Controls on Alluvial Fan Long-Profiles

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 studied 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 diffi 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 channel reach slope. For instance, some fl ume stud- Figure 3. Conceptual model of factors that control alluvial-fan slope. Figure 3A illustrates 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. distal margins, data describing exceptional clast 50 sizes are not a suffi cient description to understand fan sediment entrainment or transport. The same lack of data makes it diffi cult to evaluate recent theoretical models developed by conditions with channelized fl ow, constant grain Hooke and Rohrer (1979). However, these mod- Parker et al. (1998a, 1998b) and by Whipple et size, and subsidence rate, these elegant models els have not yet been tested on fi eld-scale alluvial al. (1998). These studies used sediment transport show that slope declines downfan as active chan- fans because of the lack of data. models with thresholds to investigate long-pro- nel width decreases and fl ow depth increases. In the following sections, we describe fi eld fi les of tailing basin fans and, for the fi rst time, Under these conditions, slope is proportional to sites, methods, and models that we use in an explored Drew’s hypothesis with a modern under- the ratio of sediment to water discharge, a fi nd- attempt to test two alternate hypotheses for fan standing of sediment transport. Under equilibrium ing consistent with earlier experimental work by long-profi les (Fig. 3) in four gravel-dominated

Geological Society of America Bulletin, May/June 2008 621 Stock et al.

fans in the arid southwest USA (Fig. 4). These fans are characterized by steep source catchments with hillslope, debris fl ow, and fl uvial processes that supply a wide range of grain sizes to fan heads, and ephemeral fl ow and sedi-

) s

( ment transport by traction (ﬂ owing water) in

e 1 c 7 n fan channels. We focus on channelized alluvial 9 e

r 1 0

e fans, whose high-albedo threads (e.g., Figs. 1 , f 0 r e 0

e r

2 9 d and 4) represent focused sediment transport

e , 7 n l s 9 a p l l l i 1

r down channels, not sector inundation and sheet-

m , e M r a

b ; x e ﬂ ooding that may occur on some fans (e.g., r 2 O E

h 7 d o

9 Blair, 2000). We measure grain size and channel n

1 R

d d ,

t t d 0 n n hydraulic geometry down active channel threads n n n 0 u u o o a 0 o o

1985; Harvey, 1987; Harvey, 1987; Harvey, 1985; 2002 f f 2 ,

to test two proposed hypotheses for fan-channel m m e

l , e e k u u e r i n n o a a s a Hubert and Filipov, 1989; Hubert and Filipov, Harvey, 2002 Mather and Hartley, 2005 2005 Hartley, and Mather Marchi et al., 1993 1993 Marchi et al., Beaumont, 1972; Kostaschuk et al., 1986; et al., 1986; Kostaschuk 1972; Beaumont, slope changes. For instance, Shields’ bed-mate- o o o l e e e

B B N B N K

Ritter et al., 1993; Blair, 1999, 2000; 2000; Blair, 1999, 1993; al., et Ritter

Bull, 1962; Bull, 1964b; Denny, 1967; Hooke, 1968; Hooke, 1968; Denny, 1967; Bull, 1964b; Bull, 1962; Eckis, 1928; Chawner, 1935; Blissenbach, 1952; 1952; Blissenbach, 1935; Chawner, Eckis, 1928; 1985; Kesel, Bluck, 1964; Beaty, 1963; 1993 al., et Ritter H rial threshold entrainment model predicts that channel slope should increase as bed material

a a i i n i n n becomes coarser for a given hydraulic radius. We r r izona al., 2004 Pearthree et Kesel, 1985; a r o o p f f i A i

l l

S test this statement of the threshold hypothesis by ; /

n a a a a a o C C i c c c

t i i i measuring fan-channel slope, hydraulic radius, , , r r r a y y e e e c e e l l

o and bed-material diameter to see whether they l m l m m L a a A A A

l l l V V follow Shields’ prediction for a threshold chan-

a a a

a ern U.S. ern U.S. 1978; Nummedal, and Boothroyd Lustig, 1965; ern U.S. ern U.S. Harvey, 1987 1977; Wells, Denny, 1965; h r r h r i t e e t t t t s a n n n n a n r nel (Fig. 3B, upper right inset). A rigorous test Central America Central e o o e e OBSERVATIONS ON ALLUVIAL ON FANS ALLUVIAL OBSERVATIONS e e e

D N P C

N Western U.S./Persia Western U.S./Persia Canada/Spain C

D C of Drew’s transport hypothesis would compare measured unit bedload fl ux down a fan sector to that predicted by a transport equation with fi eld data inputs. For the moment, sediment fl ux measurements are too diffi cult to make in ephemeral

n o concavity? concavity? Arizona/Canada/Italy 1986; Kostaschuk et al., 1965; Melton,

i channels. So, we use a less rigorous test and t a z

i apply the ﬁ eld data to plot the unit bedload ﬂ ux l a r al Valley, California Death 1971 Oberlander, Beaumont and e c predicted by transport equations downfan, to see i t n i r e

g if this model is consistent with a reduction in – – / r

? n e ? e t o p

n bedload ﬂ ux. The tests are applied to active fan i a t u o i n a s t t

i r uent off-axis? channels Western U.S. with the understanding1979 Hooke and Roher, that departures e s r o o

or supercritical or Arizona Southwest Rahn, 1967 l l p p r a a from these conditions in the past could also e e c c t i i d t t

n i i l I r r inﬂ uence fan surface slope. So, our analyses e c c

v r r

a are strictly tests of these alternate threshold and a a r e e g

n n

d transport hypotheses on modern fan channels i w w p covariation with drainage spacing & lower b.c.? b.c.? lower & spacing drainage with covariation o o a l l that are presumed to be in equilibrium.

F F R

Width adjusted to sediment discharge? to discharge? sediment Width adjusted America; Arizona Central 2001 Field, Kesel, 1985;

Source grain size limitation size limitation Source grain 1977; Wells, 1984; Lombardo, French and

Influence of debris flows; long-profile long-profile flows; debris of Influence n FIELD SITES o i 0.87 Width adjusted to water & sediment discharge discharge West sediment & water to adjusted Width t ) a

0.5 u drain. artifactual area; controls fan Sediment yield 1 0 . . q 2 2

e We selected four alluvial fans (Fig. 4; Table 2)

/ up long-profiles? from concave Artifactual U.S. Himalaya/western 1965; 1962; Melton, 1928; Bull, Eckis, Drew, 1873; supercritical or Flow near critical Colorado Roaring River, Blair, 1987 < < n e drainage = cA

l r r in or near the Mojave Desert, California. Channel b F F a

i r < < = cA = 7.3(H/A = cA

long-proﬁ les for these fans (Fig. 2A) are concave a fanchannel 1 3 fan avg avg . . V 1

S 1 up, and slope declines from ~0.04–0.09 at fan heads, to 0.01–0.005 at axial washes, and to neg-

TABLE 1. SUMMARY OF HYDRAULIC, SEDIMENTOLOGIC, AND MORPHOMETRIC AND MORPHOMETRIC OF HYDRAULIC, SEDIMENTOLOGIC, SUMMARY TABLE 1. ligible values at playa boundaries (Fig. 2B). The n W a f

n fans have radial form and active channel threads drainage with plane bed or low-relief alternate bar and S mid-channel bar morphology. Work by Pearthree

drainage et al. (2004), Vincent et al. (2004), and Pelletier

clasts? exceptional on dependant Slope not West slope? with covariation Artifactual Western U.S./ Kesel, 1985; Bluck, 1964; Blissenbach, 1952; d power function of function power

max e 7 max t et al. (2005) has shown that ﬂ ow in such settings . a 0

> is focused into narrow threads, with deposition n r u n i common in overbank areas, where unconﬁ ned

s d e a

v ﬂ ow may be referred to as sheetﬂ ooding. e x a n r u o h l w i t f

t Bedforms in these channels are composed of

l g a d e : ?—indicates our hypothesis to explain observation to explain our observation : ?—indicates hypothesis n v i a r n imbricated material transported by water trac- d o e n l n s a d Minimum Fr number, author reports a range of 1.6 to 2.2 in text. in 2.2 to 1.6 of a range reports author Fr number, Minimum a ? Note b

supply/discharge

source basin (width, depth, slope) slope) (width, depth, source area h e t tion. We identiﬁ ed deposits from traction trans- Fan slope steepest off of main axis main off of Fan slope steepest freq Transport less

Slope independent of D Slope independent O Fan slope decreases with A with Fan slope decreases

Sand fraction (weight) increases dow Sand fraction (weight) B Slope increases with D Slope increases Hydraulic and sedimentologic Maximum grain size fines downfan downfan fines Maximum grain size slope controls grain size Maximum Western U.S./ Morphometric Fan depositional area is area Fan depositional Downfan channel hydraulic geometry geometry hydraulic Downfan channel Channel widens with increase in sediment in sediment with increase Channel widens Grain size may show no pattern no pattern show Grain size may flows/tectonics debris of Influence Western U.S. 1965; Denny, 1965; Lustig, Bull, 1964a; Fan channel width increases with A with increases width Fan channel Fan slope increases with relief of of relief with Fan slope increases

Standing waves; F Standing S Standing waves (antidunes?) waves (antidunes?) Standing Fr > 0.7 Some runoff sourced on fan on sourced Some runoff flow depth ~ Moved particle diameter predomi Drag forces hydraulics? Pedogenesis affects Western U.S. 1968 Beaty, 1967; Denny, 1965, C port (which could include hyperconcentrated

622 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les

fl ow) using the following criteria: clast sup- Figure 4. Luminescence, radiocarbon, and U- ported deposits and imbrication at both large Th dates (e.g., Clarke, 1994; McDonald, 1994; and fi ne gravel fractions (pervasive imbrica- Wang et al., 1994; Mahan et al., 2007; Sohn et tion). Although coarse particles in debris-fl ow al., 2007) indicate latest Pleistocene to Holo- Brady, 1986 levees and snouts can be imbricated (e.g., cene deposition times for similar deposits. Hewett, 1956 Albee et al., 1981 Major and Iverson, 1998), we have observed Intermediate age deposits display desert Goldfarb et al., 1988 that fi ner grains in their interior lack pervasive pavement with interlocking, varnished surface imbrication. We identifi ed debris-fl ow deposits clasts overlying a platy-structured Av horizon in catchments above fan heads using criteria composed primarily of silt. The varnish cre- based on analysis of historic deposits (e.g., ates the low albedo that characterizes much of granite granite, qtzite metavolcanics Gneiss, granitics,

Costa, 1984; Stock and Dietrich, 2003): matrix the deposit in the images of Figure 4. Varying Argillite, dolomite, Adamellite, gneiss, supported clasts and a lack of pervasive imbri- amounts of red to brown clay accumulation and Dominant lithologies Reference(s) / cation at all grain scales. iron oxidation yield a Bt argillic horizon. The • Each fan also has a series of older alluvial calcic horizon typically is stage II to stage III Station 1958-2005 1955-2005 Granitoids, volcanics 1955-2005 Granitoids, 1961-2005 deposits within which the modern active chan- (Gile et al., 1966; Machette, 1985). Most depos- 1953-1971 nels are inset. For this reason, ﬂ ow and sedi- its with desert pavements have been dated as record length ment transport are necessarily focused in nar- late Pleistocene, with some falling in the range row threads, and large amounts of sediment of late-middle Pleistocene (Bull, 1991; Wang Average cannot have been deposited in modern channel et al., 1994; Sowers et al., 1988; Mahan et al., rain (mm/a) beds since abandonment of these older surfaces. 2007; Maher et al., 2007; Sohn et al., 2007).

This timescale is important because it limits Deposits classiﬁ ed as old alluvial fan deposits (7.5') slope the time over which the boundary conditions form linear ridges (whalebacks) whose rounded Fan head on the fan (e.g., sediment supply rates, reach lateral margins indicate substantial erosion slope) can be considered relatively steady. The (Peterson, 1981). Remaining soil horizons are relative age of these older fan deposits can be primarily the stage IV calcic horizon (e.g., Gile

inferred from pedogenesis and surface charac- et al., 1966; Machette, 1985) and thin, degraded Fan relief* teristics, which generally covary with elevation argillic horizons (e.g., Yount et al., 1994). (top)-(bottom) = (m) above the modern channel. Yount et al. (1994) Because petrocalcic material is present at the ‡ established three main, time-based divisions for surface, these features may have high albedo. alluvial deposits—young, intermediate, and old. The 0.74-Ma Bishop ash is the most diagnos- (m) Arc length

Brieﬂ y, we review the evidence for the age of tic tephra layer observed regionally in these old # ) these deposits, following Miller et al., 2008. deposits (McDonald, 1994). 2 (km

In the Globe fan area, active alluvial chan- TABLE 2. FIELD SITES nels and adjacent sandy bars or terraces that Globe Fan area ) are episodically inundated by overbank ﬂ ow 2 are the two youngest subdivisions of the young Globe fan (Figs. 1 and 4–7) carries sediment Fan head category. These deposits lack desert pavements from low-relief (<500 m) headwaters cut largely area (km and rock varnish, have a relatively high albedo in Proterozoic gneisses and Mesozoic gran- in aerial photographs (e.g., Fig. 4), and have ite. No Quaternary faults are known in either

been active since the oldest wagon roads were source area or fan. Field observations revealed Mean (std. dev) established in this area, ca. 1–2 centuries ago. that Holocene debris-ﬂ ow deposits terminate at DEM slope Older young alluvial fan deposits form adja- valley slopes of ~0.10, high above the fan head. ? cent, higher surfaces that only rarely show Strongly indurated fanglomerates overlying evidence for recent ﬂ ood inundation and have gneiss are exposed in the channel thalweg just (m) area relief denser vegetation including creosote bush, upslope from the fan head (near A in Fig. 5), Max. source white bursage, cryptogamic mats, and annual indicating that the Globe channel here has not grasses. These deposits display very weak incised bedrock over late Pleistocene to Holo- fault

pavements in some cases, slight varnish, weak cene time. High-fi ll terraces at the fan head dis- inactive? 1043 .33 (.19) 44.29 64.9 8200 1090 - 800 = 290 0.057 214 045890/ vesicular A (Av) horizons, and weak cam- play inconsistent pedogenesis and thin sheets of bic horizons (Bw). The oldest of the young varying ages of alluvium as young as the oldest category of deposits have noticeable desert young deposits (latest Pleistocene). These rela- pavements that do not interlock and whose tions indicate that fan head was episodically California pebbles are varnished and possess rubifi cation deposited over a long period of Pleistocene time (reddening). Underlying Av and Bw horizons and was dramatically incised in early young are persistent features, with calcic morphol- alluvial deposit time. Channels of the lower ogy of stage I. Deposits of this category are fan are disrupted by engineering berms meant darker toned on aerial photography than the to divert and concentrate fl ow under railroad next younger category, due to increased rock bridges. These channels subsequently feed varnish and decreased plant cover. The young Kelso Valley axial wash, an ~0.01 slope, braided From 1-m (Globe) or 10-m DEM of catchment above fan head. Approximated using DOQQ. Straight line from fan head to margin along central axis. Second area is for pre-Holocene far area shown in Figure 4. NOAA station number, from www.wrcc.dri.edu. # ‡ • ? *Fan head contour minus margin contour. Fieldsite Range State Bounding Bounding State Fieldsite Range Globe Lucy Gray Providence Lucy Gray California Nevada/ inactive? 586 .41 (.18) 12.39 9.7/17.1 6500 1040 - 710 = 330 0.060 274 045721/ Sheep Creek Avawatz California thrust 1444 .52 (.21) 20.60 20.2/22.3 5800 430 - 110 = 320 0.068 69 040437/ deposits occupy the bulk of the lighter areas in sand channel. Hanuapah Panamint California Normal? 3055 .57 (.20) 74.05 16.0/45.3 5800 256 - (-82) = 338 0.062 58 042319/

Geological Society of America Bulletin, May/June 2008 623 Stock et al.

Figure 4. (A) Globe fan in the Providence Mountains, Califor- nia; (B) Hanaupah fan, Death Valley, California; (C) Lucy Gray fan, California and Nevada; A GlobeGlobe and (D) Sheep Creek fan in the Avawatz Mountains, California. B HanaupahHanaupah Digital orthophotographs of alluvial fans with source catchments shown as hillshaded 1-m (Globe) or 10-m DEMs, on which are draped 2-m (Globe) or 20-m slope estimates. North is to the top of the images. Blue cells are slopes below 0.10, where channel processes may predominate; green cells are slopes between 0.10 and 0.65, where creep and overland ﬂ ow transport processes may predominate; and red cells are slopes above 0.65 (~35°), an approximation for the angle of repose. Solid blue lines approximate the lateral boundar- C LucyLucy GGrayray ies of active sediment transport on the fans; dashed blue lines approximate former boundaries of the fans. Channel cross sections are shown by white dots, starting with 1 at distal end and increasing sequentially upfan. Variations in image albedo on fans represents, in part, differential development of desert varnish on deposits that are largely Pleistocene or older. Note active fault on Hanaupah fan trun- cating older surface. Source catchments with active faults mapped at IndexIndex their boundaries (Hanaupah and Nevada Sheep Creek) tend to have larger California source catchment areas above angle-of-repose slopes. Hanaupah Sheep Creek

Lucy Gray Globe

Arizo na D SheepSheep CCreekreek

624 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les

N width

50 m active

Figure 5. Maximum-fall slope over 4 m, superimposed on 1-m hillshaded LiDAR topography of Globe fan. Channel slopes decline from 0.07 at fan head (greens) to 0.01–0.02 (reds) at junction with axial wash. Cross-section number in Table 3 is shown above white dots representing ﬁ eld cross-section locations. Dashed white line shows crest of older terraces adjacent to modern wash (see Fig. 23). Note fan disruption by railroad and linear artifacts from LiDAR swath lines (~0.3 m in relief). Dashed arc through cross section 9 shows path along which we measured ~50 m of active channel widths, ~10 m more width than present at cross section 16 (see Table 3).

Fan deposits to the south of Globe have been meter. Studies of channel hydraulic geometry Lucy Gray extensively studied by McDonald (1994) and and scour depth are underway in this area (yel- by McDonald et al. (2003), who mapped eight low circles, Fig. 5), and these studies allow The Lucy Gray Mountains and the McCullough alluvial units whose deposition they argued to crude estimates of the depth of sediment stor- Range expose gneissic Early Proterozoic gran- be a function of regional climatic transitions. age in fan channels. itoids, associated pegmatitic dike rocks, and Alluvial fan deposits of Globe and nearby We sampled fi ne sediment at two sites in the Tertiary mafi c volcanic rocks whose detritus piedmonts are the focus of numerous current Globe area for luminescence dating to constrain comprise the Lucy Gray fan. The two ranges are interdisciplinary studies, including geoecol- deposition age—a 5.0-m–high fl uvial fi ll terrace separated by a low-relief belt of early Pleisto- ogy (Phelps et al., 2005; Bedford et al., 2007; (A, Fig. 5) whose age represents the last time cene and older alluvium, where Hewett (1956) Miller et al., 2007) and vadose zone hydrology of substantial aggradation at Globe fan head proposed the pre-Quaternary McCullough fault, (Miller et al., 2002; Miller et al., 2004; Schmidt and one of the widest channels to the south (B, with ~6 km of normal displacement, east side et al., 2004; Perkins et al., 2005; Nimmo et al., Fig. 5), to estimate the age of the oldest deposits up. Snow is not uncommon in the higher eleva- 2005). Figure 7 illustrates surfi cial mapping on the Pleistocene soils. At the fi ll terrace, we tions of the McCullough Range. Drainage from on the southern side of the fan (Miller, 2006, sampled a fi ne-grained pocket of eolian sedi- the McCullough Range transports sediment unpublished mapping), where deposits are ment ~2 m above the active channel. We took through a bedrock canyon cut in the Lucy Gray mapped into four broad categories on the basis adjacent bulk samples from 0.1- to 0.2-m–thick Range, westward toward Ivanpah Valley, where of inset relations and pedogenesis as described imbricated gravel beds with sand matrix. At a it is deposited in the Lucy Gray fan and the con- in the previous section. Historically active 20-m wide channel south of Globe fan, we nected Roach and Ivanpah playas (~800 m). deposits (blue) are inset into older deposits that excavated a 0.95-m–deep trench to an underly- Channel proximal surfaces in the Lucy Gray fan extend to the distal fan as narrow ribs, indicating oxidized, iron-rich soil (Btk) that is likely have been mapped as Holocene to late Pleisto- ing long-term persistence of fan geometry. Soil Pleistocene. Overlying sediments are thin- to cene (Schmidt and McMackin, 2006), and var- pits in active channels on the southern side of medium-bedded (1- to 10-cm) sands locally nished (low albedo in Fig. 4), undissected depos- the fan revealed indurated, reddened soils with stratifi ed and cross bedded, with sparse imbri- its over most of the fan indicate minor Holocene Bw to Bt horizons, at depths less than one cated gravel lenses. subsidence. The fan lacks evidence for Holocene

Geological Society of America Bulletin, May/June 2008 625 Stock et al.

xs-15 xs-12

hb = 0.81 m hb = 0.70 m

d50 = 10 mm d50 = 14 mm S = 0.050 S = 0.056 sand fraction = 0.22 sand fraction = 0.32

xs-1 xs-7

hb = 0.21 m hb = 0.37 m

d50 = 4 mm d50 = 12 mm S = 0.017 S = 0.040 sand fraction = 0.52 sand fraction = 0.39

Figure 6. Images of locations on Globe fan, illustrating decline in bankfull depth hb downfan. Note that while slopes declines in the ﬁ rst three panels from ~0.06 m to ~0.04 m, median gravel size d50 does not change signiﬁ cantly. However, sand fraction increases monotonically downfan, consistent with progressive off-channel deposition of gravel.

or late Pleistocene faulting, although a short rem- channels join Amargosa Wash, a sand bedded Hooke (1972), Dorn et al. (1987), and Hooke nant fault scarp in Pleistocene deposits lies south channel. Mapping by Clayton (1988) shows four and Dorn (1992) have mapped between two and of the fan (K. House, 2006, oral commun.). The older, varnished surfaces at Sheep Creek fan, fi ve older fan deposits that darken with desert playa provides a local source for eolian sand that which lie above the active channel. A clay-rich varnish with age (Fig. 4). These units are prob- is mixed with alluvium in the distal reaches of alteration zone in the catchments headwaters ably young to intermediate in age. Using rock the fan (Schmidt and McMackin, 2006). produces unusually fi ne mudfl ows (e.g., Schmidt varnish dating, Dorn (1996) proposed that four and Menges, 2003) whose fi ne-grained deposits units above the active wash were deposited dur- Sheep Creek (sandy/clay mixtures) can be found at the Amar- ing 0.5–9.0 ka, 14–50 ka, 105–170 ka, and 490– gosa River at slopes as low as 0.005. By contrast, 800 ka. This technique have been challenged by The left-lateral Garlock fault terminates at the coarse-grained debris-fl ow deposits are not evi- Wells and McFadden (1987), who questioned Avawatz Mountains, a high-relief (e.g., >1000- dent in the active channels past the fan head. its utility and application to units such as these. m) range north of Baker, California. Paleozoic Recent luminescence work by Sohn et al. (2007) and Proterozoic carbonate and siliciclastic Hanaupah showed that deposits equivalent to the youngest deposits and Mesozoic granitic and metavol- two units discussed above have dates of 3.7– canic rocks (Brady, 1986; Spencer, 1990) are Hanaupah fan sediments are derived from 13 ka, and 25 ka and older. As with other fans source rocks for alluvial fans on the northern and steeplands cut in Proterozoic rock (quartzite, we studied, Pleistocene deposits extend nearly eastern range fronts. Reverse faults along these dolomite, and argillite) and Tertiary granites of the length of the fan. However, the down-to-the- margins place rock onto unconsolidated Pleis- the Panamint Mountains (Albee et al., 1981), a east Hanaupah fault (Hunt and Mabey, 1966) tocene deposits at fan heads, indicating active normal fault-bounded block. This source region truncates heavily varnished deposit surfaces in rock uplift (e.g., Brady, 1986). On the northern produces annual snowmelt runoff that may occa- the distal fan (Fig. 4), indicating Pleistocene front, Sheep Creek fan (Fig. 4) is bounded by sionally reach the upper fan. Above active chan- subsidence. Leveling by Sylvester (2001) indi- active reverse faults at its fan head, and its distal nels, Denny (1965), Hunt and Mabey (1966), cates that this fault had no detectable surface

626 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les

Figure 7. Surﬁ cial geologic map of Globe fan and adjacent Hayden piedmont. Bedrock in purple, intermediate-age deposits are shown in yellow, older young deposits in green, and historic active deposits, including active channel bed, in blue. Background is 1-m Digital Orthophoto Quarter Quadrangle (DOQQ). Blue line encloses estimate of Holocene sediment pathways for the entire fan; yellow line encloses area over which sediment passing through cross section 16 is deposited over Holocene.

creep between 1970 and 2000. Lacustrine channel and most deposits exposed in cut banks pair of contours (typically 40 ft) down the main deposits mark former Lake Manly shorelines in are composed of clast-supported, pervasively thread to the axial valley or the playa boundary. the most distal portion of the fan. imbricated gravels with a sandy matrix. Imbrica- We mapped the boundaries over which sedi- Denny (1965) hypothesized that over an tion is consistent with traction transport by water, ment from the highest cross section could be appropriately long timescale, deposition upfan rather than inertial transport by debris fl ows. dispersed as a fan sector (e.g., yellow polygon of the Hanaupah fault might be balanced by in Fig. 5). Within this sector, we mapped high- erosion from older deposits. He measured par- METHODS albedo, active channels along a contour that ticle sizes in the active channel by collecting 25 includes each measured cross section. Summing particle diameters at equal spacing along cross Thread and Sector Selection these channel widths along each arc provides an sections. He found that geometric mean grain estimate of total active channel width, through size varied down the active channel between 11 To measure hydraulic and grain-size prop- which the sediment passing the uppermost cross and 24 mm, with no systematic pattern. Work by erties downfan, we selected the most active, section could be distributed as it travels down- Ibbeken at al. (1998) used photography to exam- continuous channel thread traversing the fan fan. Field traverses indicated that these esti- ine planform grain-size distributions at four pro- center. Center threads have minimal sediment mates of total widths are within several tens of fi les down the lowest fan surface, which includes input from fan-marginal sources (e.g., adjacent meters of fi eld-measured values. active channels. They presented these results as steeplands or other fans), simplifying the pattern composite distributions and reported no strong of sediment supply to the channel. We chose Channel Measurements Down-Thread downfan trends in grain size. Gravel composi- the most active thread by fi nding the widest, (Bankfull Depth, Width, Reach Slope, tion is ~20% granite, 60% quartzite, and 10% least vegetated (high albedo) thread on recent Scour Depth, and Bed Texture) carbonate and argillite (Hunt and Mabey, 1966). (1980s) U.S. Geological Survey DOQQ (Digital Hanaupah is widely reported as an example of Orthophoto Quarter Quadrangle) coverage. We We selected cross-section sites within straight a debris-fl ow dominated fan (e.g., Blair and defi ned fan head as the point where the single- reaches with representative bed texture and McPherson, 1994b; Ibbeken et al., 1998). Occa- thread channel fi rst bifurcates downfan (called abundant self-formed banks. We avoided areas sional deposits of coarse boulders and matrix- intersection point by Hooke, 1967, and by Was- where bank heights were infl uenced by dense supported lenses in older fi ll terraces are likely son, 1974). Starting just above the fan head, vegetation. We defi ned bankfull fl ow top using debris-fl ow deposits. However, both the active we chose cross sections between every other the highest banks with recent deposition along

Geological Society of America Bulletin, May/June 2008 627 Stock et al. a reach, including slackwater fi nes and gravel channels, this represents one or more traverses; were responsive to optical techniques and that lobes that were unvegetated or vegetated with we completed a traverse even after 100 measure- the proper temperatures were used in produc- annuals. We defi ned bankfull depth hb as the dif- ments to avoid biasing sampling to a particular ing the De values. The fi ne-grained (4–11 µm) ference between this height and the lowest point channel area. We found that we needed at least extracts from all samples were dated using the along the cross section. We avoided cut banks, 75 counts to reach stable median values in the total bleach, multiple-aliquot additive-dose which represent past incision rather than the most poorly sorted channels that we sampled (MAAD) method (Singhvi et al., 1982; Lang, desired estimate of current fl ow trim lines. To (Fig. 9). We calculated bed sand fraction as the 1994; Richardson et al., 1997; Forman and estimate reach slope, we surveyed long-profi les number of <2-mm counts divided by the total Pierson, 2002). using either handlevel, stadia rod, and tape or number of counts. We calculated bed-material, laser and tape (Hanaupah). For each long-pro- grain-size distributions excluding sand or fi ner MODEL FORMULATION fi le, we surveyed about ten points over a reach fractions, unlike most previous studies that length approximately fi ve times the observed lumped all grain sizes together to calculate non- Threshold and Transport Equations for bankfull width. The long-profi les were symmet- parametric measures (e.g., d50). Steep Alluvial Fans ric about the cross section. For channels over ~20 m wide, we used the maximum tape length LiDAR Topography Shields’ criterion predicts that at the threshold of 100 m to survey the long-profi le. These cri- of motion for a uniform grain size D, teria resulted in point spacing varying from We contracted with Airborne 1 to acquire ττρρ=−* ( ) several meters for narrow channels to 10 m for LiDAR topography at Globe with 1-m or denser ccswgD (1), the widest channels. Contour map slopes are average point spacing, with the intention of defi n- ρ ρ 77%–230% of reach slopes (Table 3; Fig. 8), ing channel dimensions and slopes as accurately where s and w are sediment particle and water τ illustrating the need for local measurements. as possible. We fi ltered the last return-point data density, g is the gravitational constant, c is Slopes derived from 1-m LiDAR (light detection set using TerraSolid Software’s TerraScan® pro- critical fl uid shear stress for initial motion, and τ* and ranging) contour maps closely approximate gram to approximate a bare-earth topographic c is a dimensionless number characterizing fi eld values (Fig. 8). surface. We kriged these points using Golden resistance to motion (Fig. 3). Approximations As part of an ongoing study on channel scour Software’s Surfer® with linear averaging over a of shear stress using the product of hydraulic ρ depth at Globe fan, we dug trenches between 10-m window, yielding a 1-m grid approximat- radius R and slope S ( wgRS) are known to be the banks of a second set of randomly selected ing bare-earth topography. Visual inspection locally inaccurate, but may capture reach-scale channels to estimate the depth of recent sedi- and fi eld checking indicate that the resulting variations in shear stress. Using this approxi- ment over older soils (Fig. 5). Recent fl uvial DEM (digital elevation model) contains many mation, (1) can be recast in terms of a threshold sediments are planar beds of sand and gravel local linear artifacts (scale errors?) from laser slope at bed-material entrainment: that range from 1 to 10 cm thick and lack the swath lines with systematically different eleva- red color or fi nes content of underlying pedo- tions (e.g., Fig. 5). ⎡ρρ− ⎤ D genic layers. The underlying soils are typically S = τ* ⎢ sw⎥ (2), c ρ R brown to red with Bt and Bw horizons and have Luminescence Dating ⎣ w ⎦ been dated elsewhere as Pleistocene (e.g., Sow- ers et al., 1988; Wang et al., 1994; Sohn et al., We sampled the base of the widest active assuming form drag is insubstantial. Addition 2007). Depth to this older horizon is an estimate channel in the fi eld area (B, Fig. 5), and a pocket of sine correction terms for steep slopes has a of long-term maximum scour depth for active of fi ne-grained (eolian?) sediment near the negligible effect on values (third decimal place channels and approximates the amount of Holo- base of the deepest bank exposure (A, Fig. 5) or lower). If fan slope is set by threshold condi- cene material stored beneath channel beds. We at the fan head. We sampled these deposits at tions, and D is represented by the median grain selected straight reaches to avoid excessive night, driving a PVC (polyvinyl chloride) tube size d50, a plot of d50/R against S should be non- τ* effects of fl ow convergence or divergence. We into sample areas and capping the ends of the random for a constant c. We collect these data sampled channels from ~0.1-m to ~20-m bank- tube. Shannon Mahan (U.S. Geological Sur- from the cross sections shown in Figures 4 and full width to examine variations in long-term vey, Luminescence Dating Laboratory, Denver, 5 and plot it to test this hypothesis. scour depth with channel size. Colorado) processed samples in her laboratory. To test the transport hypothesis, we input We assumed that during bank forming, likely She discarded end material (~3 cm) from each hydraulic and grain-size data from the cross supercritical fl ows, sands behave as suspended tube and prepared samples using standard sections below fan heads to a bedload transport load. We estimated coarse bed-material (>2-mm) procedures with appropriate modifi cations model and estimate unit bedload fl uxes down- size distributions and bed-surface sand fraction (Millard and Maat, 1994; Roberts and Wintle, fan. We use a gravel transport equation that

Fsand using a random walk technique across the 2001; Singhvi et al., 2001). Blue-light OSL Wilcock and Kenworthy (2002) modifi ed after entire channel bed (excluding banks) over an (optically stimulated luminescence) was done a surface-based transport model proposed by area ~4 m on either side of the cross section. on fi ne sand-size (125- to 105-micron size) Parker (1990). The expression was calibrated We chose 4 m as an arbitrary value that captures samples, and IRSL (infrared stimulated lumi- using measured bedload fl ux (e.g., Fig. 10) cross-section local conditions but is wide enough nescence) was done on the fi ne silt-fraction under a variety of fl ow conditions: that we do not persistently resample the same (4- to 11-µm) samples. All sand-size samples area. Using a pencil point, we collected inter- were analyzed by single-aliquot regeneration ⎛ ρ ⎞ ⎡ ⎤45. s − mediate axis measurements for grains >2 mm procedures (SAR) (Murray and Wintle, 2000; ⎜ 1⎟ gqbg ⎢ ⎥ ⎝ ρ ⎠ x until we had at least 100 measurements. For nar- Banerjee et al., 2001) with blue-light excita- W * = w =−A⎢1 ⎥ (3), g ⎛ τ ⎞ ⎢ ⎛τ ⎞ 025. ⎥ row or sandy channels, this resulted in covering tion. Dose recovery and preheat plateau tests F b ⎢ ⎜ b ⎟ ⎥ sand ⎝⎜ ρ ⎠⎟ ⎝ τ ⎠ the area multiple times. For very wide, gravelly were performed to ensure that the sediments w ⎣ rrg ⎦

628 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les * τ 50 /d b h (continued) # σ (mm) † IQR (mm) 2 (mm) Mode 84 d 75 d 25 d 16 d 5 6 24 34 12 0.38 24 34 18 0.53 6 5 5 8 53 70 21 0.27 53 24 45 0.47 8 5 5 6 23 34 13 0.38 23 32 17 0.53 6 5 50 – – – – – – – – – – – – – d 17 5 6 36 50 16 0.32 36 30 – 6 17 5 – (mm) 44 5 3 3 9 15 6 6 0.45 54 0.41 54 15 6 0.45 6 9 3 3 0.92 44 5 70 12 5 0.41 4 7 3 2 42 4 Sand fraction fraction R (m) 0.43 0.24 35 11 16 61 76 61 0.43 0.24 16 31 35 11 45 0.38 23 0.44 b 81 0.45 0.22 10 4 6 16 49 0.85 23 0.34 10 14 79 0.35 0.32 4 10 6 23 35 0.42 46 42 7 56 11 14 81 21 1.38 29 0.29 31 110 0.35 1.79 35 0.51 88 0.56 0.28 30 52 8 62 11 25 41 0.36 29 0.56 96 0.54 0.30 33 52 5 67 10 22 42 0.27 29 0.65 34 0.16 0.39 10 4 5 26 51 0.35 37 0.27 11 13 5 21 6 23 0.33 31 12 34 0.47 17 0.40 41 0.91 96 0.70 0.15 21 75 0.43 0.26 41 6 48 11 27 23 0.54 0.22 18 41 6 46 11 27 30 20 48 9 0.35 56 14 30 24 46 0.36 1.37 34 28 0.40 0.66 46 0.86 44 0.24 0.61 10 28 0.18 0.59 3 4 19 24 56 0.30 38 0.44 4 10 6 25 49 70 0.43 79 0.42 3 20 18 5 24 64 0.28 94 4 43 19 44 6 54 0.23 67 0.34 59 19 15 87 0.46 0.18 0.24 0.18 48 22 40 57 0.38 0.24 0.26 70 8 0.29 85 19 29 35 91 0.44 0.28 30 63 6 0.46 88 12 61 51 26 111 94 8 0.31 21 38 51 22 0.26 73 0.37 0.27 16 15 0.34 0.24 39 0.23 0.44 15 4 6 27 45 14 21 0.30 27 0.40 70 0.42 0.33 16 70 0.29 0.32 6 7 14 33 47 0.32 46 0.33 6 16 7 12 33 65 0.41 49 0.22 5 26 16 7 10 22 0.36 31 5 26 12 45 6 21 0.35 29 0.96 15 11 50 0.40 0.71 15 41 0.42 0.85 65 1.36 98 0.50 0.30 19 5 9 41 49 17 32 0.32 52 1.17 33 0.22 0.47 13 5 6 25 34 13 19 0.38 25 0.41 78 0.43 0.34 33 54 8 65 13 26 41 0.35 24 0.37 69 0.42 0.17 20 6 8 42 55 19 34 0.33 35 0.71 h (m) b w (m) (7.5') Slope (50-m) TABLE 3. CROSS-SECTION HYDRAULIC AND GRAIN SIZE AND GRAIN DATA HYDRAULIC CROSS-SECTION TABLE 3. w/d Slope (m) 7290 30 0.017 0.020 6.2 0.21 0.14 0.52 4 2 2 7 10 4 5 0.45 53 0.36 53 0.45 5 4 10 7 2 2 4 0.52 0.14 0.21 6.2 0.020 0.017 7290 30 x Downfan – 7469 1630 0.015 0.015 750 0.46 0.46 – – – – – – – – – – – – – – – – – – – – – 0.46 0.46 750 0.015 0.015 1630 – 7469 50 (m) width active wash Kelso Sample Total Amargosa – 5286 1027 0.006 0.01 380 0.37 0.37 – 0.37 0.37 380 0.01 0.006 Amargosa – 1027 5286 22 – -2256 60 0.063 0.067 38.3 0.64 0.28 0.23 15 5 6 32 42 14 26 0.35 44 0.72 44 0.35 26 14 42 32 6 5 15 0.23 0.28 0.64 38.3 0.067 0.063 60 -2256 – 22 8 65 3342 35 0.048 0.038 11.9 0. 11.9 0.038 0.048 35 3342 65 8 16 100 402 – 0.057 0.069 54.1 – – 0.22 11 5 7 25 36 13 18 0.37 – – – 0.37 18 13 36 25 7 5 11 0.22 – 0. – 1. 0. 47.6 54.1 96.6 56 0.060 0.069 0.060 0.057 0.050 0.057 0.047 0.054 59 – 65 71 786 402 0 60 -378 100 100 – 15 16 17 18 20 – -1134 78 0.053 0.049 48.6 0.62 0.37 0.33 18 4 7 30 44 15 23 0.30 34 0.66 0.48 0.98 34 25 56 0.30 0.37 0.37 23 35 21 15 21 14 44 51 36 30 44 28 7 9 7 4 7 5 18 25 14 0.33 0.23 0.18 0.37 0.34 0.37 0.62 0.62 0.79 48.6 44.6 44.8 0.049 0.056 0.060 0.053 0. 0.058 0.062 78 13.6 72 57 -1134 -2580 – 0.068 -3066 – 20 – 23 0.073 24 14 1062 20 14 16 21 534 20 0.068 0.079 18.8 0. 0. 18.8 29.9 0.079 0.084 0.068 0.069 20 40 534 288 21 56 16 17 18 35 0 21 0.071 0.068 25.6 1. 2 28 5010 18 0.023 0.026 8.05 0. 0. 0. 0. 0. 8.05 0. 9.1 20.8 0. 16.5 0.80 0. 14.2 0.026 23.2 0.031 0.043 17.7 24.3 0.049 14.4 0.062 0.023 0.051 0.031 0.04 0.051 0.071 0.046 0.129 18 0.052 33 0.052 37 0.055 0.059 21 5010 0.056 4770 16 4380 41 28 19 3612 28 30 42 21 2982 2 2574 49 3 2280 4 1962 35 1650 28 6 49 35 8 35 9 10 11 12 7 50 3708 43 0.040 0.052 16 0.37 0.25 0.39 12 0.39 0.25 0. 0.37 31.2 16 0.054 0.052 0.042 0.040 80 43 4098 3708 70 50 6 7 9 70 2904 65 0.046 0.062 24 0.37 0.21 0.34 11 0.34 0.21 0.37 0. 0. 0. 0. 0. 24 23.5 50.2 57.2 26.2 37.5 0.062 0.057 0.069 0.052 0.056 0.052 0.046 0.053 0.058 0.056 0.050 0.055 65 46 72 82 56 2904 58 2544 2010 1866 70 1500 80 1158 70 70 9 50 10 80 11 12 13 14 Sheep Creek 19 – -738 – 0.051 0.051 69.4 – – 0.28 – – 28 69.4 0.21 0.051 0.31 0.051 0.67 – -738 21 – 19 0.068 0.071 31 822 49 15 21 – -1524 84 0.050 0.054 80.9 0.96 0.46 0.31 20 7 9 40 49 19 31 0.38 48 0.70 48 0.38 31 19 49 40 9 7 20 0.31 0.46 0.96 80.9 0.054 0.050 84 -1524 – 21 1 49 5286 27 0.014 0.013 7 0.26 0.15 0.70 9 3 4 20 55 11 16 0.23 29 0.14 29 0.23 16 11 55 20 4 3 9 0.70 0.15 0.26 0. 7 0.013 9.4 0. 0.014 0.062 26.1 27 0.05 0.071 5286 11 0.065 49 3096 1 27 20 1404 7 20 13 3 50 5502 34 0.034 0.033 9.1 0.27 0.18 0.41 6 3 4 15 22 7 11 0.37 45 0.63 45 0.37 11 7 22 15 4 3 6 0. 0.41 0. 0.14 0.18 0.17 0.27 0.27 0.28 0. 5.3 9.1 12.9 6.4 0.022 0.033 0.043 0.054 0.024 0.034 0.036 0.040 20 34 46 19 6294 5502 4860 4284 50 50 80 60 2 3 4 5 Location Globe 1 5 35 4014 28 0.043 0.038 19.5 0. 19.5 0.038 0.043 28 4014 35 5

Geological Society of America Bulletin, May/June 2008 629 Stock et al. *

τ 50

/d b h #

σ (mm) †

IQR (mm) maps. ′

(mm) Mode

84 contour maps are maps are contour ′ d 75

25 d 16

eld. d (continued) (continued) 50

eld channel lengths tend to be longer eld channel lengths tend to be longer d (mm)

SIZE DATA 70 7 3 4 12 16 7 8 0.43 30 0.20 30 16 7 0.43 8 12 4 3 70 7 02 30 8 15 54 55 24 39 0.38 53 1.43 55 24 39 54 15 8 02 30 Sand Figure 8. Plot of reach slope against contour-map slope against contour-map 8. Plot of reach Figure local measure- for slope, illustrating requirement 7.5 ments. Channel slopes from values, often higher 77%–230% of reach-slope because ﬁ 7.5 sinuous) than estimates from (i.e., more Globe are By contrast, LiDAR slopes shown for because they values, closer of reach 89%–115% of the channel length approximations better are observed in the ﬁ fraction fraction

R (m) 0.10 0.43 13 35 27 5 5 13 22 0.37 16 0.09 0.11 0.30 27 60 43 6 8 21 35 0.32 17 0.05 LIC AND GRAIN b

43 0.33 0.44 23 7 46 65 10 40 0.46 0.40 22 24 36 7 61 83 11 0.32 24 19 0.42 50 0.29 17 0.55 33 0.16 0.47 6 3 4 9 10 6 5 0.54 55 0.19 89 0.53 0.14 24 19 0.69 0.16 8 80 99 12 24 27 7 55 81 11 24 68 0.28 44 37 0.29 0.89 50 1.02 h (m) 0.55 0.35 0.42 17 6 8 40 54 18 33 0.33 32 0.45 0.20 0.13 0.62 10 0.33 0.10 0.61 5 6 15 19 23 5 11 6 31 36 14 13 0.46 25 20 0.37 0.21 22 0.12 0.38 0.37 0.41 16 6 7 44 0.51 0.42 54 0.37 17 23 8 48 10 71 37 0.33 24 25 38 0.52 0.33 22 0.49 b

w (m) Globe Sheep Creek Hanaupah Lucy Gray Globe, LiDAR

(7.5’) Slope

(50-m) TABLE 3. CROSS-SECTION HYDRAU CROSS-SECTION TABLE 3.

reach slope over ~50 m w/d Slope 1:1 (m) 4990 16 0.005 0.005 2.3 0.14 0.14 1.00 1 1 1 1 1 0 1.00 140 0.42 140 1.00 0 1 1 1 1 1 1.00 0.14 0.14 2.3 0.005 0.005 4990 16 4584 36 0.019 0.020 7.6 0.21 7.6 0.020 0.019 4584 36 4689 22 0.012 0.006 7.35 0. 7.35 0.46 0.006 7.2 0.012 0.020 4689 22 0.021 4500 16 x Downfan 0.001 0.01 0.1 0.1

0.01

50 50 50 50

(m)

slope slope 7.5 0.001 width active ′ Interquartile Range Range Interquartile Standard deviation deviation Standard † #

3 610 6195 44 0.026 0.038 8.8 10.3 0. 0.038 0. 0.036 11.2 0.026 32.5 0.028 0.061 44 31 0.049 0.047 6195 5495 0.048 610 550 26 3 81 4 1355 445 180 70 12 14 10 49 2636 25 0.059 0.052 29.8 1. 29.8 0.052 0.059 25 2636 49 10 3 4 2 340 7035 57 0.025 0.029 12 0.21 0.09 0. 0.09 0.21 12 0.029 0.025 Total Location Sample 57 2 7035 340 Lucy Gray 2 1 400 7545 47 0.014 0.024 7.1 0.15 0.15 0.80 7 4 4 13 17 8 9 0.49 21 0.18 6 84 3819 14 0.045 0.049 12.1 0.84 0.46 0.30 49 5 14 102 141 33 88 0.19 17 0.26 0.31 0.64 17 19 37 0.19 0.28 0.28 0.13 0.23 0.17 0.16 0.60 88 23 51 13 10 65 10 0.49 39 33 0.48 27 0.37 26 0.30 0.34 29 0.33 141 15 90 32 112 44 40 0.40 22 12 102 66 19 21 21 73 26 15 26 50 67 14 15 61 17 8 46 22 41 43 52 5 7 50 6 29 7 9 49 34 30 8 26 10 7 6 0.30 7 0.28 8 6 0.26 7 5 11 20 0.46 7 0.33 22 0.50 21 14 0.62 0.50 15 0.50 0.84 0.58 0.44 0.97 0.37 0. 0.17 0.44 0.17 0.16 8 12.1 0.16 10.5 0.30 30.7 0.30 0.24 0.26 0.041 0.21 0.049 0.20 0.059 0.52 0.074 8.4 0.44 6.3 0.046 9.55 0.045 13.5 0.055 9 0.067 10.1 0.043 14 23.6 0.045 14 0.053 11 0.053 0.048 34 4015 0.055 0.029 3819 0.055 0.026 3672 0.038 56 0.034 3105 84 0.044 63 35 0.041 5 24 0.036 77 6 45 7 68 17 9 4840 23 4280 43 3705 3225 420 2755 390 8 2260 330 1805 180 5 77 180 6 140 7 140 3500 8 9 17 130 0.66 0.20 0.058 0.067 16 28 0.94 88 10 23 10 7 78 0.23 11 41 1.01 15 130 0 72 0.044 0.069 36.7 Hanaupah 1 13 80 915 46 0.036 0.053 17.6 0.053 0.036 46 915 11 80 – 13 -2674 57 0.068 0.074 90.2 1.58 1.04 0.

630 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les

1000

100 (mm) 50

10 median grain szie d

1 10 20 30 40 50 60 70 80 90 100 # grains counted

Figure 9. Plot of median value of cross-channel point count as a function of the count number for cross sections of the Hanau- pah fan, the most poorly sorted channels of the study sites. Median values tend to stabilize above ~75 counts, indicating that 100 counts are probably a minimum number of gravel counts to reach a stable median value in this environment.

10 1 Parker (1990)

eq. 3 Figure 10. An attempt to validate the use of low-gradient, gravel-trans- (see Wilcock and port equations for high-gradient, supercritical fl ow on fans. Plot of nondi- Kenworthy, 2002) τ τ mensional shear stress for gravel fraction ( b/ rg) against nondimensional * gravel-transport rate Wg , after Wilcock and Kenworthy (2002). Small sym- 10 –1 antidune bols represent gravel-transport rates measured in lower gradient (typically <0.03 slope) gravel-bed rivers and used to calibrate versions of Equation (3). upper planar Larger stippled circles and squares are gravel transport rates from super- and transition critical fl ow in a fl ume (Ikeda and Iseya, 1988), with τ calculated using * rg Wg Wilcock and Kenworthy’s (2002) subsurface sand-fraction model (their Equation 8). Thick solid line is Wilcock and Kenworthy’s Equation 4 for gravel transport [Equation (3) in this paper], with A = 115. Thin line is the Parker (1990) transport function. We fi nd that an exponent of −6 in Equa- 10 –3 tion (4), rather than the suggested −25, best fi ts the experimental data to the bold line representing Equation (3), but the difference is not striking. Close fi t of supercritical fl ume data to transport rates observed elsewhere is consistent with the use of Equations (3) and (4) to model gravel transport on steep fan channels with supercritical fl ow. Note the underprediction of transition and upper planar transport rates, probably as a result of reduced fl ow resistance. 10 –5 10 –1 10 0 10 1 τ τ / rg

Geological Society of America Bulletin, May/June 2008 631 Stock et al.

* where Wg is nondimensional unit gravel ﬂ ux qbg, • Variations in ﬂ ow intermittency downfan low slope (<0.02) perennial gravel-bedded riv-

Fsand is surface bed sand fraction, and the param- are small enough to have marginal effects on ers with low relative roughness values (e.g., hb/ eters A ( = 115) and x ( = 0.923) have values transport rate. d50 values of 100–1000) on which (3) has been estimated in Wilcock and Kenworthy (2002). There is evidence that is consistent with some of calibrated. Many models of roughness on fan τ τ The quotient b/ rg is the nondimensional critical these assumptions. For instance, older deposits channels suggest critical or supercritical fl ow τ τ* shear stress *c for gravel entrainment, or rg. above active washes (Fig. 7) and shallow chan- (e.g., Dawdy, 1979; Edwards and Thielmann, τ* A further adjustment reduces rg as surface- nel scour depths indicate that subsidence can- 1984; French, 1987; Blair and McPherson, bed sand fraction increases, using an expression not be invoked to store large amounts of gravel, 1994a). The sole direct measurements that we proposed by Wilcock and Kenworthy (2002): at least over Holocene timescales. We assume are aware of for fl ow on alluvial fans (Rahn, that production of sediment on the fan is neg- 1967; Beaumont and Oberlander, 1971) indicate (ττ**) − ( ) ligible compared to source catchment, so that standing waves during a fl ow event and Froude ττ**= ( ) + rg01 rg (4), unit gravel fl ux declines monotonically down- (Fr) numbers far greater than 0.8. These observa- rg rg 1 []F exp 14 sand fan. We know this assumption is locally violated tions suggest that the major sediment transport because we have observed small catchments fl ows on most fans with substantial sand likely τ where *rg is the reference nondimensional from desert pavement areas on the fan contribut- involve standing waves, antidunes, and chutes- stress for gravel at the threshold of motion, ing sediment. However, these volumes appear to and-pools, at Fr greater than 0.8 (Table 1). τ ( *rg)1 is this stress for a bed of 100% gravel, be subordinate to that produced from an actively We know of no fi eld data that can be used to τ and ( *rg)0 is the equivalent stress for a bed eroding source catchment. The assumption that evaluate the effect of supercritical fl ow condi- with negligible gravel. Following Wilcock and channel slope is adjusted to transport gravel fl ux tions on conventional bedload transport equa- τ Kenworthy (2002), we set ( *rg)0 = 0.0350 and rate is consistent with the observation that gravel tions. Therefore, we use data from the nearest τ ( *rg)1 = 0.011. fractions tend to cover most of the bed (Table 3). fl ume analog to ask the question whether (3) We apply Equations (3) and (4) to alluvial We test the assumption that (3) is an unbiased is defensible to use under steep, supercritical fan channels, using the cross-section values of estimate of bed-material transport down ephem- fl ow. We substitute Ikeda and Iseya (1988) data τ shear stress b and sand fraction Fsand (Table 3) eral, steep fan channels in the next section. We into (3) and plot predicted, nondimensional τ*r and g values calculated from (4). At the upper- evaluate the assumption that spatial variations in shear stress against measured bedload fl ux rate most fan cross section (e.g., 16 in Fig. 5), we form drag do not dominate transport rate down- (larger, stippled symbols in Fig. 10), and against calibrate (3) by fi nding the unit fl ux that repro- fan by plotting the variation in relative roughness unit bedload fl ux predicted using the subsur- duces the observed channel slope for the mea- downfan in the Results section. We assume that face model of Wilcock and Kenworthy (2002). sured bed material and hydraulic radius R. At fl ow conditions at bankfull set channel slope, a This subsurface model differs from the surface the next downfan cross section, we repeat this proposition that has been argued in perennial model (4) in that it uses volumetric sand frac- procedure, with the additional step of partition- rivers, although it has yet to be shown in arid tion, the parameter reported by Ikeda and Iseya ing fl ux across the total active channel width in regions. Experimental studies by Hooke (1967) (1988) and a slightly different fi t for the infl u- τ the sector. We repeat this procedure downfan to and Hooke and Rohrer (1979) indicate channel- ence of sand on *rg (Equation 8 with subsurface generate a plot of unit bed-material fl ux versus bed aggradation during transport followed by parameters from Wilcock and Kenworthy, 2002, τ downfan distance. degradation on the declining hydrograph limb. Table 3). We calculate b from the depth-slope This procedure has a number of implicit To the degree this sequence occurs on natural product of fl ume fl ow and correct for side-wall assumptions, which can be summarized in fans, we overestimated fl ow depths from active drag using the formula from Williams (1970). approximate order of decreasing supporting bank heights. However, the downfan pattern of Plots of the Ikeda and Iseya (1988) data in Fig- evidence: reduced bank height implies similar decreases ure 10 are well approximated by Equation (3) • For purposes of late Holocene timescale in fl ow depth, even if we overestimate its mag- predictions, which are shown as a bold line. This sediment budgets, there are negligible amounts nitude. We also apply mainstem values of shear is consistent with the use of (3) to model gravel of gravel deposited in active channel beds. stress to other active channel threads across the transport on steep alluvial fan channels where • For purposes of transport calculations, there arc. This has the tendency to overestimate fl uxes bankfull fl ow is probably also supercritical. is negligible production of sediment on the fan on smaller channels, although the proportion of so that unit bedload fl ux does not increase below smaller channels across many arcs tends to be RESULTS the fan head. low (Table 3). To distribute sediment down the • Coarse particle (gravel) fl ux controls slope. fan segment, we assume that gravel fl ux is dis- Table 3 lists fi eld data from the channels of • Equation (3) provides an unbiased estimate tributed by proportion to channel width, which alluvial fans in Table 2. To our knowledge, data of bedload fl ux. we have no way of testing. Nor can we evaluate in Table 3 represent the most complete data set • Spatial variations in form drag downfan are the role of the frequency of fl ow and transport yet collected for alluvial fan channels domi- negligible. events (intermittency) downfan. nated by traction transport. For this reason, we • Bankfull fl ows set transport rates that con- illustrate patterns of hydraulic and texture data trol reach slope. Test of Transport Equation Using Flume from the subset of the data in Figures 11–9. • Observed bankfull depths correspond to Data depth during active transport. Hydraulic Geometry • Mainstem shear stresses are characteristic of We can test whether Equation (3) is reason- smaller threads. able to apply to fan channels that are steep Figure 5 illustrates the downfan decrease in • Gravel fl ux from fan-head channel can be (0.01distributary channels downfan in critical fl ow with high relative roughness (hb/d50 LiDAR topography. Slopes are close to fi eld- proportion to channel width. <100). These conditions are different from the surveyed reach slopes (Fig. 8); therefore, this

632 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les

0.8 Globe Sheep Creek Hanaupah Lucy Gray 0.6

0.4 hydraulic radius (m)

0.2

0 0 2000 4000 6000 8000 10000 meters from fan head Figure 12. Gravel bar near cross section 8 on Globe fan, illustrating Figure 11. Downfan decline in hydraulic radius. movement of bed material overbank.

LiDAR topography does approximate fi eld of Globe fan narrowing more rapidly downfan Granulometry measures of channel slope. Although fi eld-sur- than they shallow. veyed slopes may be locally different from those Figure 15 shows the lack of gravel fi ning measured from U.S. Geological Survey 7.5′ Scour Depth over the upper 60%–80% of fan-channel length. topographic maps (e.g., Fig. 8), the pattern of Sheep Creek fan has the coarsest median val- downfan slope decline from ~0.10 at fan heads Figure 14 illustrates recent sediment deposits ues of ~30 mm, roughly three times larger than to slopes approaching 0.01 at distal channels is that increase in depth from centimeters to ~100 cm those observed at similar downfan distances not substantially different. as channel bankfull width increases from ~0.1 m with the Globe fan. Other percentiles (Table 3) Figure 11 illustrates a downfan decline in to 20 m. Episodic burial and channel scour into have similar patterns, showing no detectable hydraulic radius, from values of 0.5–0.9 m at the underlying, moderately indurated soils post- fi ning in the upper half or more of the fan. All fan heads, to values of 0.1–0.2 m at distal fan dates Pleistocene to earliest Holocene-aged soils fans show fi ning after ~4 km downfan, distances channels. These values lie within the range at these sites (Fig. 5). Luminescence dates from where bankfull depth on all four fans converge reported by other authors (e.g., Wasson, 1977; the base of one 20-m–wide channel (B in Fig. 5) to values of ~1/3 m, and reaches have 50%–60% Field, 2001). Downfan from these channels, range from 5.5 ka (OSL) for the sand fraction gravel, as well as sandy patches. Figure 16 illus- banks are poorly expressed or not present. to 12.7–15.5 ka (IRSL) for the fi ne silt fraction trates a decline in bed-area gravel fraction, from For all fans, there is a rapid, often exponential (Table 4). The fi ne-silt mode with the older IRSL values of 0.6–0.9 at fan heads, to values below decline in hydraulic radius. For fans with no age is distinct from the rest of the sample. The 0.5 at lower fan boundaries. The pattern is con- recent faults (Globe and Lucy Gray), the expo- fi ner material is plausibly a separate, partially sistent with fi eld observations of overbank lobes nential decline is followed by a linear reduction bleached population deposited from a turbid of coarse gravel (Fig. 12), indicating off-chan- in hydraulic radius at a rate of ~10−5 m/m in the water column into the pores of a fully bleached nel deposition of part of the coarse bed mate- lower part of fan. sand layer. The dates indicate late Pleistocene to rial over older Active or Young alluvial deposits. Locally, banks have been overridden by Holocene deposition in the channel, with the most On Globe fan, patches of fi nes begin to appear pulses of bed material that are present as narrow likely time of peak scour at ca. 5 ka. Together in the channel at gravel fractions below 60%– tongues of gravel just overbank (Fig. 12). These with Figure 14, relative and absolute age dating 70%, corresponding to slopes <0.05. Figure 15 deposits are commonly found overbank near indicates that large amounts of sediment cannot indicates that (1) substantial gravel deposition τ channel bends, where the inertia of bed mate- be stored in Globe channel beds over late-Pleisto- occurs downfan, and (2) *c can be expected to rial appears to have carried it up and over the cene to Holocene timescales. decline downfan as bed sand fraction increases channel bank into older Active or Young alluvial By contrast, sample A from the fan-head fi ll (e.g., Wilcock and McArdell, 1993). This reduc- terraces. Similar deposits can be found as wake- terrace is at least 72 ka, and possibly older, as tion would tend to increase bedload fl ux in the zone effects downstream from large bushes, it is either saturated or close to saturation for absence of other changes. boulders, or other channel obstructions. luminescence (see dose rates in Table 4). This Width-to-depth ratios for most fan channels minimum date indicates that deposition (and Test of Threshold are between 10 and 100 (Table 3; Fig. 13). Many probably sediment-supply rate) at the Globe fans have channels with approximately constant fan head has been lower for many multiples We test the threshold explanation for fan- width-to-depth ratios (parallel lines in Fig. 13). of Holocene time. It also indicates that the last channel slope (2) by plotting reach slope against As bankfull depths grow shallower downfan, incident of bedrock-channel incision at this the ratio of median grain size to hydraulic τ they grow proportionally narrower. Exception- location must be substantially older than 72 ka radius, assuming constant *c. If the reduction ally, channels of the Globe and Lucy Gray have because of the heavily indurated fanglomerate in median grain size is the primary control on more variable width-to-depth ratios, with those in the channel thalweg nearby. reach slope, we expect a nonrandom, positive

Geological Society of America Bulletin, May/June 2008 633 Stock et al.

1000 10

0.794 Sc = 0.085w 0 max b = 100 / h b w b 1

(m) 100 B b

0.1 = 100 / h b w b Globe fan 10 Globe basin bankfull width w Sheep Creek 0.01 maximum scour depth (m) Hanaupah confined, Globe 10 unconfined, Globe = Lucy Gray / h b unconfined, other fans w b axial washes

1 0.001 0.1 1 10 0.1 1 10 100 bankfull depth hb (m) bankfull width (m) Figure 13. Plot of bankfull depth versus width for fan channels. Lines Figure 14. Plot of recent deposit depth over Pleistocene soils against show constant values of width-to-depth ratios. With the exception of bankfull width for channels of Globe fan. Optically stimulated lumi- Lucy Gray and Globe, the channels on fans tend toward a constant nescence (OSL) date at the base of the widest channel (B) is ca. 5 ka, ratio between 10 and 100. consistent with Holocene age for channel ﬁ lls.

correlation between local slope and the ratio of widths, this exercise predicts that fan-channel where slope declines, although sand fractions do grain size to hydraulic radius. A plot of these slope must increase downfan as R declines, even increase downfan. In the case of fl uvial trans- τ variables from the four fan mainstem channels with compensating effects of reductions in *c port, there are no established theories to predict in Figure 17 indicates that there is no strong and spreading of fl ux into distributary channels. channel gradient as a function of the largest correlation of reach slope with the ratio of grain These results indicate that any transport expla- grain size. So, most previous studies arguing for size to hydraulic radius. The data are inconsis- nation for these fan slopes must incorporate a maximum grain-size control on fan slope (e.g., tent with a threshold entrainment explanation term for reduction in gravel fl ux downfan, as Blissenbach, 1952; Bluck, 1964; Denny, 1965; for fan-channel slope. hypothesized by Drew (1873). Kesel, 1985; Kesel and Lowe, 1987; Hubert and Following the method outlined above, we ask Filipov, 1989) cannot be interpreted using the Test of Transport what values of unit bedload fl ux are required to available understanding of sediment transport. match observed reach slope at each of the fan Observed patterns in bed-material grain size and To illustrate downfan patterns in resistance to sites. The resulting graph (Fig. 21) illustrates the sand fraction (Figs. 15 and 16) may also require transport, and transport capacity, we plot rela- pattern of the reduction in bedload fl ux required, some modifi cations to established models of tive roughness (Fig. 18) and τ* (Fig. 19) down if there are: (i) no errors in fi eld data; (ii) the fan sedimentology that depend on progressive the main axial channels of the four fans. There transport Equation (3) is appropriate; (iii) the grain-size fi ning downfan. τ is no strong pattern in relative roughness. How- *c correction is appropriate; and (iv) smaller Drew’s hypothesis that concave-up fan long- ever, transport capacity declines exponentially channels along the contour arc hydraulic radii profi les result from progressive downfan depo- down all fans as hydraulic radius decreases, equivalent to the mainstem. The pattern is domi- sition is consistent with calculations. Transport while median gravel size remains approxi- nated by an exponential decline in required unit solutions for channel slopes require exponential mately constant in upper and middle reaches. gravel fl ux downfan. The length over which gravel-deposition rates downfan. These rates These relations indicate that in the absence of half of the load is lost varies from ~800 m to result from the combination of rapid decline other effects, decreases in hydraulic radius sub- 1400 m for fans with no intersecting faults, and in hydraulic radius with a negligible change in stantially reduce transport rates downfan. Only is much shorter for the Hanaupah fan (200 m) median gravel-particle diameter and an expan- τ τ if *c reductions with increasing sand fraction with a mid-fan normal fault, and a nearby lacus- sion ratio and *cr reductions that are not suf- are large, or channels bifurcate very rapidly trine lower boundary. These loss rates are likely fi ciently large to counteract the decline in τ*. so that the load in any one channel is greatly minimums because condition (iv) overestimates Even with this decline, fl ow depths far less than reduced, can total bedload fl uxes be relatively the transport capacity of smaller channels along observed bankfull depths would be required to constant downfan. the contour arc. deposit bed material in-channel. Instead, obser- We explore the effects of channel bifurcation vations of overbank deposition on low terraces τ and *c reductions on transport at Globe fan by DISCUSSION or abandoned channels suggest that most deposi- using (3) to predict changes in channel slope tion occurs where bed material moves overbank down Globe fan, assuming no gravel deposition The commonly cited explanation for decreas- to cover older Active or Young alluvial depos- downfan as the null hypothesis. We calculate ing slope downfan as a result of grain-size fi ning its. This would explain the thin, late Holocene this fl ux as the value in (3) that reproduces the is not consistent with data from the fi eld sites deposits (Fig. 14) observed in most channels. observed reach slope at the fan-head cross sec- that we have investigated (Fig. 17). Median It would also be consistent with experiments tion. As Figure 20 indicates, even with declin- bed-material grain sizes do not change detect- by Sheets et al. (2002) showing that the major- τ ing *c and modest increases in total channel ably over large fractions of the channel length ity of deposition occurs during short-duration,

634 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les

TABLE 4. GAMMA SPECTROMETRY, COSMIC AND TOTAL DOSE RATES, AND EQUIVALENT DOSES AND AGES Sample Sediment K Th U Water Cosmic dose Total dose De nφ Age number typeα (%) (ppm) (ppm) content rate rate (Gy) (ka) (%) (Gy/ka)# (Gy/ka)‡ KS04-IV-33 Fluvial 3.76 ± 0.22 12.7 ± 0.37 1.43 ± 0.13 9.0 ± 0.4 0.18 ± 0.02 5.07 ± 0.13 28.0 ± 1.15d 7 (8) 5.52 ± 0.54 7.66 ± 0.21 97.0 ± 1.07e – 12.7 ± 0.72 119 ± 0.80e – 15.5 ± 0.83 KS05-IV-12 Eolian 3.56 ± 0.07 20.0 ± 0.40 2.06 ± 0.17 10 ± 0.5 0.23 ± 0.02 4.62 ± 0.08 > 231 ± 25.4d 13 (24) > 50.1 ± 6.76 6.10 ± 0.12 439 ± 32.9e – 72.0 ± 4.14 > 464 ± 58.0e – > 76.0 ± 6.17 αDiscrete beds contained within the alluvial fan or wash #Cosmic doses and attentuation with depth were calculated using the methods of Prescott and Hutton (1994) ‡Total dose rate is measured from moisture content of 10% as an average between field moisture and full saturation moisture values. dDose rate and age for fine-grained 250-105 um quartz sand using single-aliquot additive dose. Linear regression used on age, errors to one sigma eDose rate and age for fine-grained 4-11 um polymineral silt (IRSL) using multiple-aliquot additive dose. Exponential regression used on age, errors to one sigma. Correction for fading made using a g value of 5.27%/decade thus equivalent dose as given is original and not corrected φNumber of replicated equivalent dose (De) estimates used to calculate the mean. Second number is total measurements made including failed runs with unusable data.

1 Globe 60 Sheep Creek Globe Sheep Creek Hanaupah 0.8 Hanaupah Lucy Gray Lucy Gray

40 0.6 (mm) 50 d

0.4 gravel fraction 20

0.2

0 0 2000 4000 6000 8000 10000 0 0 2000 4000 6000 8000 10000 meters from fan head meters from fan head Figure 15. Plot of median gravel diameter versus distance from fan Figure 16. Plot of bed area gravel fraction versus distance from head. Resolvable ﬁ ning occurs in the last ~25%–50% of the fan- fan head. channel lengths. Trends for 16th, 25th, 75th, and 84th percentiles are similar (Table 3).

0.1

Globe Sheep Creek 140 Hanaupah Globe Lucy Gray Sheep Creek 120 Hanaupah Lucy Gray 100 0.01 50 80 / d b reach slope h 60

20 0.001 0.001 0.01 0.1 1 d50 / R 0 0 2000 4000 6000 8000 10000 Figure 17. (A) Plot of reach slope against ratio of median grain size meters from fan head to hydraulic radius as a test of Shields’ threshold explanation for fan- τ Figure 18. Plot of relative roughness (ratio of bankfull depth to channel slope, assuming constant *c. Slopes of lines through each data set are indistinguishable from zero, indicating that these statements of median gravel diameter) versus distance from fan head, illustrating the threshold channel are inconsistent with observed data on fans. reduction in hydraulic radius.

Geological Society of America Bulletin, May/June 2008 635 Stock et al.

2 cate that (1) Holocene subsidence rates are neg- Globe ligible; (2) hydraulic radius decreases downfan; Sheep Creek (3) total active channel width is variable; and (4) Hanaupah Shields’ stresses (τ*) decrease downfan. These Lucy Gray differences require deposition of large gravel volumes downfan to match observed gradual slope reduction (e.g., Fig. 2). So, over times- ∗ 1 τ cales at which we can estimate bankfull depths from fi eld data, Mojave fans depart signifi cantly from published models. Over longer, Pleisto- cene timescales, higher terraces in Figure 7 do record aggradation at rates that decrease downfan, as recorded by decreasing elevations above modern channel fl oors. This pattern requires a 0 model for variable downfan aggradation rates, 0 2000 4000 6000 8000 10000 missing at present because of a need for an over- meters from fan head bank, bed-material deposition model. Figure 19. Plot of τ* versus distance from fan head, illustrating reduc- At Holocene timescales, the declining tion in transport capacity downfan as hydraulic radius decreases. hydraulic radius downfan is the most important infl uence on the transport calculations. Shal- lowing could result from any combination of (i) bifurcation of source catchment fl ow into distributary channels, (ii) transmission losses of 1 water, or (iii) reductions in local runoff genera- Meyer-Peter Mueller eqn. tion downfan. For instance, after storms in 2004 eqn. 4 (modified Parker) and 2005, surfaces where pedogenesis created 7.5’ slope strong Av or Bt horizons of low permeability produced minor runoff and sediment transport (Schmidt et al., 2004) that dissipated downfan. These observations raise the possibility that at 0.1 least in the present climate, fan-channel runoff slope is locally sourced from older Pleistocene fan deposits, rather than from the source catchment. This may be one of the fundamental differences between temperate fans that depend on source catchment runoff and arid region fans with biologic soil crusts, silt-rich Av horizons, and 0.01 0 2000 4000 6000 8000 impermeable subpavement B-horizons (e.g., meters from fan head Denny, 1965; Beaty, 1968) that can generate local Horton overland fl ow. Figure 20. Plot of slope predicted by Equation (3) for Globe fan, The interpretation of fan long-profi les as assuming no gravel deposition downfan. MPM refers to Meyer- statements about sediment supply links the Peter and Mueller bedload equation (see Julien, 1998, equation 9.3a) history of hillslope sediment production rates is a common averaged shear stress bedload-transport equation. to fan surface slopes. For instance, surfaces of Increase in slope indicates that despite spreading of channels and Pleistocene deposits (e.g., >72-ka sample from

reduction of t*c, substantial gravel deposition is required to match Fig. 5A) have higher slopes at Globe and adja- observed slopes with existing theory. cent fans (Fig. 22) than active channels. Test pits show that they also may have fi ner bed material. This combination is consistent with the hypoth- unchannelized fl ow. Field observations indicate Parker et al. (1998). Solutions for an idealized esis that the older, higher surface at Globe repre- three possible mechanisms for overbank, bed- channelized fan in Parker et al. (1998) assume sents aggradation due to greater sediment supply. material transport: (1) particle inertia ejection constant subsidence rates, fl ow depths that As an illustration, if the modern Globe channel as the channel changes direction; (2) particle increase dramatically downfan as active channel had the slope of the Pleistocene deposit adjacent inertia ejection at in-channel obstructions like widths decrease due to deposition, and constant to it for the fi rst seven cross sections (Fig. 22), vegetation; and (3) local loss of banks leading to Shields’ stresses. The resulting long-profi les are its unit bedload fl ux from Equation (4) would fl ow expansion and deposition. At present, we largely statements about the planform geom- be ~8% larger than that currently required to have no expectation about which process pre- etry of the subsiding space to be fi lled and the match its fan-head slope. Although one cannot dominates on a given fan. increase in fl ow depth as sediment fl ux decreases rule out alternate hypotheses (e.g., wider chan- Field data from alluvial fans depart substan- downfan and channels narrow for a constant dis- nels and shallower fl ow at the same sediment tially from idealized parameters presented in charge. Field data from the fans in Figure 4 indi- supply) to explain the steeper slopes observed

636 Geological Society of America Bulletin, May/June 2008 Alluvial fan long-proﬁ les

100 on these older deposits, systematic increases in sediment supply from hillslopes in the past (e.g., Bull, 1991) might be a more testable hypothesis 10

given current uses of cosmogenic radionuclides /sec) to estimate past lowering rates. 2

New views of the transport of the coarse size (m s fractions (e.g., Solari and Parker, 2000) may also 1 yield new interpretations of old observations. For instance, downfan decreases in maximum clast size observed in previous studies could 0.1 refl ect decreasing bankfull depths downfan. Globe x1/2 = 900 m Field and fl ume experiments show that rapid Sheep Creek x1/2 = 800 m fl ow over surfaces that are smooth relative to the 0.01 Hanaupah x = 200 m unit bedload flux q 1/2 maximum particle size can transport particles Lucy Gray x1/2 = 1400 m that approach, or even occasionally exceed fl ow depth (e.g., Fahnestock and Haushild, 1962; 0.001 100 1,000 10,000 Fahnestock, 1963; Beaumont and Oberlander, meters from fan head 1971). This is probably a statement about the predominance of drag forces in steep channels Figure 21. Plot of minimum required unit bedload fl ux at each cross section with relatively low friction angles and fast, shal- necessary to reproduce observed local slope. Downfan reductions can be low fl ow (e.g., critical to supercritical fl ow in characterized by exponential curves with the distance over which half the channels with substantial sand cover). Alluvial load is lost, varying from ~200 m for Hanupah to ~1400 m for Lucy Gray. fan channels in parts of Death Valley transported large particles during 2004 rainfall events (e.g., Fig. 23), and previous observations indicate that 5 channels on these fans can transport particles that approach and sometimes exceed fl ow depth high terrace at supercritical fl ow (Beaumont and Oberlander, low terrace 4 1971). In another instance, Stock observed fl ow and sediment transport over a small alluvial fan near Pescadero, California, in 2005. Shallow –0.00078 (<3-cm), rapid fl ow rolled waves of gravel down 3 ΔZ = 4.88e a fl ow thread, with particle tops often exposed above water as they rolled along a sandy bed. In this case, the combination of steep slope 2 (~0.05), fast fl ow, and smooth channel-bed sur- terrace height (m) –0.00016 face resulted in gravel moving at speeds that ΔZ = 1.27e approached that of the fl uid velocity. Particles 1 accumulated behind inequant, slower rolling particles, and the sediment mass would move downstream until inertia would carry the mass 0 0 2000 4000 of particles to deposit as a lobe overbank at a meters from fan head bend, or in an area of fl ow separation. If such conditions prevail widely during sediment trans- Figure 22. Height of older surfaces above current Globe mainstem chan- port on fans, particle inertia could play a strong nel (white line in Fig. 5), from 1-m LiDAR. Higher elevation and slope role in off-channel gravel deposition at bends or (+0.002) on these deposits is consistent with higher sediment supply at the in fl ow separation zones. Wasson (1974) specu- time of their deposition. For modern bankfull depth and median grain lated that this effect limited the movement of size, this increased slope would require a unit sediment fl ux ~8% higher coarser particles downfan during fl ood surges than the modern fl ux, sustained across the upper fan area. in a steep Australian fan. It is clear that isolated boulders or groups of boulders downfan need not be interpreted solely as reworked debris- the techniques used by Wilcock and colleagues measuring it. A study by Griffi ths et al. (2006) fl ow deposits. (Wilcock and McArdell, 1993; Wilcock et al., using impounded sediment to date overland fl ow Uncertainties in the transport calculations of 2001; Wilcock and Kenworthy, 2002; Wilcock events offers one innovative possibility to esti- τ this study illustrate opportunities to understand and Crowe, 2003) to reduce *c as sand frac- mate at least the long-term fan runoff frequency. mechanics of steep-channel sediment transport tion increases capture this effect under super- Together, these and other uncertainties mean during episodic fl ash fl oods. We are not aware critical fl ow conditions on steep fan channels, that closing the problem on the control of allu- of any fi eld measurements of bedload fl ux dur- where drag forces may predominate. We have vial fan slopes will require additional detailed ing alluvial fan fl oods that could be used to also ignored the role of sediment and fl ow pro- studies of the hydraulics of fan channels, both in validate using Equation (3). Nor is it clear that duction on the fan, because of the diffi culty in the laboratory and in the fi eld.

Geological Society of America Bulletin, May/June 2008 637 Stock et al.

Geographers, v. 53, p. 516–535, doi: 10.1111/j.1467- 8306.1963.tb00464.x. Beaty, C.B., 1968, Sequential study of desert fl ooding in the White Mountains of California and Nevada: U.S. Army Natick Laboratories Technical Report 68-31-ES, 96 p. Beaumont, P., 1972, Alluvial fans along the foothills of the Elburz Mountains, Iran: Palaeogeography, Palaeo- climatology, Palaeoecology, v. 12, p. 251–273, doi: 10.1016/0031-0182(72)90022-3. Beaumont, P., and Oberlander, T.M., 1971, Observations on stream discharge and competence at Mosaic Canyon, 0.44-m0.44-m deepdeep fflowlow Death Valley, California: Geological Society of Amer- ica Bulletin, v. 82, p. 1695–1698, doi: 10.1130/0016- 7606(1971)82[1695:OOSDAC]2.0.CO;2. Bedford, D.R., Miller, D.M., Schmidt, K.M., and Phelps, G.A., 2007, Landscape-scale relationships between surfi cial geology, soil texture, topography, and creosote bush size and density in the Eastern Mojave Desert of California: in Webb, R.H., Fenstermaker, L.F., Hea- ton, J.S., Hughson, D.L., McDonald, E.V., and Miller, D.M., eds., The Mojave Desert: Ecosystem processes and sustainability, University of Nevada Press. Blair, T.C., 1987, Sedimentary processes, vertical stratifi cation sequences, and geomorphology of the Roaring River alluvial fan, Rocky Mountain National Park, Colorado: Journal of Sedimentary Petrology, v. 57, p. 1–18. Blair, T.C., 1999, Cause of dominance by sheetfl ood vs. debris-fl ow processes on two adjoining alluvial fans, Death Valley, California: Sedimentology, v. 46, p. 1015– 1028, doi: 10.1046/j.1365-3091.1999.00261.x. Figure 23. View up road inundated by Desolation Canyon fan channel in 2004, Death Val- Blair, T.C., 2000, Sedimentology and progressive tectonic unconformities of the sheetfl ood-dominated Hell’s ley, California. A 0.6-m boulder (next to author) rests atop an old road surface, which slopes Gate alluvial fan, Death Valley, California: Sedimen- at 0.043; a trim line on the road berm to the left of this boulder indicates a maximum tary Geology, v. 132, p. 233–262, doi: 10.1016/S0037- 0738(00)00010-5. fl ow depth of 0.44 m. The image indicates that exceptionally large particles may be mobile Blair, T.C., and McPherson, J.G., 1994a, Alluvial fans and on steep, fi ner grained fan channels where fl ow approaches maximum particle diameter. their natural distinction from rivers based on morphol- Observed decreases in maximum particle size down traction-dominated fans may be state- ogy, hydraulic processes, sedimentary processes, and facies assemblages: Journal of Sedimentary Research, ments about decreasing fl ow depth. Section A, Sedimentary Petrology and Processes, v. 64, p. 450–489. Blair, T.C., and McPherson, J.G., 1994b, Alluvial fan processes and forms, in Abrahams, A.D., and Parsons, CONCLUSION rates of overbank, bed-material deposition. Pre- A.J., eds., Geomorphology of desert environments: London, Chapman and Hall, p. 354–402. dicted deposition rates downfan are also sensi- Blissenbach, E., 1951, The geology of alluvial fans in Ari- We used fi eld measurements of hydraulic tive to details of sediment transport formula, and zona [M.S. thesis]: University of Arizona, 86 p. geometry and granulometry on four alluvial to an evolving understanding of the role that the Blissenbach, E., 1952, Relation of surface angle distribution to particle size distribution on alluvial fans: Journal of fans in the arid American southwest to test two abundance of relatively fi ne grain sizes plays in Sedimentary Petrology, v. 22, no. 1, p. 25–28. hypotheses to explain slopes of active alluvial increasing transport rates. Blissenbach, E., 1954, Geology of alluvial fans in arid- regions: Geological Society of America Bulletin, v. 65, fan channels. A lack of bedload fi ning down p. 175–190, doi: 10.1130/0016-7606(1954)65[175: ACKNOWLEDGMENTS middle and upper fan channels, and the failure GOAFIS]2.0.CO;2. of Shields’ criterion to explain observed reduc- Bluck, B.J., 1964, Sedimentation of an alluvial fan in south- We thank Alison Duvall, Sarah Godfrey, Maiana ern Nevada: Journal of Sedimentary Petrology, v. 34, tions in these channel slopes, lead us to con- Hanshaw, Leslie Hsu, Heather Lackey, Sarah Robin- p. 395–400. clude that grain-size reduction alone does not son, Johnny Sanders, and John Vogel for fi eld assis- Boothroyd, J.C., and Nummedal, D., 1978, Proglacial control fan-channel long-profi les. Declining tance. Shannon Mahan did the luminescence sampling braided outwash: A model for humid alluvial fan and dating. Stock thanks the U.S. Geological Survey deposits: Canadian Society of Petroleum Geologists, channel slopes downfan can be explained with Memoir 5, p. 641–668. Mendenhall Program and the Desert Southwest Surfi - reductions in transport rate downfan as gravel Brady, R.H., 1986, Cenozoic geology of the northern cial Mapping project for fi nancial and logistical sup- Avawatz Mountains in relation to the intersection of fractions deposit, a view fi rst espoused by Drew port. We thank Mark Reid, Michael Singer, Frank Paz- the Garlock and Death Valley fault zones, San Ber- in 1873. We fi nd that with our current under- zaglia, Terence Blair, and Dru Germanoski for their nardino County, California [Ph.D. thesis]: University standing of sediment transport, the bed-mate- helpful reviews. Peter Wilcock generously provided of California, Davis, 292 p. the source data for Figure 10. 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640 Geological Society of America Bulletin, May/June 2008