Diagnosing Climatic and Tectonic Controls on Topography: Eastern Flank of the Northern Bolivian Andes

Diagnosing climatic and tectonic controls on topography: Eastern flank of the northern Bolivian Andes

Nicole M. Gasparini1,* and Kelin X. Whipple2,* 1DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, TULANE UNIVERSITY, NEW ORLEANS, LOUISIANA 70118, USA 2SCHOOL OF EARTH AND SPACE EXPLORATION, ARIZONA STATE UNIVERSITY, TEMPE, ARIZONA 85287, USA

ABSTRACT

High relief and steep rainfall gradients make the eastern flank of the northern Bolivian Andes an excellent location for deciphering the relative roles of tectonics and climate on erosion and landscape evolution. We seek to resolve the climate versus tectonics debate in this location by linking topographic analyses and erosion rate data with fluvial bedrock incision theory and numerical landscape evolution modeling. We find that patterns in the channel steepness index (channel slope normalized for drainage area) in both transverse channels that drain across the rainfall gradient through the driest and wettest parts of the landscapes, and frontal channels that drain only the wettest regions are indicative primarily of a gradient in rock uplift rate, although climate likely plays a secondary role in shaping these channel profiles. Pre- viously published erosion rates from 23 watersheds vary with the proposed rock uplift gradient and inversely with rainfall rate, suggesting that increased rainfall is not driving increased rock uplift and erosion. The channel steepness index in an additional 35 tributary watersheds increases with the proposed rock uplift gradient. Simulations from a landscape evolution model that isolate the signatures of rainfall and uplift patterns on landscape morphology corroborate our interpretation that the morphology of this landscape is primarily controlled by a gradient in rock uplift rate, with rainfall rates playing a secondary role. Model results also suggest that the differences between channel steepness values in the transverse and frontal channels cannot be explained by the uplift and rainfall patterns alone. Differences in lithology may be contributing to the higher channel steepness values in the transverse channels, or the transverse channels may be affected by a transient oversteepening phenomenon seen in tools-and-cover river incision models. The conclusions are possible only after detailed comparisons among real and modeled rivers of different sizes that drain different locations. We present best practices for future studies that seek to resolve the relative imprint of rock uplift and rainfall on a landscape.

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MOTIVATION Bookhagen and Strecker, 2008), and areas of ences in tectonic style and regional morphology high erosion rates are often in areas with both also correlate with, and are arguably driven by, Spurred by early insights into the potential high rainfall rates and high relief. The relative variations in the thickness of sedimentary rock role of erosion in geodynamics (e.g., Dahlen contribution of relief and rainfall to localized packages associated with the paleogeography and Suppe, 1988; Koons, 1989; Molnar and high erosion rates remains an open question. of Paleozoic depositional basins (e.g., Sheffels, England, 1990; Beaumont et al., 1992), the Whereas some studies have documented a cli- 1995; Allmendinger and Gubbels, 1996; Baby tectonic geomorphology community has spent matic influence on erosion rates (e.g., Moon et et al., 1997; Kley, 1999). In addition, there is the last couple decades exploring connections al., 2011; Bookhagen and Strecker, 2012), many much debate regarding the inference of plateau between climate and tectonics through erosion. have found no detectable relationship between surface uplift from isotopic, paleobotanical, and Models that assume that climate strongly affects rainfall and erosion rate (e.g., Ahnert, 1970; geomorphic records that could be biased by cli- erosional unloading show a strong link among Riebe et al., 2001; Montgomery and Brandon, mate change (e.g., Barnes and Ehlers, 2009). rainfall, rock uplift rates, and resulting land- 2002; Aalto et al., 2006; Goswami et al., 2012). At a smaller scale of observation, a parallel scape form (e.g., Willett, 1999; Beaumont et Contrasts in the morphology and exhumation debate has focused on both the uplift history and al., 2001; Whipple and Meade, 2004; Roe et al., history between the wet Bolivian Andes north of the relative influence of climate and tectonics 2006; Whipple, 2009). Despite model predic- 17°S and the dry Bolivian Andes south of 18°S on the morphology and patterns of erosion rate tions, firm demonstration of a tectonic response have long been held as a prime example of the across strike in the northern Bolivian Andes. to climate has remained elusive (Whipple, hypothesized climatic influence on tectonics, Whereas Barnes et al. (2012) argued that middle 2009). Indeed, even the requisite connection but no consensus has been reached. Numerous Miocene to recent exhumation patterns vary between rainfall and erosional efficiency is studies have argued that climate has played an with rainfall in the northern Bolivian Andes, not fully understood (e.g., Molnar et al., 2006; important role in shaping this contrast in mor- neither Safran et al. (2005) nor Insel et al. (2010) DiBiase and Whipple, 2011). Moreover, rain- phology and tectonic history (e.g., Masek et al., detected a climatic control on millennial-scale fall and relief are often correlated due to oro- 1994; Horton, 1999; Montgomery et al., 2001; erosion rate patterns in this region. Safran et al. graphic enhancement of rainfall (e.g., Anders Strecker et al., 2007; McQuarrie et al., 2008; (2005) showed that erosion rates correlate best et al., 2006; Bookhagen and Burbank, 2006; Norton and Schlunegger, 2011; Schlunegger et with the channel steepness index and suggested al., 2011; Barnes et al., 2012). However, many that nonuniform rock uplift drives the observed *[email protected]; [email protected]. researchers remain unconvinced because differ- distribution of erosion rates. Similarly, Aalto et

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al. (2006) found that 90% of the variability in Bolivian Andes. Second, the highly concave is a noticeable change in the elevation distribu- decadal-scale erosion rates could be explained shape of profiles below the upper convexity tion of the landscape downslope of the foot of by relief and lithology, and that rainfall was not could only result from: (1) a spatial gradient the escarpment, where the downslope gradient significantly correlated with erosion rates. How- (decreasing from west to east) in rock uplift in mean, maximum, and minimum elevations ever, these authors acknowledge that the strong rate (e.g., Kirby and Whipple, 2001; Safran et abruptly decreases. These changes in topog- correlation between relief and rainfall may have al., 2005; Gasparini and Brandon, 2011), (2) a raphy are highlighted by changes in channel inhibited their ability to isolate a climatic influ- spatial gradient in erosional efficiency (increas- steepness (Fig. 1), local relief, and hillslope ence. In contrast, Schlunegger et al. (2011) ing from west to east either due to progressively angles across the landscape (Fig. 2B). have recently argued that the complex convexo- greater runoff or more erodible substrate; e.g., Changes in the topography are linked to concave profiles of large rivers draining the Roe et al., 2002; Duvall et al., 2004; Craddock the rainfall pattern revealed in satellite data Cordillera Real result not from the pattern of et al., 2007), (3) a temporal decline or cessation (Fig. 2A; Bookhagen and Strecker, 2008; rock uplift but rather from the distribution of of rock uplift (e.g., Whipple and Tucker, 2002; Nesbitt and Anders, 2009). The rainfall rate orographic rainfall revealed by satellite remote Baldwin et al., 2003), (4) oversteepening below begins to increase ~20 km NE of the foot of sensing (e.g., Bookhagen and Strecker, 2008). the knickpoint associated with the upper con- the escarpment and rises to a peak at ~15 km The unusual morphology of these rivers, how- vexity due to a shortage of abrasive tools (e.g., SW into the escarpment (Fig. 2A, inset), where ever, could reflect either patterns in rock uplift Whipple, 2004; Chatanantavet and Parker, 2006; mean elevation reaches ~2300 m. At higher or a combination of uplift and rainfall patterns, Crosby et al., 2007; Gasparini et al., 2007), or mean elevations, rainfall rates decline, but and the relative contributions of these two driv- (5) some combination of these. Although this list they remain above 1500 mm/yr until the mean ers have not been assessed. encompasses a wide range of possibility space, elevation reaches ~3300 m, some ~25 km SW we will demonstrate that a combination of topo- of the escarpment front (Fig. 2A). Farther SW APPROACH AND SCOPE graphic analysis and numerical modeling in into the high mountains, rainfall rates continue which the contrasts between adjacent rivers and to decline, dropping below 500 mm/yr at eleva- We seek to resolve this debate for the spe- their smaller tributaries are carefully examined tions exceeding 4500 m on the crest of the Cor- cific example of the northern Bolivian Andes can greatly narrow the range of possible expla- dillera Real. and simultaneously advance understanding of nations, and even reveal the dominant factor or The study area is underlain primarily by the limits of what can and cannot be deduced factors controlling the landscape morphology. Ordovician, Silurian, and Devonian sedimen- about the drivers of erosion from the study of After a brief description of the setting and tary rocks that have been variably metamor- landforms. Despite the divergence of prior inter- uplift history (“Setting” section), we present phosed (Martinez and Tomasi, 1978; Martinez, pretations, the details of river profiles, and their detailed analyses of river profiles over a range of 1980; Guarachi et al., 2001). Little variability relation to the distribution of rainfall and erosion sizes (drainage areas of 10–1000 km2), landscape in rock strength at a scale that could influence rates in the northern Bolivian Andes present an positions, and substrate lithologies in relation to landscape morphology is recognized in these excellent opportunity to resolve the controversy, modern rainfall patterns and millennial-scale Paleozoic rock packages (Safran et al., 2005; shed light on the coupling of climate and tec- erosion rates (“Topographic Analysis” section). Syrek and Barnes, 2011). An important excep- tonics in general, and illustrate best practices The results of these analyses motivate numeri- tion is the granitic and high-grade metamorphic for the study of tectonic geomorphology of ero- cal simulations that are used to decipher the rocks exposed in the core of the Cordillera Real sional landscapes. Most critical to our approach relative contributions of climate, tectonics, and (Fig. 2B), which have the highest Schmidt ham- is a systematic and theory-guided analysis of lithology (“Numerical Modeling” section). From mer readings (a proxy for compressive strength; river profiles with different drainage area, land- this analysis, we demonstrate that the spatial and Selby, 1982) in the region (Safran, 1998; Aalto scape position relative to regional topographic temporal pattern of rock uplift exerts the domi- et al., 2006). With the exceptions of the Rio and rainfall patterns, and erosion rate. nant control on landscape morphology and ero- Consata and Rio La Paz, the large channels that The convex to highly concave shape of the sion rate patterns, and that some combination of drain across the escarpment incise into these large river profiles highlighted by Schlunegger rainfall, lithology, and transient oversteepening apparently harder rocks, a complication not con- et al. (2011) is a key observation because rela- of transverse drainages plays a secondary role on sidered in some previous work in the region, but tively few circumstances can produce such land- landscape morphology (“Discussion” section). that is considered here. forms. First, the upper convexity could reflect In addition, we develop a general theory on how Overlying the Paleozoic rocks, there is a (1) an abrupt downstream increase in rock uplift patterns of channel steepness in rivers of a range section of Neogene gravels known as the 11–7 rate (e.g., Kirby and Whipple, 2001), (2) a tran- of sizes can be compared and contrasted to deci- Ma Cangalli Formation (Fig. 2B; Fornari et al., sient response to an increase in rock uplift rate pher the dominant evolutionary driver. 1987; Mosolf et al., 2011). The Cangalli Forma- (e.g., Kirby and Whipple, 2012), and/or (3) the tion rests unconformably over the folded and greater erosive power of glaciers (e.g., Brozović SETTING faulted Paleozoic sequence on a surface with up et al., 1997; Brocklehurst and Whipple, 2002). to 500 m relief and is associated with perched The first would require active deformation on We focus on a region of high relief in the low-relief erosion surfaces that apparently grade either a west-vergent thrust or east-dipping nor- northern Bolivian Andes that we call the Beni to the top of the basin-fill level (Fornari et al., mal fault at roughly the position of the 3800 m Escarpment, a topographic step of greater than 1987). Besides constraining the timing of the contour, but no candidate structures nor breaks 3 km that runs from north of the Rio Consata to exhumation of granitic rocks in the Cordillera in thermal histories have been recognized (e.g., the vicinity of the Rio La Paz drainage, where Real and termination of shortening in this part McQuarrie et al., 2008; Barnes et al., 2012). The it becomes less distinct, fading away to the of the orogen (Mosolf et al., 2011), the distribu- second and third alternatives speak to the ques- southeast (Fig. 1). Over a cross-strike distance tion of the Cangalli Formation carries important tion of whether there is geomorphic evidence of of 45 km, the mean elevation drops from ~5 to implications for the style and timing of recent rapid post–10 Ma surface uplift of the northern 1.5 km (Fig. 1, inset topography swath). There uplift and deformation. Although we cannot

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69°W 68°W 67°W elevated low-relief 12–9 Ma San Juan de Oro erosion surface, now perched above deeply °S incised canyons, suggests much of this uplift 15 N has been contemporaneous with deformation in the Subandean fold-and-thrust belt since ca. 10 25 km Ma (Gubbels et al., 1993; Kennan et al., 1997; Barke and Lamb, 2006). This geomorphic and geologic evidence accords with paleobotanical Consata evidence of 2–3 km of surface uplift since ca. 14 Ma in southern Bolivia (Gregory-Wodzicki Elevation et al., 1998; Graham et al., 2001). The paleo- (masl) botanical evidence is in turn entirely compat- 6404 ible with two independent isotopic estimates of 209 surface uplift of the Altiplano (Garzione et al., = 0.45 2006; Ghosh et al., 2006). °S ksn , θref Controversy arises, however, because (1) all

16 0 - 100 100 - 350 published estimates of surface uplift carry large 350 - 500 uncertainties, (2) the geomorphic evidence pre- > 500 sented to date is not definitive, and (3) a combination of late Cenozoic cooling and isotopic changes in rainfall in response to gradual plateau uplift could masquerade as rapid changes 6 Beni in surface elevation (Barnes and Ehlers, 2009; 5 Escarpment Ehlers and Poulsen, 2009). In accordance with the observed high elevation, low-relief sur- La Paz 4 faces in the study area (Whipple and Gasparini, 2014), we will show that the details of landscape 3 morphology in the Beni escarpment are most °S consistent with >3 km of rock uplift relatively

17 2 elevation (km) recently, or over, at most, the last ~ 15 m.y. 1 TOPOGRAPHIC ANALYSIS 0 40 30 20 10 0 −10 −20 −30 distance from escarpment front (km) Theory and Methods

Figure 1. Shuttle Radar Topography Mission (SRTM) 90 m digital elevation model (DEM) of the The goal of our topographic analysis is to study area, draped over a hillshade model. River lengths with a drainage area >10 km2 are shown illuminate trends in channel form across the

colored by ksn. The large valleys of the La Paz and Consata Rivers are labeled for context. The 3800 Beni escarpment and to relate these trends to m contour is shown in black on the map. The inset plot at lower left summarizes elevation changes current rainfall, local lithology, millennial-scale across the white box on the map; the solid black line is the average elevation, and the dashed erosion patterns, and a hypothesized rock uplift lines are the maximum and minimum elevations across the box. Elevation data are binned in 500 pattern. We use a 90 m projected Shuttle Radar m distances. Distance in the inset elevation plot is calculated in a perpendicular direction from the diagonal white line running from northwest to southeast across the map. Data northeast of the Topography Mission (SRTM) digital eleva- white line are plotted as negative distances in the inset elevation plot, and data southwest of the tion model (DEM) for the topographic analysis white line are plotted as positive distances. Where the white reference line runs through the white (USGS, 2006). We use the TRMM (Tropical box, it marks the front of the Beni escarpment. The inset plot at upper right shows the location of Rainfall Measuring Mission) 2A25 data from the study area in South America. years 1998–2007 to explore rainfall patterns (Nesbitt and Anders, 2009). Safran et al. (2005) used in situ cosmogenic radionuclide data to be certain of the accuracy of the mapping of Uplift History Controversy calculate watershed-averaged erosion rates in these gravels in detail, the spatial distribution the region, and we use their measured rates. We of the Cangalli Formation suggests significant The surface uplift history of the margin of hypothesize that there is a linear increase in rock deformation and uplift associated with the Beni the Central Andean Plateau in Bolivia is contro- uplift rate over a 30-km-wide zone SW of the escarpment—remnants of these gravels are versial despite many independent lines of evi- foot of the Beni escarpment, consistent with the mapped as capping ridges up to ~2500 m eleva- dence that point to significant (>2 km) post–10 mapped distribution of the 11–7 Ma Cangalli tion some ~20 km SW of the foot of the escarp- Ma surface uplift. Study of the Cenozoic stra- Formation (Fig. 3), and that this trend, along ment (Figs. 2B and 3). In addition, Fornari et tigraphy of southern Bolivia indicates signifi- with patterns in rainfall, can be seen in the chan- al. (1987) reported that the base of the Cangalli cant surface uplift because megafans deposited nel morphology. Formation preserved along the Rio Tipuani sug- near sea level in the Eocene and Oligocene are Our study focuses on patterns in both the gests significant postdepositional tilt, as is illus- now preserved at plateau elevations (Horton, large and small channels that flow across the trated schematically in Figure 3. 2005). Geomorphic evidence consisting of the Beni escarpment. The headwaters of the large

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69°W 68°W channels reach back into the driest parts of the A study area, and these channels flow roughly perpendicular to the escarpment front. We refer N to the large channels as the transverse channels (labeled in Fig. 2). The transverse channels have 20 km an upstream length from the escarpment front of Consata greater than 50 km (Fig. 4). The headwaters of i the largest transverse channel, the Consata, have Tipuan eroded farther back into the Altiplano than the other four transverse channels. We contrast the

°S Challana morphology of the transverse channels with that

16 k , θ Trib sn ref = 0.45 of five shorter channels that also flow roughly 0 - 350 perpendicular to the escarpment front but that 350 - 500 Challana do not reach back into the driest parts of the > 500 Zongo landscape (Fig. 2). We refer to these channels Annual Rainfall (mm) 4,000 as frontal channels, and they have an upstream 0 - 330 length from the front of less than 30 km. 330 - 660 3,000 The transverse channels are very steep 660 - 1000 2,000 where they flow across the escarpment, and 1000 -1500 all of the channels have knickpoints or broad 1500 - 2000 1,000 convexities (a downstream increase in chan- 2000 - 2500 annual rainfall (mm) 0 nel slope) between elevations of ~4 and 2.5 km 2500 - 3000 40 30 20 10 0 −10 −20 −30 °S (Fig. 4). The upper elevation of the convexities 3000 - 3603 distance (km) 17 roughly corresponds with the lowermost snow- 69°W 68°W line during the Last Glacial Maximum (Klein B a' et al., 1999), which is illustrated by the 3800 m contour in Figures 1 and 2, and is generally N above the elevation of the highest rainfall rates (blue band in Fig. 2A). The largest transverse 20 km channel, the Consata, has the smallest convex- a ity, and below the convexity, the channel is less steep than the other four transverse channels. We explore patterns in channel form using the normalized channel steepness index. °S Numerous studies have found that channels 16 are generally concave, and the channel slope, S, decreases systematically with downstream drain- Slope (%) age area, A (e.g., Hack, 1957; Flint, 1974; Snyder 0 - 10 et al., 2000). This relationship is expressed as: 10 - 25 25 - 45 −θ 45 -70 Sk= s A , (1) 70 - 304

Cangalli Formation - where q is the concavity index, and ks is the chan- gravel deposits nel steepness index (Wobus et al., 2006a; Kirby Granitic rocks and Whipple, 2012). The concavity index, q, is defined as positive in concave channels and typi- High-grade °S metamorphic rocks cally varies between ~0.3 and 0.6 (Tucker and 17 Whipple, 2002). In channel reaches where chan- Figure 2. Maps of (A) mean annual rainfall, from TRMM (Tropical Rainfall Measuring Mission) satel- nel slope increases downstream due to the pres- lite data (Nesbitt and Anders, 2009) and (B) landscape slope (in percentage calculated from Shuttle ence of knickpoints, knick zones, or local over- Radar Topography Mission [SRTM] 90 m digital elevation model [DEM]) with locations of mapped steepening in channel slope, is negative, and the granite, high-grade metamorphic rocks (Martinez and Tomasi, 1978; Martinez, 1980; Guarachi et al., q 2001), described by Safran et al. (2005) as staurolite, andalusite, or silimanite-grade metasediments profile is convex (Wobus et al., 2006b). Knick- surrounding the granitic rocks, and the Cangalli Formation of coarse-grained sediments (Fornari et points are often transient features that indicate al., 1987, Guarachi et al., 2001; Mosolf et al., 2011). Maps are draped over the same hillshade model a change in uplift rate (e.g., Hoke et al., 2007), used in Figure 1. Both maps cover the same area, which is a subset of the area shown in Figure 1. base-level fall rate (e.g., Whipple, 2001; Crosby The reference line that runs through the front of the Beni Escarpment is shown as a gray line in and Whipple, 2006), or possibly a change in cli- each of the maps. The transverse and frontal study channels are illustrated in each of the maps and mate (e.g., Bonnet et al., 2001; Whipple, 2001). are shaded by ksn. The ksn legend in A also applies in B. The 3800 m contour is shown as a thin black line in both maps. The inset plot in A illustrates the maximum, minimum, and mean annual rainfall Following Wobus et al. (2006a), we explore across the gray box. Rainfall data are binned in 500 m distances. The dashed thick black line in B (a patterns in the normalized channel steepness

to a′) illustrates the approximate location of the ridgeline profile shown in Figure 3. index, ksn, derived from the following relationship,

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Hypothesized Rock Uplift of the surrounding rocks and that an influx of 5 Ridgeline Topography coarse, durable sediment from these rock units Cangalli Formation may inhibit channel incision for a considerable 4 distance downstream. a Base of Cangalli Formation Critical factors for discriminating among the

m) Inferred paleo-river bed

3 potential causes for the highly concave regions in Projected Tipuani River Prole the channels are the relative steepness and con- 2 cavity of adjacent rivers of different size, position

Elevation (k relative to the rainfall pattern, and position rela- 1 a' tive to rock-type boundaries. Because estimates of local profile concavity are highly sensitive to 0 the length scale over which they are measured 010203040506070 (e.g., Wobus et al., 2006a), and because chan- Distance (km) nels of different sizes crossing the same spatial Figure 3. Elevation profile along ridgeline (shown in Fig. 2B) containing the Cangalli Formation gradient in rock uplift, rock strength, or rainfall (Fornari et al., 1987; Guarachi et al., 2001; Mosolf et al., 2011) (areas shown in gray). The profile is will exhibit systematically different concavity not drawn perpendicular to the escarpment front. The Tipuani River profile is projected into the indices for the same change in channel steepness a–a′ cross section. The escarpment front is at ~55 km, where the gradient of the paleoriver bed is across this zone (reflecting simply the different inferred to change. This profile provides a qualitative approximation of the uplift pattern as inferred from deformation of the Cangalli Formation. relationships between the logarithm of drainage area and distance), the spatial pattern of ksn values is the more effective tool for quantifying 6 variations in channel form in response to lateral 1 Consata I n gradients in boundary conditions. 5 Tipuani   ksn =  m  , (3) Challana Trib  KP  4 Challana Study Design Zongo 3 where I represents the local bedrock incision We explore the ksn patterns in five large trans-

elevation (km) 2 rate; K represents the rock erodibility (harder verse channels and five smaller frontal channels rocks = smaller K value); P is the upstream spa- that drain across the Beni escarpment, 23 tribu- 1 tially averaged runoff rate within the watershed tary or subtributary watersheds of the transverse 0 100 75 50 25 0 −25 −50 (assumed equal to the spatially averaged rainfall channels in which Safran et al. (2005) measured 10 streamwise distance from reference line (km) rate within the watershed for simplicity), and m erosion rates based on concentrations of Be Figure 4. Transverse channel profiles. The zero and n are the area and slope exponents of the in river sands, and an additional 35 tributary or location is where the channels flow across the stream power incision rule (Roe et al., 2002; subtributary watersheds for which we do not reference line that runs through the front of the Whipple, 2009). At steady state, the incision have erosion rate data but that are exclusively Beni escarpment (see Figs. 1 and 2). Note that the rate equals the rock uplift rate, U, and U can on the Beni escarpment. The tributary water- vertical scale is exaggerated by a factor of ~16. replace I in Equation 3. Accordingly, a relation- sheds are smaller, and so they may be more

ship between ksn and either erosion or rock uplift representative of uniform uplift and are more rate has been shown in a number of studies representative of uniform rainfall conditions, in −θref Sk= sn A , (2) (Kirby and Whipple, 2001; Wobus et al., 2006a; comparison with both the transverse and frontal Harkins et al., 2007; Hilley and Arrowsmith, channels, which have a wider range of rainfall

where qref is a reference concavity value that is 2008; Ouimet et al., 2009; Cyr et al., 2010; DiB- rates and possibly rock uplift rates within their

fixed in order to effectively and quantitatively iase et al., 2010). Importantly, ksn is not directly watersheds. We include an additional two chan- compare channel steepness values from river to related to local changes in rainfall rate; rather, it nels south of the Rio La Paz where the Beni river regardless of drainage area and local varia- is affected by the discharge, and, as a result, it is escarpment has largely lost definition (Fig. 2). tions in the concavity index, q (e.g., Wobus et inversely related to the upstream spatially aver- Where the escarpment loses definition, rainfall al., 2006a; Kirby and Whipple, 2012). The nor- aged rainfall rate within the catchment raised to becomes more uniform (Fig. 2A), as may the

malized channel steepness index, ksn, is known the m/n power (typically m/n is ~0.5). spatial pattern of rock uplift (Whipple and Gas- to vary with rock uplift rate, lithology, and cli- Harder rocks and coarser bed material parini, 2014). The two southern catchments are mate (e.g., Kirby and Whipple, 2012, and ref- should result in smaller values of K, although included in the study exclusively to help assess erences therein). Higher normalized channel there is not an exact mapping between rock type the potential role of lithologic heterogeneity.

steepness values are associated with faster rock and K value (Stock and Montgomery, 1999), We calculated ksn across all channels in our uplift and incision rates, stronger rocks, and less and much remains to be quantitatively under- study area that have a drainage area greater erosive climatic conditions (lower mean annual stood about the role of rock strength and the than 10 km2, and we display the results from runoff and/or less variable runoff; Tucker, 2004; properties of coarse bed material (e.g., Sklar all the channels, but we focus primarily on the Lague et al., 2005; Molnar et al., 2006; DiBiase and Dietrich, 2004). Based on observations by patterns from our 70 study channels and tribu- and Whipple, 2011). Safran (1998), we consider the possibility that tary watersheds. We used a freely available tool

The well-known detachment-limited unit the granitic and high-grade metamorphic rocks that calculates ksn over a moving window in all stream power river incision model is useful to exposed in the core of the Cordillera Real are channels greater than the prescribed minimum

illustrate the expected dependencies of ksn: harder and more resistant to erosion than all size (http://www.geomorphtools.org). Channel

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slopes are smoothed over a 2 km moving win- all reaches in the watershed with drainage area just downstream of the 3800 m contour (Figs. 1 2 dow, and ksn is calculated over a 2 km distance greater than 1 km to represent the normalized and 2). However, the Consata, which also flows at every point in the channel. When illustrating channel steepness of the watershed. In the 35 through the zone of high rainfall, does not have

only the data from the Bolivian channels, qref additional watersheds, we only include those a similar reach of high ksn values. Many of the is set to 0.45, a value commonly used in other stretches of the tributary channel that are below tributaries to the transverse channels (includ- studies and therefore convenient for compari- 3800 m in order to ensure that we sample the ing tributaries to the Consata; Fig. 1) also have 0.9 sons with other locations (Kirby and Whipple, adjusted or adjusting portions of the landscape ksn values above 500 m in this zone. Channel 2012). When comparing the Bolivian channels that we can confidently interpret to be fluvially steepness values decline toward the escarpment

with landscape evolution model data, qref is set dominated. The mean annual rainfall across all front in all of the transverse channels and in to 0.5 because the intrinsic concavity (m/n) of of the tributary watersheds is also calculated. their tributaries. Most of the frontal channels 0.9 the model channels is 0.5. Because there is a All of our data analysis and modeling are have ksn values below 350 m ; there are only a 0.9 linear relationship between ksn values calculated based on the modern rainfall data. What mat- few short reaches with ksn values above 350 m , 0.9 with qref = 0.45 and qref = 0.5, patterns in ksn with ters most for our study is the pattern of rainfall, and none with ksn values above 500 m (Fig. 2). distance are the same regardless of the reference which is dictated by orographic precipitation Downslope of the escarpment front, the channel concavity value (Fig. 5). mechanics, and this pattern is unlikely to change steepness index is generally less than 150 m0.9 In the 23 watersheds that Safran et al. over the time scale of our erosion rate data. As (Fig. 1). (2005) sampled, we compare the relationships we will show, our analysis indicates a secondary

among ksn, distance from the escarpment front, dependence of topography on rainfall, which Transverse and Frontal Channels watershed-averaged rainfall rate, and erosion minimizes the sensitivity of our analysis to pos- Channel slope versus drainage area data from rate. Safran et al. (2005) investigated trends sible paleoclimate change. Further, we fully the transverse channels have some similar trends in erosion rate with normalized channel steep- acknowledge that climate and orographic pre- (Fig. 6). All of the transverse channels have a ness index calculated using a slightly different cipitation enhancement will change at the mil- notable convexity, identified as a region in which method than we use here, and we repeat their lion-year time scale of surface uplift. By using slope increases with drainage area. In the Con- 0.9 analysis using our measurements of ksn. We use the modern rainfall pattern in the numerical sata, ksn values increase from ~100 to ~350 m a subset of the data collected by Safran et al. modeling scenarios, we maximize the predicted across the convexity (Fig. 6A), whereas in the (2005). We only consider watersheds that are influence of rainfall. Tipuani, Challana tributary, Challana, and Zongo

tributaries, or subtributaries, to a transverse Rivers (referred to herein as the high ksn trans-

channel because they integrate erosion rates Results verse channels), ksn values increase from ~100 0.9 over an area in which rainfall and erosion rates to ≥500 m (Figs. 6B–6E). The high ksn trans- are likely less variable. We also do not include General Trends verse channels incise into regions with granite data from watersheds with a drainage area less There is a zone of high channel steepness val- and high-grade metamorphic rocks, whereas the 2 0.9 than 10 km . ues (the highest values, ksn > 500 m ) roughly Consata does not (Fig. 2B). In all of the trans- For the analysis of the Safran et al. (2005) parallel to the escarpment front (Fig. 1). In four verse channels, the convexity occurs in a region

watersheds and the additional 35 watersheds, of the five transverse channels, the highest ksn where rainfall rates are less than 1000 mm/yr. we follow Ouimet et al. (2009) and DiBiase et values are just upstream of (or reach slightly In the Zongo and Consata Rivers, this convex-

al. (2010) by using the mean ksn value across into) the highest rainfall zone (Fig. 2A) and ity occurs well upstream of the point where the channel enters the region where rainfall rates are above 1000 mm/yr, but in the other three 2500 transverse channels, the downstream end of the A θ = 0.45 convexity appears to roughly coincide with the 2000 ref location where the channel enters the region θ = 0.5 ref of rainfall rates above 1000 mm/yr. In all of sn 1500 k the transverse channels, the convexity occurs 1000 Figure 5. Comparison be- between drainage area values of 10 and 100 km2 tween channel steepness (see also Fig. 4). All of the channels are primarily 500 values in the Challana concave downstream of this convexity. They have River calculated using qref 0 = 0.45 and q = 0.5. (A) The a region in which the concavity remains close to 45 40 35 30 25 20 15 10 5 0 ref trend with distance from the reference value of 0.45 and ksn is relatively perpendicular distance from reference line (km) the front of the escarp- uniform, and then downstream of this region, the ment, or reference line, is concavity increases, and ksn values decrease back 800 B the same regardless of the to ~100 m0.9 at the foot of the Beni escarpment. reference concavity. (B) The The five frontal channels do not have the 600 channel steepness values = 0.45 calculated using different convexity that is present in the transverse chan- re f reference concavity values nels (Fig. 6F). They have a relatively constant 400 2 , θ are linearly related. slope (zero concavity) until ~5 km , a feature

sn y = 0.38 x +14 k 200 r2 = 0.98 common to steep mountainous drainages that is interpreted as a dominance of debris-flow pro- 0 cesses at small drainage area (e.g., Montgomery 0 400 800 1200 1600 2000 and Foufoula-Georgiou, 1993; Lague and Davy, k θ sn, ref = 0.5 2003; Stock and Dietrich, 2003). The short

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0 10 A D

−1 10 k sn =500 k −2 sn =350 10 k sn =100

channel slope (m/m) −3 Consata Challana 10 B E −1 10

−2 10

channel slope (m/m) −3 Tipuani Zongo 10 C F −1 10

−2 10

channel slope (m/m) −3 Challana Trib Five Frontal Channels on the BE 10 6 7 8 9 6 7 8 9 10 10 10 10 10 10 10 10 10 10 drainage area (m2) drainage area (m2) 0 10 G )

−1 10

k sn =500

k sn =350 −2 10 Tipuani

channel slope (m/m North Challana Trib k Challana to sn =100 Zongo South

−3 10 6 7 8 9 10 10 10 10 10 10 drainage area (m2)

Figure 6. Slope-area data from the five transverse channels (A–E) and five frontal channels that flow across the Beni escarpment (BE) (F). (G) Plot includes the data from B–E in one plot for comparison. All of the plots have the same limits on the x

and y axes and show the same three constant ksn (qref = 0.45) reference lines (solid lines, labeled in D and G). A–E illustrate data from a single channel; the light-gray zone marks the region in which the rainfall rate is greater than 1000 mm/yr; the darker-gray zone marks the region in which the rainfall rate is greater than 2000 mm/yr. Both the Tipuani and Zongo Rivers end in an area with less than 1000 mm/yr precipitation. The Consata flows into and out of a region with less than 1000 mm/yr rainfall, and this detail is not illustrated here. The five frontal channels (F) are the same five frontal channels that are shown in gray in Figure 7, and the same data symbol is used for each frontal channel in the two figures. Note that this 2 figure illustrates that the frontal channels have some reaches with ksn values above 350 at drainage areas less than 10 km , but this detail is not shown in Figure 2.

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interval over which ksn increases downstream on remains high or decreases (roughly between 30 band in Figures 7A and 7B highlights the pat-

these channels is artifactual, simply reflecting and 20 km), and a region in which ksn decreases tern in the four high ksn transverse channels.

the low concavity in this reach (less than the ref- (roughly between 20 and 0 km) (Fig. 7A). Similar to the high ksn transverse channels, ksn erence value of 0.45). At drainage areas exceed- These three regions correspond to the convex, decreases toward the escarpment front in the five ing 5 km2, the frontal channels have a relatively concave, and high-concavity regions, respec- frontal channels on the Beni escarpment (gray

uniform, high concavity. Because the channel tively, illustrated in the slope-area data (Figs. symbols in Fig. 7A). However, ksn values in the

concavity is higher than the reference value, ksn 6B–6E and 6G). Even though ksn increases and frontal channels are lower than those in the high

decreases downstream. then decreases from ~40 to 15 km, the rainfall ksn transverse channels. The five frontal channels The patterns in the slope-area data are illus- rate increases throughout this zone toward the on the Beni escarpment do not incise into any

trated in the ksn trends with distance from the escarpment front. From ~15 km to the escarp- areas of mapped granite, and only their headwa-

escarpment front (Fig. 7). The four high ksn ment front, ksn and rainfall rates both decrease. ters reach into the highly metamorphosed rocks.

transverse channels have a region in which ksn Downslope of the escarpment, ksn values remain The light-gray band in Figures 7A and 7B high-

increases toward the escarpment front (roughly low, and rainfall rates continue to decrease with lights the ksn pattern across the escarpment in the

between 40 and 30 km), a region in which ksn distance from the escarpment. The dark-gray frontal channels. Downslope of the escarpment,

ksn values in the frontal channels are similar to the nearly uniform values in the transverse channels. 1000 The ksn pattern in the Consata, a transverse A Tipuani channel that does not incise into any of the 800 Challana Trib mapped areas of granite or high-grade meta- Challana morphic rocks (Fig. 2B) and has a larger drain- 600 Zongo 3000 = 0.45 f age area than the high ksn channels, differs from re annual rainfall θ the other transverse channels (Fig. 7B). There is , 400 2000 sn

k an increase, leveling off, and decrease in the ksn 200 1000 values in the Consata, but the locations of these changes are farther away from the escarpment 0 front than in the high k channels. Further, k 40 30 20 10 0 −10 −20 −30 sn sn never goes above ~400 m0.9 in the Consata, in 1000 comparison with maximum values above 600 0.9 B Consata m in the high ksn transverse channels. 800 annual rainfall The two channels south of the Beni escarpment drain an area with nearly uniform rainfall, 600 3000 = 0.45 f are immediately adjacent to each other, are of re θ

, 400 2000 similar size, but one drains an area of mapped sn

k granite, and the other does not (Figs. 2 and 7C). 200 1000 Although there is a great deal of scatter, channel steepness values do not appear to vary system- 0

mean annual rainfall [mm] atically with distance from the reference line 80 70 60 50 40 30 20 10 0 −10 −20 −30 in these two channels. However, at a given distance from the reference line, the channel that 1000 C granite channel flows across an area of mapped granite has a higher k value than the channel that does not 800 non-granite channel sn annual rainfall drain any of the mapped granite. On average, the granite channel has a ksn value that is 1.6–

= 0.45 600 f

re 1.9 times higher than the nongranite channel θ

, (depending on the area over which k is aver-

sn 400 2000 sn k aged in the granite channel), which can be taken 200 1000 as an indication of the magnitude of the potential lithologic effect. 0 40 30 20 10 0 −10 −20 −30 Tributary Watersheds perpendicular distance from reference line [km] Mean channel steepness indices, mean Figure 7. Channel steepness index (data points; values on left axis) and mean annual rainfall in the annual rainfall, and erosion rates in the sampled region around the channels (solid lines; values on right axis) as a function of distance perpendicular tributary watersheds from Safran et al. (2005) to the reference line running through the front of the Beni escarpment (see Figs. 1 and 2). (A) Data all have noticeable trends with distance from

from the four high ksn transverse channels (black data) and the five frontal channels (gray data) the reference line along the foot of the escarp- on the Beni escarpment. (B) Data from the Consata River only. Note that the limits of the x axis in ment (see Fig. 8 for watershed locations and B differ from those in A and C. The dashed box in B marks a region in which the digital elevation Fig. 9 for the data). Mean k and erosion rate model (DEM) is poorly resolved, resulting in artificially higher k values. In A and B, the dark- and sn sn in the tributary watersheds generally increase light-gray areas highlight the range in observed ksn values in the four high ksn transverse channels and the five frontal channels, respectively. (C) Data from the two frontal channels south of the Beni with distance upslope (SW) of the reference line

escarpment (see Fig. 2 for locations). (Figs. 9A and 9C). The value of ksn remains low

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68°W of 2) with the contrast seen in the two channels south of the Beni escarpment, which appears to 25 km be attributable to a lithologic effect (Fig. 7C), and the difference in means in the tributary N watersheds is supported by a one-way analysis of variance (ANOVA) test (p < 0.0001). Inter- estingly, the tributary channels entirely within granitic and high-grade metamorphic rocks have

significantly lower ksn values than the main-stem transverse drainages in the same landscape position, even where the mean rainfall rate is less than 1000 mm/yr (Fig. 10). This suggests that °S the very high ksn on the transverse channels is 16 unlikely to be purely a lithological effect.

The relationship between rainfall rates and ksn in the tributary watersheds is similarly complex. As seen in the subset of watersheds studied by

Safran et al. (2005), ksn values decline with average annual rainfall (Fig. 10C), although another possible interpretation is that there is a threshold Erosion rate, mm/yr at ~2000 mm annual rainfall, resulting in channels with generally lower channel steepness val- 0.60 - 0.77 ues above this threshold. However, the wettest watershed has a similar k value (308 m0.9) as 0.40 - 0.60 sn the driest watershed (316 m0.9), and the appar- 0.20 - 0.40 ent threshold at ~2000 mm/yr coincides with the lithologic boundary discussed already. Erosion 0.04 - 0.20 rate patterns and the distribution of the Cangalli Formation (Figs. 9 and 3, respectively) suggest Figure 8. Map illustrating the location of the watersheds sampled by Safran et al. an underlying tectonic driver for the spatial distri- (2005) that are used in this study. The watersheds are colored by the erosion rate, bution of tributary channel steepnesses. The trib- and the channels in these watersheds are colored by k using the same legend sn utary morphometric data are arguably consistent used in Figure 1. The background map is annual rainfall, using the same rainfall shading as was used in Figure 2A, draped over the same hillshade model used in with this interpretation, but not uniquely so. The Figures 1 and 2. The gray line running from northwest to southeast follows the strong spatial correlations among rainfall rate, front of the Beni escarpment. The thin dark blue line is the 3800 m contour. lithology, channel steepness, and erosion rate decidedly complicate interpretation. Fortunately, careful, systematic investigation with a landscape downslope of the reference line, where erosion Mean channel steepness values from the 35 evolution model can help us to determine the sce- rates are also low but rainfall rates are high (Fig. tributary watersheds on the Beni escarpment nario that best matches all available data. 9B). Rainfall rates are lowest at a distance of 30 could be indicative of a spatial rock uplift gradi-

km (and greater) upslope of the reference line, ent, or a change in ksn with lithology, but not both Inferences and the Need for Numerical where the highest erosion rates are measured. (Fig. 10). Although channel steepness is gener- Modeling Several interesting trends are observed among ally higher in watersheds underlain by granitic

tributary watershed ksn values, mean annual rain- or high-grade metamorphic rocks (290 and 420 Based on our topographic analysis, we make fall, and erosion rate. Converse to expectation m0.9) than in those underlain by weaker sedimen- the following initial interpretations: and common assumptions, erosion rates gener- tary and low-grade metasediments, the weaker (1) The foot of the Beni escarpment marks ally decrease with increasing watershed-aver- rocks experience greater rainfall rates (Fig. 10C) an uplift boundary. The channels and

aged annual rainfall (Fig. 9D). Erosion rates do, and exhibit a well-defined increase in mean ksn tributary watersheds downslope of the 0.9 however, generally increase with mean ksn (Fig. from ~150 to ~300 m over a 20 km distance escarpment front have uniformly lower

9E), as predicted by Equation 3. Although there upslope of the escarpment front (Fig. 10B). The ksn values and erosion rates in comparison are not many points to generate trends, it is worth higher channel steepness values in the stronger with the channels on the escarpment.

highlighting that, for a given mean ksn, erosion rocks fall on the same trend of increasing ksn with (2) The change in rock uplift rate at the base rates are higher in the driest watersheds (illus- distance from the range front, reaching ~400 m0.9 of the escarpment is likely an uplift gra- trated by the fit lines in Fig. 9E). This is counter at 30 km upslope (Fig. 10B), suggesting a com- dient (increasing to the west) and not an to what is predicted by the theoretical relation- mon response to an uplift gradient as suggested abrupt step change in rock uplift rate as ship (Eq. 3). We note that the rainfall pattern by the pattern in erosion rates and deforma- would be expected from an active thrust varies southeast of the Beni escarpment, and the tion of the Cangalli Formation (Figs. 9C and 3, fault (no fault is mapped). The erosion

rock uplift pattern in this area likely also varies respectively). However, the difference in mean rate and ksn patterns, along with the slope (Whipple and Gasparini, 2014), and this could channel steepness between stronger and weaker on the surface of the Cangalli Forma- contribute to some of the scatter in the trends. rocks is commensurate (approximately a factor tion, support an uplift gradient.

238 www.gsapubs.org | Volume 6 | Number 4 | LITHOSPHERE

550 0.8 D 5 450 A 0.6 350

= 0.4

f re 0.4 θ 250 ,

k 150 0.2 2

erosion rate (mm/yr) r = 0.31 50 0 70 60 50 40 30 20 10 0 −10 −20 −30 0 500 1000 1500 2000 2500 2500 annual rainfall (mm)

2000 B

1500 P ≤ 500 mm/yr 1000 500 < P ≤ 1500 mm/yr 500 1500 mm/yr < P

annual rainfall (mm) 0 70 60 50 40 30 20 10 0 −10 −20 −30

0. 8 C 0.8 E 0. 6 0.6

0. 4 0.4

0. 2 0.2 2

erosion rate (mm/yr) r = 0.36

0 erosion rate (mm/yr) 0 70 60 50 40 30 20 10 0 −10 −20 −30 50 150 250 350 450 550 perpendicular distance from reference line (km) ksn ,θref = 0.45

Figure 9. Data from the tributary watersheds studied by Safran et al. (2005). (A–C) Mean channel steepness index of all reaches across a watershed (with 2 standard errors about the mean), watershed averaged annual rainfall, and erosion rate (with 10% error bars), respectively, as a function of the watershed outlet distance from the front of the Beni escarpment (shown in Figs. 1 and 2). Negative distances indicate that the watershed outlet is downslope from the escarpment. (D) Erosion rate as a function of the watershed averaged annual rainfall. (E) Erosion rate as a function of the mean channel steepness index. Solid lines, when shown, are best-fit linear regressions to all of the data, and r2 values (coefficient of determination) are shown with these regression lines. Linear regressions for subsets of the data based on watershed averaged annual rainfall are shown in E only. The r2

values for the best-fit linear relationship between erosion rate and ksn are 0.053, 0.58, and 0.66 for rainfall (P) < 500 mm/yr, 500 < P < 1500 mm/yr, and 1500 mm/yr < P, respectively. All of the plots use the same legend (above E) for the data symbols based on the watershed averaged annual rainfall.

(3) The upper convexity in the transverse cial erosion, or both. These upper con- uncertain the relative roles of lithology, channels is unlikely to record a down- vexities do not appear to be controlled tectonics, and rainfall rate (Figs. 9 and stream (eastward) increase in rock by the rainfall pattern as suggested by 10). Additional analyses are required to uplift rate: (a) Knickpoint positions are Schlunegger et al. (2011) because there make further progress. defined by drainage area and elevation is no consistent relationship between the We use a numerical model to more quan- (~3800 m) and do not trace a likely position of the convexities and the mod- titatively assess the dominant drivers of land- structural boundary (Fig. 1); (b) defor- ern rainfall pattern (Fig. 6); this hypoth- scape evolution across the Beni escarpment mation within the Eastern Cordillera is esis will be more rigorously tested in our and to extract a general understanding of how, thought to have ceased by late Miocene numerical simulations. and to what limits, the dominant drivers can be

time (e.g., Barnes et al., 2012; Lease and (4) The difference in ksn values between inferred from study of topography and erosion Ehlers, 2013); and (c) neither cosmo- transverse and frontal channels that incise rate patterns. We use the models to test the fol- genic nor thermochronometric data indi- into the granite/high-grade metamorphic lowing questions: (1) Has the landscape reached cate a sudden change in erosion rate at rocks and those that do not suggests that steady state, or is the morphology indicative this position (Fig. 9; Safran et al., 2005; lithology may play a significant role in of a transient response (or in other words, has Insel et al., 2010). Thus, these upper landscape morphology. However the there been significant uplift since the late Mio- convexities are either transient features, relationships among channel steepness, cene?)? (2) Could rainfall patterns alone cause

or the low ksn regions at high elevations lithology, rainfall rate, and erosion rate in a large convexity in these channels, as sug- have reduced slopes as a result of gla- tributary channels are complex, leaving gested by Schlunegger et al. (2011)? (3) Can an

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68°W A granitic and high grade metamorphic rocks N other

500 6 km B 400 = 0.45 f re 300 , θ sn k 200

100 average 0 40 30 20 10 0 perpendicular distance from escarpment front (km) °S

500 16 C 400 = 0.45

f re 300 , θ sn k 200

100 r2 = 0.52 average 0 0100020003000 watershed averaged annual rainfall (mm)

Figure 10. Locations and data from 35 tributary watersheds. (A) Map illustrates the locations of the 35 tributary watersheds, outlined in thick white lines, with rainfall map (legend in Fig. 2A) and locations of granitic and high-grade metamorphic rocks (legend in Fig. 2B) for reference. The thin black

line illustrates the 3800 m contour. Channels are shaded by ksn using the same scale as used in Figure 1. Note that the northwest-most tributary watershed is the only one that contains a tributary larger than 10 km2 within it. However, because we only use those portions in the tributary channels that are below the 3800 m contour (in order to stay within the fluvially adjusted or adjusting region), we only include data from the main-stem channel of this watershed. (B) Mean channel steepness index of all reaches across a watershed (with 2 standard errors about the mean) as a function of the

mean watershed channel distance perpendicular from the escarpment front. (C) Average watershed ksn (with 2 standard errors about the mean) values as a function of watershed averaged annual rainfall. The r2 value is for the best-fit linear relationship to all of the data, shown with the solid black line. Legend above B applies to both B and C.

uplift gradient alone cause the observed chan- ent uplift patterns that we use are designed to ment during transient conditions (Whipple and nel morphology? (4) How much of the variation test whether there is a gradual or step change Tucker, 2002; Baldwin et al., 2003). in channel steepness values between the frontal in uplift rate at the foot of the escarpment. Our The geometry of the modeled landscape is and transverse channels could be explained by experiments use two different rock strength val- designed to explore only the 45 km upslope of lithology, or variations in rock strength? ues to explore the possible influence of variable the escarpment front, which would encompass lithology. Only fluvial processes are modeled, approximately the drainage divide at the edge NUMERICAL MODELING or in other words, we do not resolve hillslopes of the plateau to the foot of the escarpment for in our model, and glacial processes are not sim- most of the transverse channels (upslope/south- Model Description ulated. Because we do not simulate glacial ero- west portion of the swath topography in Fig. 1). sion, we are only modeling scenarios in which All of the modeled landscapes have the same We use the CHILD landscape evolution the upper convexity reflects the rainfall gradi- dimensions, resolution, and boundary condi- model (Tucker et al., 2001a, 2001b) to explore ent, a transient adjustment to a change in rock tions (Fig. 11A). The spacing between points on the topographic signatures of different uplift uplift rate, or both. The potential role of glacial the landscape is ~200 m. and rainfall scenarios and to explore the poten- processes is discussed in section on “Role of Elevation change at each point across the tial role of lithologic variations. We perform a Tectonics, Rainfall, and Lithology on the Mor- landscape is a combination of vertical uplift and number of numerical experiments in which we phology of the Beni Escarpment.” We include fluvial incision: systematically vary the spatial rock uplift pat- both detachment-limited and transport-limited dz()xy, dz()xy, tern, spatial rainfall pattern, or both the rain- fluvial processes in all of the model simula- = Ux(), y + , (4) dt dt fluvial fall and uplift patterns, in order to illuminate tions, which allow us to capture sediment depo- () the signature that each of these forces could sition in channels with large drainage areas that where z(x, y), and U(x, y) are the elevation have on landscape evolution. The two differ- may be inundated with large volumes of sedi- and rock uplift rate at a point (x, y) in space,

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Figure 11. Steady-state numeri- uniform rainfall, steady-state topography nonuniform rainfall, steady-state topography cal simulations with uniform rock uplift. The left column illustrates A B the steady-state topography (A), rainfall pattern (C), and normalized elevation distribution (E) from experiment X1 with uniform rainfall across the landscape, and it is also the initial condition for all of the transient experiments (X2–X8). The right column illustrates the steady-state topography (B), rainfall pattern (D), and normalized elevation distribution (F) from experiment X2 with nonuniform rainfall. In the topography figures (A and B), red shading is relatively high elevation, and blue shading is relatively low ele-

vation. Data from the highlighted 50 km, open boundary or mountain front, x direction channels (in black) are shown 45 km, y direction in Figure 12. Rainfall rate only 3000 3000 changes in the y direction, mov- 2000 CD2000 ing away from the model open 1000 1000

boundary, or mountain front. All rainfall rate (mm/yr) of the model experiments have 1 1 the same configuration as shown d d in A, and all of the model bound- 0.8 0.8 aries are closed (no sediment or water can cross the boundary), 0.6 0.6 except for the one side labeled open boundary. The bottom two 0.4 0.4 figures (E and F) compare swath elevation data from the modeled

elevation normalize 0.2

elevation normalize 0.2 landscape (thin black lines are maximum and minimum values; EF 0 0 thick black line is the mean) with 40 30 20 10 0 40 30 20 10 0 the real landscape (gray line and distance (km) distance (km) area). The thick gray line is the mean elevation value across the Beni escarpment, and the gray area represents the range of observed elevations. This is a subset of the data shown in the inset in Figure 1. The data in E and F are normalized to vary between 0 and 1.

respectively, and t is time. The second term on white noise. We present six transient experiments The transient experiments (X3–X8) are de- the right-hand side of Equation 4 is the fluvial (X3–X8), all of which start from the topography signed to explore three of the four plausible incision or deposition rate at a location (x, y) resulting from X1, a low-uplift, higher-erodibil- conditions discussed earlier that are necessary

in space. The details of the fluvial processes ity, low-relief, steady-state topography. Because to create the broad convexity and general ksn pat- equations are provided in the supplemental all of the simulated landscapes evolve from the terns that are observed in the channels on the materials.1 same initial white-noise surface, they all have Beni escarpment. We discuss, but do not attempt the same channel network. Four of the six tran- to model, the potential role of a shortage of Experimental Design sient runs are focused on exploring the four high abrasive tools in generating the very high steep-

ksn transverse channels between the Consata ness index values along the transverse drainages The experiment details are given in Table 1. and La Paz Rivers (Fig. 1) that largely define given the limitations of current tools-and-cover Two of the experiments (X1 and X2) are the overall morphology of the landscape. These models to capture the observed transient behav- designed to contrast the effects of rainfall pat- channels incise into the granitic and highly ior of many large rivers (e.g., Gasparini et al., terns on steady-state morphology, but our topo- metamorphosed units, and the simulations use 2007; Kirby and Whipple, 2012). In our tran-

graphic analysis suggests that the landscape is lower erodibility (K and Kt) values (X3–X6). An sient experiments, we systematically vary the

transient. These two experiments start from the additional two experiments with higher K and Kt spatial uplift pattern, temporal uplift pattern, same initial nearly flat topography with 0.5 m of values explore whether a lithologic difference rainfall pattern, and rock erodibility in order to may affect the difference in steepness values quantify the effects that each of these variables between the frontal and transverse channels (X7 have on landscape morphology. 1GSA Data Repository Item 2014171, Supplemental and X8) by comparing the morphology of the In the model, rainfall gradients drive dis- Figures 1–6 and text, is available at www.geosociety frontal channels from these runs with that of the charge gradients, which may alter both the tran- .org/pubs/ft2014.htm, or on request from editing@ geosociety.org, Documents Secretary, GSA, P.O. Box transverse channels from comparable runs with sient erosion pattern (Han et al., 2014; Colberg 9140, Boulder, CO 80301-9140, USA. lower erodibility (X5–X6). and Anders, 2014) and the steady-state channel

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TABLE 1. SUMMARY OF NUMERICAL EXPERIMENTS

† § Experiment K, Kt values* Rock uplift pattern Rainfall pattern Run duration (m.y.) X1 High Uniform (0.1 mm/yr) Uniform (1500 mm/yr) Until steady state is reached X2 High Uniform (0.1 mm/yr) Nonuniform Until steady state is reached X3 Low Uniform, temporally varyingUniform (1500 mm/yr)3.3 m.y. constant rock uplift rate (1 mm/yr); 1 m.y. rock uplift rate linearly declining in time (from 1 to 0 mm/yr); 1 m.y. zero uplift X4 Low Uniform (1.0 mm/yr) Nonuniform 3.3 m.y. X5 Low Ramp Uniform (1500 mm/yr) 3.3 m.y. X6 Low Ramp Nonuniform 3.3 m.y. X7 High Ramp Uniform (1500 mm/yr)3.3 m.y. X8 High Ramp Nonuniform 3.3 m.y.

*High K, Kt values are 9e–7 (units vary between K and Kt); low values are 5e–7. †Ramp uplift pattern described by Equation 7 in supplemental material. §Nonuniform rainfall pattern described by Equation 6 in supplemental material.

slope (e.g., Craddock et al., 2007). We explore Results and Comparison with Data transverse channels on the Beni escarpment two different rainfall patterns (Figs. 11C and (Craddock et al. [2007] observed similar pat- 11D) in the model experiments, uniform and When comparing between the model and terns in the Marsyandi River in Nepal). In the nonuniform, based on the observed pattern across Bolivian data, we use a reference concavity area where rainfall rates are nonuniform in the the Beni escarpment (Fig. 2A). In the transient value of 0.5. The model channels have a known modeled landscape (between 0 and 40 km from experiments that include nonuniform rainfall, the reference concavity value (m/n), and the results the mountain front), the upstream spatially aver- rainfall pattern is implemented at the start of the are easier to interpret when this value is used. aged rainfall rate increases toward the mountain

transient simulation. Ehlers and Poulsen (2009) Because the channel steepness index values are front. As a result, ksn declines downstream (Eq. found that the current orographic rainfall pattern sensitive to the reference concavity, for com- 3), and channel concavity is greater than the ref- was not established until the topography reached parison with model output, we recalculate the erence value, even in the region where rainfall 75% of its current elevation; therefore, if any- Bolivian data with the same reference concavity rates decline toward the mountain front in the

thing, we likely overestimate the effects of the used in the model (qref = 0.5). As noted earlier, transverse channel. This pattern matches the ob- rainfall pattern on the channel morphology. The the trends in channel steepness in the Bolivian served pattern, but it is more subdued. Models

equations for the nonuniform pattern are given in data are the same regardless of the reference predict only a factor of 2 decline in ksn along the the supplemental material (see footnote 1). concavity value (Fig. 5). transverse drainages, i.e., far less than observed. We explore two different rock uplift patterns Similarly, the nonuniform rainfall model repro- in the model experiments: uniform and a ramp- Steady-State Landscapes duces some key attributes of the frontal chan- uplift pattern. The ramp-uplift pattern is a linear The topographies of the two steady-state, nels but not others. The higher mean rainfall rate increase in rock uplift rates for the first 30 km uniform-uplift landscapes, one with uniform in the modeled frontal channels yields frontal into the escarpment, at which point the rock uplift rainfall (X1) and the other with nonuniform channels that are less steep than the transverse rate becomes uniform. The equations describing rainfall (X2), are very different, as evidenced channels, as observed. However, the upstream the uplift pattern are given in the supplemental simply by looking at the topographic maps spatially averaged rainfall rate in the frontal material (see footnote 1). All of the transient (Figs. 11A and 11B). In the landscape with channels decreases toward the mountain front, simulations explore the consequences of a total nonuniform rainfall, less of the topography is and this results in a slight downstream increase

of 3.3 km of rock uplift; this amount is dictated at the highest elevations (less red and yellow in ksn (Fig. 12, black dashed line and x symbols), by the ~3.8 km elevation of the upper convexity areas in the topographic map). The differences i.e., the opposite of what is observed on the Beni in transverse channels, ~3.3 km above the ~0.5 in elevation distribution across the landscapes escarpment (Fig. 7). The clear implication is km elevation of rivers exiting the foot of the Beni are clearly illustrated in the topographic swaths that a rainfall gradient alone cannot explain the escarpment. There is a direct trade-off between (Figs. 11E and 11F). In the landscape with uni- morphology of the Beni escarpment, but that the

rock uplift rate and erosional efficiency (K, Kt), form rainfall, the maximum elevation gradu- rainfall gradient may contribute to differences

and we calibrate K and Kt, based on the uplift ally decreases from the back of the mountain in the ksn values (at a given distance from the values, to best match observed channel steepness toward the front (Fig. 11E). In contrast, in the escarpment front) between the frontal and trans- and relief. In other words, the outcomes of our landscape with nonuniform rainfall, there is an verse channels. numerical experiments do not and cannot con- abrupt decline in the maximum elevation of the Neither of the steady-state, uniform-uplift- strain either the rate of rock uplift or the duration landscape at the point where the rainfall rate rate scenarios results in landscapes with much of the simulated young phase of uplift. For sim- begins to increase toward the mountain front (at resemblance to the upper reaches of the Beni plicity, we set the maximum rock uplift rate to 1 40 km) (Fig. 11F). In both cases, the modeled escarpment. Both scenarios result in transverse mm/yr, and consequently we simulate a 3.3 m.y. elevation distributions are noticeably different channels with no knickpoints or convexities duration of rock uplift. Note that the simulation from that observed in the Beni escarpment. (Fig. 12). Contrary to the argument put forward that explores the effect of a decline in uplift rates The rainfall pattern drives a region of in by Schlunegger et al. (2011), the rainfall pat- through time (X3) is run for slightly longer after creased concavity in the modeled steady-state tern alone cannot create a convexity at steady the initial 3.3 km of uplift, when we impose a transverse channel (Fig. 12, black solid line state, nor does one emerge during the evolu- decrease in rock uplift rate, and the topography and squares) that bears some similarity to the tion toward this steady state. Moreover, any begins to decay. observed region of increased concavity in the enhancement of rock uplift in the high rainfall

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Frontal Channel, Uniform P 1]). In addition, the larger, transverse channels Transverse Channel, Uniform P respond more quickly than the frontal channels, Frontal Channel, Non−uniform P and the slope in the transverse channels near Transverse Channel, Non−uniform P the mountain front quickly declines toward the

1200 transport slope necessary to move the sediment 1000 delivered from upstream (Whipple and Tucker,

800 2002). This is evident in the nearly constant ksn values near the mountain front in the transverse 600 Figure 12. From top to bot- channel (green squares in Fig. 14A). The fron- 400 tom, steady-state channel profiles, channel steepness tal channels remain steeper because they do not Elevation [m ] 200 index versus distance from respond as quickly as the transverse channels 0 open boundary (or moun- (green x symbols in Fig. 14A). As a result, near 65 60 55 50 45 40 35 30 25 20 15 10 5 0 tain front), and slope-area the mountain front, there is a region in which Distance from mouth [km] data from steady-state the frontal channels have higher k values than simulations (X1 and X2) sn 160 with uniform rainfall (gray) the transverse channels, which is the opposite of and nonuniform rainfall the trend observed in the channels on the Beni 120 pattern (black). The same escarpment. These results suggest that a tempo- legend applies for the bot- ral decline in rock uplift is not responsible for = 0.5

ref 80 tom two plots. In the uni- the morphology of the Beni escarpment. θ

, form rainfall case, the fron- Experiment X4 isolates the effects of the sn tal and transverse channels k 40 rainfall pattern on a transient landscape, and have the same slope-area relationship and channel it also has noticeable contrasts with the real 0 landscape. The mean, peak, and minimum 45 40 35 30 25 20 15 10 5 0 steepness value, making perpendicular distance from front [km] it nearly impossible to elevations are higher than the normalized Beni distinguish between the escarpment topography near the mountain front Frontal Channel, Uniform P two channels in the plots (Fig. 13B). The rainfall pattern does result in a Transverse Channel, Uniform P because the data overlap. decrease in k values near the mountain front The locations of the chan- sn Frontal Channel, Non−uniform P in the transverse channels (blue squares in Fig. Transverse Channel, Non−uniform P nels on the model topography and the rainfall pattern 14A). However, the decline in ksn is not as steep 10−1 are shown in Figure 11. P— in the modeled transverse channel as it is in the rainfall. Beni escarpment transverse channels, and as a result, the modeled channels are not as concave as the Beni escarpment channels (supplemen- 10−2 tal Fig. 2 [see footnote 1]). The rainfall pat-

slope [m/m ] tern causes the frontal channels to have lower

ksn values than the transverse channels at the same perpendicular distance from the moun- 106 107 108 109 tain front, but k increases toward the front in 2 sn drainage area [m ] the modeled frontal channels (same patterns as observed in steady-state experiment X2). Both the transverse and frontal modeled channels zone, such as that suggested by Schlunegger et channel morphologies observed on the Beni have an upper convexity. The upper convexity al. (2011), would act to damp the high concav- escarpment. in the modeled transverse channel is much more ity of the modeled transverse drainages, which discrete (occurs over a shorter channel length) is one of the ways in which the nonuniform Nonsteady Landscapes than the convexities in the Beni escarpment rainfall simulation actually bears a resem- Experiment X3 isolates the effects of a tem- transverse channels. Unlike the modeled fron- blance to the topography of the Beni escarp- poral reduction in a uniform rock uplift rate and tal channel, the Beni escarpment frontal chan- ment. Because there are no convexities in the has some similarities to the Beni escarpment nels do not have an upper convexity. Overall, transverse channels in the modeled steady- topography but also some notable differences. a transient landscape resulting from a uniform state landscapes, no evidence for an active The trend in the minimum elevation of the mod- increase in rock uplift and the onset of enhanced structure that coincides with observed convexi- eled topography is similar to the Bolivian topog- orographic rainfall does not appear to match the ties, and a general correspondence of the con- raphy, but the mean and maximum modeled ele- morphology of the Beni escarpment. vexities with the 3800 m contour, we conclude vations are too high across the entire landscape The ramp-uplift scenario with uniform rain- that the convexity in the transverse channels on (Fig. 13A). The decline in uplift rate over time fall (X5) has a number of similarities with the

the Beni escarpment is either caused by a tran- does result in a reduction in ksn values toward Beni escarpment. Peak elevations increase lin- sient response to an increase in rock uplift rate the escarpment front (green data in Fig. 14A). early upslope of the escarpment front and then or glacial erosion. Because we do not simulate However, this decline is very localized in the almost level off (Fig. 13C). A similar pattern is glacial erosion, the rest of the numerical exper- modeled channels, and as a result, the channels observed across the Beni escarpment, although iments explore different transient scenarios to are more concave in this region than the Boliv- the location at which peak elevations level off determine possible drivers responsible for the ian channels (supplemental Fig. 1 [see footnote differs between the two landscapes. Mean eleva-

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1 Figure 14. Comparison of A channel steepness data A X3. temporal decline in uplift rate, uniform rainfall 0.8 from a transverse channel X4. uniform increase in uplift rate, nonuniform rainfall from the model simula- X5. ramp uplift, uniform rainfall 0.6 tions (colored data with 2500 square symbols), a frontal 0.4 channel from the model 2000 simulations (colored data 0. 5 0.2 = 1500 temporal decline with x symbols), the Chal- re f θ

in uplift, uniform rainfall lana River (gray square , 1000 symbols), and one Bolivian sn 1 k B frontal channel (gray x sym- 500 0.8 bols). The locations of the model channels are shown 0 0.6 in Figure 11. Only data across the Beni escarp- X6. ramp uplift, non-uniform rainfall 0.4 ment are shown for the B X7. high K value, ramp uplift, uniform rainfall Bolivian channels. The dark- 0.2 X8. high K value, ramp uplift, nonuniform rainfall uniform uplift, gray and light-gray bands 2500 nonuniform rainfall in the channel steepness plot represent the range of 2000

C channels steepness values 0. 5 0.8 observed in the Bolivian = 1500 elevation normalized re f

high ksn transverse chan- θ 0.6 nels and frontal channels, , 1000 sn

respectively. These same k 0.4 500 bands are shown in Figure 7, but because the refer- 0.2 ramp uplift, 0 uniform rainfall ence theta value is different 45 40 35 30 25 20 15 10 5 0 0 in this figure, the channel perpendicular distance from front [km] D steepness values are not 0.8 the same as those shown in Figure 7. The model scenarios are described in the legends above the plot, and unless otherwise stated, the low K value was used in the experiment. Note that in some of 0.6 the model scenarios, there is some noise in the ksn values near the escarpment front, and this is from small instabilities in the transport-limited model. 0.4

0.2 ramp uplift, in ksn is similar between the modeled and Beni do in the ramp uplift–only simulation (cf. Figs. nonuniform rainfall 0 escarpment channels; that is, both modeled and 13D and 13C). The pattern in ksn in the modeled 40 30 20 10 0 real channels have a linear rise in k into the transverse channel nearly matches the pattern distance from front (km) sn escarpment (red data in Fig. 14A). However, in the Challana River (compare blue squares Figure 13. Comparison between the modeled we highlight some notable differences between and gray squares in Fig. 14B), and this is also elevation distribution (black lines) and elevation distribution in the study area (gray area, the modeled and real data. The ksn values of the evident in the slope-area and channel profiles from white box shown in Fig. 1; only the eleva- frontal and transverse channels in the model are (supplemental Fig. 4 [see footnote 1]). In con- tion distribution on Beni escarpment is shown). the same near the downstream edge where they trast with the previous experiment, the decline The thick lines are the average elevation mov- have adjusted to the uplift pattern, whereas the in ksn toward the mountain front is not linear, ing away from the escarpment front for the field Beni escarpment frontal channels have smaller and this is because the spatially averaged rain- data (gray line) or open boundary for the model k values than the Beni escarpment transverse fall rate does not increase linearly toward the data (black line). The thin black lines show the sn minimum and maximum modeled elevation at channels at a given distance from the escarp- mountain front. The modeled frontal channels a given distance. The data are normalized so ment front. Further, the upstream portion of the also have slightly lower ksn values than the trans- that the minimum elevation value is zero, and modeled frontal channel has a notable convexity verse channels, although the difference between the maximum elevation is one for easier com- (increase in ksn) that records the same transient ksn values in the transverse and frontal channels parison between the model and field data. (A) stage of channel profile development as the con- is much less in the modeled topography than in Experiment X3. (B) Experiment X4. (C) Experi- vexities in the transverse channels, but this is the Beni escarpment. The upper convexity in the ment X5. (D) Experiment X6. not observed in the frontal channels of the Beni frontal channels is much smaller in this experi- escarpment. This scenario appears to capture ment (in contrast with X5). The addition of the many, but not all, of the quantified morphologi- rainfall pattern with the ramp uplift improves tions rise more linearly and are slightly higher in cal details of the Beni escarpment. the match with the real data; however, the rela-

the modeled landscape in comparison with the The simulation with the ramp-uplift scenario tive difference between the ksn values in the Beni escarpment. The simulated and observed and nonuniform rainfall pattern (X6) has even transverse and frontal channels is not as large as transverse channels are generally similar in greater similarity with the morphology of the it is in the real channels. form, and the location of the channel convexity Beni escarpment than the previous experiment Runs X7 and X8 were designed to explore in the modeled transverse channel is nearly the (X5). Mean, maximum, and minimum eleva- the possibility that lithology is responsible for, or

same as in the Challana channel (Supplemen- tions of the model topography rise at a slightly contributes to, the distinctly different ksn values tal Fig. 3 [see footnote 1]). The general trend lower gradient into the escarpment than they between the transverse and frontal Beni escarp-

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ment channels. Although where side-by-side downstream distribution of fluvial discharge, a wave of increased incision that was spurred by

the transverse channels (higher ksn) are cutting more extreme rainfall gradients would be increased uplift rates. Such a wave of incision through the same rocks as the frontal channels, required for rainfall to be the sole driver of the will migrate upstream at a rate proportional to the transverse channels are transporting coarse, observed morphology of the transverse chan- the local drainage area and would be expected to durable cobbles and boulders derived from the nels. The local rainfall rate does not directly be at roughly the same drainage area (Whipple granitic and high-grade metamorphic rocks impact the channel slope (Eq. 3). Rather, inci- and Tucker, 1999) and elevation (Niemann et upstream and may be steeper as a consequence. sion rates are controlled by the fluvial discharge, al., 2001) in all of the transverse channels of the Rather than attempting to recreate this com- which is a function of the upstream spatially same size. The upstream convexities in modeled plex scenario in a model, we simply compare averaged rainfall rate within the catchment, and transverse channels with low K values, ramp the morphology of the frontal channels from a not the local rainfall rate. At steady state with uplift, and either with or without the rainfall set of runs with enhanced erodibility (X7–X8) uniform rock uplift rates, modeled transverse gradient (X5 and X6; supplemental Figs. 3 and with that of the transverse channels from earlier, channels from the nonuniform rainfall simula- 4 [see footnote 1]) have similar forms to those

comparable runs with lower erodibility (X5– tion (X2) do not have any areas in which slope observed in the high ksn transverse channels on X6). Naturally, the larger erodibility values in increases downstream (no upper convexities), the Beni escarpment, with the upper convexities

experiments X7 and X8 result in lower ksn values and modeled channel concavities are weaker occurring at consistent elevations and drainage in both the frontal and transverse channels (red than those in the Beni escarpment (Fig. 12). areas. Although we cannot rule out an important and green data in Fig. 14B) in comparison with Similarly, during a transient adjustment to role of glacial scour above ~3800 m elevation, the earlier ramp-uplift experiments with either uniform faster rock uplift, modeled transverse the fact that the upper convexity in the Consata uniform rainfall (X5; red data in Fig. 14A) or channels subjected to nonuniform rainfall (X4) River occurs at the same elevation and approxi- with nonuniform rainfall (X6; blue data in Fig. do exhibit the upper convexity as expected; they mate drainage area suggests that the primary 14B). Both X7 and X8 have a ramp-uplift pat- do have an ~25-km-wide zone of enhanced control is the amount of transient surface uplift. tern, and X8 also includes the rainfall pattern. concavity, but again the simulated concavities This follows because the headwaters of the Con-

The rainfall pattern results in lower ksn values are far weaker than those observed in the Beni sata rise onto the low-relief and weakly eroded

near the mountain front (green data in Fig. 14B) escarpment (with ksn declining by only a factor Central Andean Plateau that lacked the powerful in comparison with the uniform rainfall case of ~2 compared to a factor of ~5). Further, the alpine glaciers of the Cordillera Real.

(red data in Fig. 14B). In both X7 and X8, ksn morphology of the frontal channels on the Beni Interpretation of the frontal channels, ero- values in the frontal channels are within the escarpment does not match the predicted and sion rate data, tributary morphometric data, and range observed in the Beni escarpment frontal modeled morphology of frontal channels with- the mapped distribution of the 7–11 Ma Cangalli channels, but in the lowest part of the observed out an uplift gradient; i.e., the Beni escarpment Formation all further support the hypothesis that range. The transverse channels are less steep frontal channels also express enhanced concavi- an uplift gradient is primarily responsible for the than the Beni escarpment transverse channels ties despite the downstream decrease in rainfall morphology of the Beni escarpment. Channel (see also supplemental Figs. 5 and 6 [see foot- rate in these smaller catchments (Fig. 2). steepness values in the frontal channels decrease note 1]) but interestingly match the steepness of Schlunegger et al. (2011) suggested that the toward the escarpment front (enhanced concav- the Rio Consata—the one transverse drainage convexity in the transverse channels between 10 ity; Fig. 7A), suggesting that the uplift gradient that does not tap into the granitic or high-grade and 100 km2 results from high uplift rates driven has a greater impact on channel form than the metamorphic rocks, breaching the Cordillera by high rainfall rates, but this is not consistent rainfall gradient, which would be associated Real where there is a gap in the outcrop of these with either observations or models. Although with a downstream increase in channel steep- stronger rocks. the downstream end of the convexity does ness in this case. Millennial-scale erosion rate roughly correspond to the zone of high rainfall data collected by Safran et al. (2005) vary sys- DISCUSSION rates (>1000 mm/yr) in three of the transverse tematically with distance from the escarpment channels, the convexity is well upstream of the front (Fig. 9C), whereas the trend with rainfall Role of Tectonics, Rainfall, and Lithology high rainfall zone in the Zongo and Consata rate is counter to what would be expected if on the Morphology of the Beni channels (Fig. 6). Moreover, we can find no higher rainfall rates were driving higher rates Escarpment mechanism by which this can occur and con- of erosion (Fig. 9D). Finally, channel steepness clude that this convexity rather reflects either values from 35 additional tributary watersheds It is tempting to assume that the concentra- a transient channel adjustment to an increase also increase with distance from the escarp- tion of high rainfall rates on the Beni escarpment in rock uplift rate as in our simulations, or pos- ment front (Fig. 10B). Channel steepness values must play a major role in setting the morphology sibly glacial scour at and above the Last Gla- in the wettest tributaries are generally smaller of the landscape (e.g., Schlunegger et al., 2011; cial Maximum equilibrium line altitude (e.g., than those in the driest tributaries, which sup- Barnes et al., 2012), but we have shown that the Brozović et al., 1997; Brocklehurst and Whip- ports the idea that rainfall rates influence chan- rainfall pattern alone cannot drive the patterns we ple, 2002), or most likely both (glacial scour nel morphology (Fig. 10C) but does not support observe in the topography of the escarpment and enhancing a transient convexity). This follows the idea that erosion rates increase with rainfall the morphology of the channels flowing across because the locations of the upper convexity rate. However, it is difficult to discern from the it. Here, we discuss all the lines of evidence that in all five transverse channels (including the tributary watershed data alone whether the rain- suggest that the morphology of the Beni escarp- Consata) are most consistent with the transient fall pattern or hypothesized uplift pattern is the ment is primarily driven by an uplift gradient, adjustment to uplift scenario, as summarized in primary driver of channel morphology, because and that rainfall patterns play a secondary role in the following text. The fact that the convexity these two variables likely covary. controlling landscape morphology. is in the same approximate drainage area range Although we argue that the uplift pattern Because rainfall gradients affect fluvial inci- in all of the transverse channels supports our dominates the channel morphology, rainfall sion patterns only indirectly by changing the interpretation that this convexity was driven by does appear to affect both channel steepness

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values in the larger channels and the overall do see an approximate twofold difference in ksn scape evolution in the Beni escarpment. None-

morphology of the landscape. The transverse similar to that between the frontal and high ksn theless, we can conclude with certainty that the channels drain the driest parts of the landscapes, transverse channels on the Beni escarpment in dominant influence on channel profiles and the so for a given distance from the escarpment two otherwise similar frontal channels southeast overall morphology of the Beni escarpment is front, the upstream averaged rainfall rate in the of the Beni escarpment with and without sig- the pattern and history of rock uplift. Moreover, transverse channels is lower than in the frontal nificant granitic rocks (Fig. 7C). However, the the model with the ramp-uplift pattern plus the

channels. In agreement with theory (Eq. 3) and ksn data from the 35 tributary watersheds (Fig. rainfall gradient comes fairly close to explaining model simulations (X2, X4, X6, and X8; Figs. 10B) either reveal the uplift gradient or they are the data (Fig. 14B, blue symbols), and transient 12 and 14), the Beni escarpment data show that consistent with a lithologic explanation for the oversteepening is a plausible explanation of the

the transverse channels have higher channel disparate ksn values in the transverse and fron- misfit in four of the five transverse channels, steepness values than the frontal channels at a tal channels, but not both, and many lines of without invoking any influence of lithology. given distance from the escarpment (Fig. 7A). evidence support the presence of an important The model results also show that the rainfall uplift rate gradient. Finally, we do not have the Implications for the Uplift History of the gradient reduces mean elevation values near the requisite data to quantify differences in rock Bolivian Andes escarpment front, resulting in a modeled gradi- strength, nor field observations to confirm that ent in mean elevation near the escarpment front transverse channels are armored with granitic Several studies have suggested that uplift of that more closely resembles that of the real land- boulders downstream of granite outcrops, as the Bolivian Altiplano has slowed or stopped scape (Fig. 13D) in comparison to the model would be required for the presence of harder (e.g., Garzione et al., 2006; Ghosh et al., 2006; simulation that does not include the rainfall gra- rocks in the upper reaches to affect the steep- Hoke and Garzione, 2008), but it is difficult to dient (Fig. 13C). ness of the transverse channels all the way down reconcile the landscape morphology of the Beni Adding the rainfall gradient alone to the to the foot of the Beni escarpment as observed. escarpment with the results from the model sim- transient ramp-uplift scenario (X6), however, Consequently, although we cannot rule it out, ulation (X3) of uplift slowing and cessation for does not fully explain important details of the we cannot conclude that there is a clear litho- a significant period of time (2 m.y.). The mini- observed landscape morphology: (1) The mod- logic influence at work. mum elevations across the modeled landscape eled frontal streams are comparatively slightly A transient oversteepening is predicted to have a similar distribution as those across the too steep, (2) modeled frontal streams exhibit occur when the upstream sediment load does Beni escarpment; however, the mean and maxi- transient upper convexities (blue x symbols in not supply enough tools for local incision rates mum elevations remain too high (Fig. 13A). Fig. 14B) not present in the Beni escarpment to keep pace with downstream incision rates that With time, the mean and maximum elevations frontal channels, and (3) there is no mechanism have increased due to an increase in rock uplift would decline, but by that time, the minimum in this model to explain the observation that the rate. The oversteepening occurs downstream of elevations in the center of the modeled escarp- steepness of the Consata is more similar to the a critical area that is a function of the original ment would be lower than those in the Beni frontal channels than to the other transverse uplift rate, the factor of increase in uplift rate, escarpment. Further, a cessation in uplift leads

channels (Fig. 7B). To best match the contrast and the K and Kt values (see Eq. 32 in Gaspa- not only to channels with nearly uniform slope

in steepness between the frontal and high ksn rini et al., 2007). The analytical relation in Gas- in the downstream reaches (supplemental Fig. transverse channels, either (1) the rainfall influ- parini et al. (2007) suggests that the parameter 1 [see footnote 1]; Fig. 14A, green data), rather ence needs to be stronger than incorporated into values that we use in this study are compatible than the highly concave channels as are observed our model (Eq. 3), or (2) the lithologic differ- with a transient oversteepening in the transverse in the Beni escarpment, it also leads rapidly to a ence between these drainages is important (X8, channels—a possibility substantiated by the configuration where transverse rivers have lower

red and green symbols in Fig. 14B), or (3) the observation of numerous fluvial hanging valleys ksn values than the adjacent frontal streams—the contrast in channel steepness reflects a tran- (an extreme case of transient oversteepening) opposite of what is observed (Fig. 14). sient oversteepening at and below the upper southeast of the Beni escarpment (Whipple and In contrast, our results suggest that ~3 km of convexity such as is predicted under some cir- Gasparini, 2014). If transient oversteepening surface uplift has occurred in at most the last ~15 cumstances with tools-and-cover models (e.g., downstream of the convexity is contributing to m.y., and that if uplift has ceased, the landscape

Chatanantavet and Parker, 2006; Gasparini et the high ksn values in four of the five transverse has not yet had time to respond to the cessation al., 2007; Crosby et al., 2007). channels, this would be consistent with the lack in uplift. The transient simulations had 3.3 km of As noted earlier, available erosion-rate data of lithologic signal in the tributary watershed maximum rock uplift in 3.3 m.y., but the simu- suggest a weaker relationship with runoff than data (assuming an uplift gradient) and could lations constrain only the total amount of uplift, assumed in our models, so appealing to a stron- explain the slight misfit between models and not the uplift rate and duration. Cosmogenic ger rainfall influence does not appear to be a data (Fig. 14B). However, this mechanism is isotope data suggest that the maximum rock likely explanation for the slight misfit between inconsistent with the Rio Consata (Fig. 7B), uplift rate (>30 km SW from the foot of the Beni model predictions and data (Fig. 14B, blue which lacks granitic outcrops and has much escarpment) is ~0.7 mm/yr (Safran et al., 2005), symbols). Moreover, a stronger rainfall influ- lower steepness values than the other four trans- and the longer-term mean is 0.4 ± 0.29 mm/ ence would be expected to reduce the concav- verse rivers (Fig. 7B) despite a pronounced yr, determined from apatite fission-track ther- ity of the frontal channels, inconsistent with our upper convexity (Figs. 4 and 6)—it ought to mochronometry (Insel et al., 2010). This lower observations. Similarly, a lithologic impact on express the same transient oversteepening phe- maximum rock uplift rate suggests a duration of channel morphology could explain the differ- nomenon, but it does not. 5–16.5 m.y. of rock uplift, consistent with previ-

ence between the high ksn transverse channels Thus, we can find no single model or mecha- ous interpretations that the recent phase of rapid and the frontal channels (Fig. 14B, red and nism that can explain all the data in full detail. rock uplift began ca. 10–12 Ma (e.g., Gillis et al., green symbols); however, a lithologic signal is Consequently, we cannot precisely quantify the 2006; Barnes et al., 2012). The age of the Can- not consistently observed in all of the data. We relative roles of rainfall and lithology on land- galli Formation (11–7 Ma) supports the obser-

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vation that the uplift is relatively young; other- rainfall rates drive high rock uplift and erosion mountain front in both transverse and frontal wise, these deposits would not be deformed and rates. Patterns in erosion rates, at steady state, channels (Fig. 15D). However, in this case, at would likely no longer be preserved on ridgelines should always follow the uplift pattern, and pre- a given distance from the mountain front, the

within the Beni escarpment. The preservation of vious studies support that the channel steepness frontal channels have a higher ksn value than the low-relief surfaces at ~4000 m (Whipple and index can be used as a proxy for erosion rate transverse channels in the model. Theory also Gasparini, 2014) and the age of the Cangalli For- (Safran et al., 2005; Harkins et al., 2007; Oui- predicts that channel steepness downslope of the mation (11–7 Ma) support the observation that met et al., 2009; Cyr et al., 2010; DiBiase et al., escarpment can be used to decipher the relative the uplift is relatively young. If the uplift were 2010). If rainfall and erosion rates also follow roles of climate and tectonics. Comparisons of older, the Cangalli Formation deposits would not the same patterns, this could mean that rainfall channels that drain the driest parts of the escarp- be deformed and would likely no longer be pre- rates are driving higher erosion rates, or it may ment with those that originate downslope of served on ridgelines within the Beni escarpment. simply mean that rainfall rates covary with uplift the escarpment (assuming this area is relatively Although we cannot constrain the exact timing of rates. However, because rainfall and uplift rates wetter) can also be helpful for quantifying the the uplift, we can say that the uplift began after affect topography in different ways, deciphering local influence of climate (Fig. 15B). 15 Ma, and if it has stopped, the channels have whether there is a rainfall signal atop of the uplift The patterns in the Beni escarpment chan- not yet had time to respond. signal requires topographic metrics along with nels, theory, and our numerical results lead to The numerical simulations and the topog- erosion rate data. In such cases, numerical mod- the following observations that can be used to raphy of the Beni escarpment suggest that this els are especially useful because it is not easy to quantitatively compare the influence of tecton- landscape has not fully adjusted to the pattern of quantitatively predict patterns in landscape mor- ics, climate, and lithology in other settings: rock uplift. The upstream convexity in the trans- phology in response to diverse drivers from the- (1) In order to explore the influence of rain- verse channels links a channel in a relatively ory alone, especially during transient adjustment. fall rates on fluvial discharge and inci-

lower local relief landscape with the downstream We find that all of the different climatic and sion rates, it is critical to compare ksn portion of the channel that is actively incising tectonic scenarios explored in this study have a values in channel reaches that have dif- into a high local relief landscape. The low-relief unique signature on channel morphology, but ferent upstream averaged rainfall rates. parts of the landscape above the upstream con- only when both transverse and frontal chan- In a landscape with a dominant rainfall vexity are currently above ~3800 m (Figs. 1, nels are considered together. This is predicted gradient in one direction, small water- 2, and 4), and this part of the landscape may by theory (Eq. 3), confirmed by our numerical sheds oriented perpendicular to this represent preserved patches of the former low- experiments, and summarized in Figure 15. gradient will each experience relatively

relief landscape that once stretched across the When only rock uplift rate varies spatially, ksn uniform rainfall rates. Different rain- Beni escarpment before it was an escarpment, varies directly with the uplift rate and should be fall rates among such small watersheds as evidenced by the Cangalli Formation (Mosolf the same in both transverse and frontal channels arranged along the rainfall gradient et al., 2011). This would also be consistent with (Fig. 15A). In contrast, when only rainfall rate will be particularly useful for decipher-

the elevated low-relief surfaces that have been varies spatially, ksn decreases downstream where ing the influence of rainfall on channel observed south of the study area (e.g., Barke and rainfall rates both increase and decrease down- morphology. Equally diagnostic will be Lamb, 2006) and NE of the Beni escarpment stream in the channels that drain the driest parts similarities or differences in profile con-

(Whipple and Gasparini, 2014). These low-relief of the landscape (transverse channels), and ksn cavity (best quantified and illustrated on

patches are also roughly coplanar with the low- increases downstream in the channels that drain plots of ksn vs. distance, not on classic relief erosional margin of the Central Andean only the wettest parts of the landscape (frontal slope-area plots) among channels that Plateau, which is well-developed at ~4000 m channels) (Fig. 15B). Further, at a given dis- drain areas with rainfall that is vari- elevation north of the Cordillera Real in southern tance from the mountain front, transverse chan- ously uniform, increasing downstream,

Peru (Lease and Ehlers, 2013). However, with- nels have higher ksn values than frontal channels and decreasing downstream. The power out additional data, we cannot rule out the possi- in this scenario. Although it may be unlikely to of such comparisons is exemplified on bility that these low-relief parts of the landscape find either of these idealized scenarios in nature, the Beni escarpment by the similar rate

were shaped by glaciers, especially in light of the they are useful for interpreting the more complex of decrease in ksn with distance in trans- fact that the equilibrium line altitude during the case of spatially varying rock uplift and rainfall verse and frontal channels despite their Last Glacial Maximum was at ~3800 m in this rates (Fig. 15C). Similar to the rock uplift gra- contrasting rainfall patterns. region (Klein et al., 1999). dient–only case, in scenarios with both uplift (2) Once channels have reached steady

and rainfall gradients, ksn in both transverse and state, and assuming all other variables

Best Practices for Diagnosing Controls on frontal channels declines toward the mountain are uniform, ksn values should always Channel Morphology front where the gradients in forcing influence increase with rock uplift rate (Kirby and channel morphology, indicating that the uplift Whipple, 2012). However, the relation-

Our theoretical and numerical results suggest pattern has a stronger control on channel mor- ship between ksn and rock uplift rate is a number of methods that when used in concert phology than the rainfall gradient. However, the affected by other variables. Ideally, to

can best determine the relative roles of climate, rainfall gradient results in frontal channels that decipher an uplift pattern, ksn data would

tectonics, and lithology in landscape evolution. have lower ksn values than transverse channels be measured in channels from different We advocate for a combined approach that uses at a given distance from the mountain front. locations in the landscape that have the

DEM analysis, numerical modeling, and erosion Further, the modeled decline in ksn in the steady- same upstream averaged rainfall rate and rate data along with geologic maps and hydro- state channel reaches is not linear due to the lithology. However, if there are other meteorologic data as available. When erosion nonlinear variation in spatially averaged rainfall confounding variables, or cross-corre- or incision rate data are available, they are par- rate. A temporal decline in uplift rate also results lations among variables, a comparison

ticularly helpful for deciphering whether high in a nonlinear decline in ksn downstream near the between ksn values in reaches of large

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and small channels that have the same in Transverse Channels ksn hypothesized uplift rates (as diagramed in Frontal Channels ksn in Fig. 15) should aid in identifying uplift Uplift Pattern patterns. In all four of the proposed sce- Rainfall Pattern narios presented in Figure 15, the com- steady-state segments

bination of the ksn data from transverse high A Ramp Uplift [U(x)] with and frontal channels is diagnostic of both Uniform Rainfall the uplift and rainfall pattern, whereas med. the data from just a transverse or frontal channel are not always diagnostic, even in these four relatively simple cases. In low many cases, deciphering the relative 50 40 30 20 10 0 -10 -20-30 -40 influences of different variables on ksn patterns in large and small catchments will require numerical modeling in which high B Uniform Uplift with variables are systematically varied, espe- Nonuniform Rainfall [P(x)] cially when patterns are more complex med. than those presented in Figure 15. te (3) Lithology may impact channel steepness indices both locally and downstream. low This follows because exposure of harder ainfall Ra rocks that produce an abundance of 50 40 30 20 10 0 -10 -20-30 -40 coarse, durable gravels and boulders te, or R

may increase channel slopes, not just t Ra high C Ramp Uplift [U(x)] with locally, but for some distance down- Nonuniform Rainfall [P(x)] , Uplif

stream. Therefore, when interpreting sn

k med. climate and tectonics from topography, it is important to consider rock strength distributions both locally and upstream. low

CONCLUSIONS 50 40 30 20 10 0 -10 -20-30 -40

Based on the combined analysis of DEM high D Temporal Decline in Uplift [U(t)] with data, erosion rate data, and numerical model Uniform Rainfall output, we find that tectonic processes exert med. the dominant control on the morphology of the Beni escarpment. The morphology of the Beni escarpment suggests that uplift rates increase low linearly into the escarpment, but whether the 50 40 30 20 10 0 -10 -20-30 -40 base of the Beni escarpment is a fault, shear zone, or fold and the way in which it relates Distance From the Mountain Front (km) to deeper structure (décollement ramp, duplex, Figure 15. Theoretical channel steepness patterns predicted in transverse and frontal or basement thrust sheet) are not known. Oro- channels for different rock uplift and rainfall patterns. Channels are assumed to flow graphic rainfall in response to surface uplift does perpendicular to the mountain front and parallel to the gradient in forcing(s). (A–C) appear to play a secondary role in shaping flu- Channels in the gray arrowed region have adjusted to rock uplift and precipitation forc- vial channel profiles and trends in topography, ings and reached steady state. (D) Rock uplift has ceased, so no uplift pattern is shown, and steady state is not possible. In B and C, the exact ksn pattern in the transverse but we find no measurable evidence for a feed- channel downstream of the escarpment (negative distance) depends on the rainfall back between rainfall rates and rock uplift rates. values throughout the entire upstream transverse watershed. We illustrate the case in We find that the four larger, transverse channels which the transverse channels downstream of the escarpment have a lower upstream draining harder lithologies (granites and high- averaged rainfall rate than the local rainfall rate, and so the locally drained channels,

grade metamorphic rocks) have steeper profiles labeled frontal channels here, have lower ksn values than the transverse channels off in comparison with the other observed trans- of the escarpment. The uplift and rainfall descriptions in each subfigure refer to the conditions on the escarpment, or where the distance values are positive in the figure. verse or frontal channels, all else being equal. However, assuming a gradient in rock uplift rates, lithology does not appear to impact the sediment tools downstream of the convexity evolution requires a comparison of river pro- morphology of smaller tributary channels. An (Gasparini et al., 2007). However, this explana- file morphology across drainages with a range alternative explanation for the higher channel tion is not consistent with the morphology of the in sizes, locations, and orientations relative to steepness in four of the transverse channels in Consata River. structural and climatic gradients. Moreover, a comparison with the frontal channels is a tran- We also demonstrate that diagnosing the rela- combined analysis of landscape morphology sient oversteepening due to a lack of abrasive tive roles of climate and tectonics in landscape (DEMs), geologic maps, and hydrometeorologic

248 www.gsapubs.org | Volume 6 | Number 4 | LITHOSPHERE

data, along with erosion rate patterns, and out- Brocklehurst, S.H., and Whipple, K.X., 2002, Glacial erosion ogy using sediment-flux-dependent river incision mod- and relief production in the eastern Sierra Nevada, Cali- els: Journal of Geophysical Research–Earth Surface, put of landscape evolution models, proves to be fornia: Geomorphology, v. 42, no. 1, p. 1–24. v. 112, no. F3, F03S09, doi:10.1029/2006JF000567. a powerful approach, especially if models are Brozović, N., Burbank, D.W., and Meigs, A.J., 1997, Climatic Ghosh, P., Garzione, C.N., and Eiler, J.M., 2006, Rapid uplift of used to test alternative hypotheses suggested by limits on landscape development in the northwestern the Altiplano revealed through 13C–18O bonds in paleo- Himalaya: Science, v. 276, no. 5312, p. 571–574, doi: sol carbonates: Science, v. 311, no. 5760, p. 511–515, analysis of field data. 10.1126/science.276.5312.571. doi:10.1126/science.1119365. 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Wobus, C.W., Crosby, B.T., and Whipple, K.X., 2006b, Hang- Moon, S., Chamberlain, C.P., Blisniuk, K., Levine, N., Rood, Stock, J.D., and Montgomery, D.R., 1999, Geologic constraints ing valleys in fluvial systems: Controls on occurrence D.H., and Hilley, G.E., 2011, Climatic control of denuda- on bedrock river incision using the stream power law: and implications for landscape evolution: Journal of tion in the deglaciated landscape of the Washington Journal of Geophysical Research–Solid Earth, v. 104, Geophysical Research–Earth Surface, v. 111, no. F2, Cascades: Nature Geoscience, v. 4, no. 7, p. 469–473, no. B3, p. 4983–4993, doi:10.1029/98JB02139. F02017, doi:10.1029/2005JF000406. doi:10.1038/ngeo1159. Strecker, M., Alonso, R., Bookhagen, B., Carrapa, B., Hilley, Mosolf, J.G., Horton, B.K., Heizler, M.T., and Matos, R., 2011, G., Sobel, E., and Trauth, M., 2007, Tectonics and climate Unroofing the core of the central Andean fold-thrust of the southern central Andes: Annual Review of Earth MANUSCRIPT RECEIVED 4 SEPTEMBER 2013 belt during focused late Miocene exhumation: Evidence and Planetary Sciences, v. 35, p. 747–787, doi:10.1146 REVISED MANUSCRIPT RECEIVED 1 MARCH 2014 from the Tipuani-Mapiri wedge-top basin, Bolivia: Basin /annurev.earth.35.031306.140158. MANUSCRIPT ACCEPTED 3 APRIL 2014 Research, v. 23, no. 3, p. 346–360, doi:10.1111/j.1365-2117 Syrek, J., and Barnes, J., 2011, Comparing spatial patterns .2010.00491.x. of thrust belt architecture and bedrock river morphol- Printed in the USA

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