surface slope, an observation that has formed the basis for much subsequent work. This threshold hypothesis is widely accepted in the literature (McGowen, 1979; Nilsen, 1982; Ethridge, 1985; Miall, 1996) and has been used to model alluvial fans (Price, 1974). These alternate hypotheses for downfan slope Transport hypothesis: slope declines with lower fluxes reduction can be restated (Fig. 3) as transport versus threshold entrainment controls on fan long-profi les. However, it is not clear which expected relation B process dominates in the fi eld, because despite Shields’ criteria over one hundred years of fi eld work and model- Rc ing (summarized in Table 1; and by Bull, 1977; slope Nilsen, 1982; Blair and McPherson, 1994a, d50 begins to move 1994b), the literature does not contain suffi cient d /R hydraulic and grain-size data to test these alter- 50 nate hypotheses. Most studies (e.g., Eckis, 1928; Beaty, 1963; Bluck, 1964; Kesel, 1985; Kesel S and Lowe, 1987; Hubert and Filipov, 1989) char- c acterized grain size as the median or average of the coarsest ten grains within a given square or distance, sampling in straight lines downfan, potentially across deposits of different age and processes. This characterization creates two dif- fi culties in using the data. First, it is diffi cult to isolate the role of individual processes. Second, there is no theory for the effect of exceptional Threshold hypothesis: slope declines with grain size grain sizes (e.g., maximum clast size) on chan- nel reach slope. For instance, some fl ume stud- Figure 3. Conceptual model of factors that control alluvial-fan slope. Figure 3A illus- trates Drew’s 1873 hypothesis that declining transport rate q (decreasing arrows) ies (e.g., Solari and Parker, 2000) indicate that at sed slopes >0.02, the coarsest particles may be excep- downfan lead to slope reduction (long-profi le modifi ed from Drew, 1873). Inset graph tionally mobile because of strong drag forces. As modifi ed from Ikeda and Iseya (1988) illustrates this effect in a supercritical fl ume a result, the assumption underlying earlier stud- whose slopes approximate those found on many alluvial fans (e.g., Fig. 2B). Figure 3B ies that grain size controls slope cannot be tested illustrates later threshold hypothesis (e.g., Blissenbach, 1952) that declining grain size with available data. Although it is known from downfan (either because of shallowing fl ow, as shown, or because of comminution) photographs and fi eld inspection that the grain leads to a decrease in slope. Inset shows an illustration of this effect as characterized by Shields’ criterion (see Equation (1) in text), where particles of d diameter begin to size of many alluvial fans varies from gravel 50 move once fl ow reaches a depth of R at S slope. Smaller inset shows predicted rela- dominated to sand or fi ner fraction deposits at c c tion between channel slope and ratio of d to R for a threshold entrainment channel.