Geomorphic Constraints on the Incision History of the Lower Kern River, Southern Sierra Nevada, California GEOSPHERE; V

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GEOSPHERE Geomorphic constraints on the incision history of the lower Kern River, southern Sierra Nevada, California GEOSPHERE; v. 14, no. 3 William C. Krugh and Blake C. Foreshee Department of Geological Sciences, California State University, Bakersfield, 9001 Stockdale Highway, Bakersfield, California 93311, USA doi:10.1130/GES01603.1

7 figures; 2 tables ABSTRACT rock uplift rates in the southern Sierra Nevada compared to those in the northern Sierra Nevada (Wakabayashi and Sawyer, 2001; Clark et al., 2005; CORRESPONDENCE: wkrugh@csub.edu Stream profile analysis of the lower Kern River and its tributaries help to Mahéo et al., 2009). Previous studies have used low-temperature thermo- constrain the landscape response to late Cenozoic tectonics in the southern chronometry and/or basin stratigraphy to place constraints on the nature CITATION: Krugh, W.C., and Foreshee, B.C., 2018, Sierra Nevada of California. In this study, we identify two relict landscapes that and timing of this exhumation (e.g., House et al., 1997, 1998, 2001; Clark Geomorphic constraints on the incision history of the lower Kern River, southern Sierra Nevada, California: have been offset from the Kern Plateau by periodic displacement along the et al., 2005; Mahéo et al., 2009; Saleeby et al., 2012, 2013; Cecil et al., 2014). Geosphere, v. 14, no. 3, p. 1101–1118, doi:10 .1130 southern Sierra Nevada fault system starting ca. 20 Ma. These remnants pro- Several of these studies have identified the northwestward migration of /GES01603.1. vide context from which to evaluate existing models of rock uplift and exhu- a delaminating lithospheric root as a first-order control on late Cenozoic mation in the region. Reconstructed channel profiles on the relict landscapes tectonic activity in the southern Sierra Nevada. Science Editor: Raymond M. Russo indicate a slow incision rate of ~0.07 mm/yr throughout most of the Miocene. The Sierra Nevada has been deeply incised by major rivers that drain Associate Editor: Jeff Lee An increase in incision ca. 6 Ma resulted in the formation of the Kern Canyon. its western flank. Major rivers of the southern Sierra Nevada include, from

Received 14 August 2017 This increase in incision likely occurred in response to east-west delamination south to north, the Kern, Tule, Kaweah, Kings, and San Joaquin Rivers. Sev- Revision received 28 November 2017 of a lithospheric root beneath the southern Sierra Nevada and is reflected in a eral studies have used constraints on incision history, stream profile analy Accepted 7 February 2018 pulse of vertical incision (~0.11 mm/yr) that has propagated upstream at a rate sis, and tectonic geomorphology to investigate the landscape response to Published online 21 March 2018 of ~7.3 mm/yr to its current position near Isabella Lake. Another two pulses late Cenozoic tectonic activity (e.g., Stock et al., 2004, 2005; Clark et al., of rapid incision were likely generated by increased rock uplift associated with 2005; Figueroa and Knott, 2010). Stock et al. (2004, 2005) used cosmogenic the northward migration of the delamination hinge after 1 Ma. Incision rates 26Al/10Be burial dating of cave sediments to constrain pulses of incision and of 0.58–1.2 mm/yr have propagated up the lower Kern River resulting in for- the development of relief along the Kings and Kaweah Rivers. Clark et al. mation of the Kern River gorge. These new constraints on the incision history (2005) used low-temperature thermochronometry and stream profile analy- of the lower Kern River provide further corroborating evidence for existing sis of tributaries along the upper Kern and Kings Rivers to constrain surface models of late Cenozoic mantle delamination and associated epeirogenic pro- uplift and incision of the high-elevation, low-relief landscape of the high cesses that have helped shape the landscape of the southern Sierra Nevada. Sierra. Figueroa and Knott (2010) used Sierran river longitudinal profiles and other geomorphic indices to identify an overall southward increase in INTRODUCTION relative tectonic activity since the late Pliocene. Results from each of these studies support a recent topographic response associated with the delami- The Sierra Nevada is a prominent NW-trending mountain range coupled nation of a lithospheric root beneath the southern Sierra Nevada. OLD G with the Great Valley in central California (Fig. 1). The mountain-valley sys- The lower Kern River and its tributaries are uniquely positioned to best tem makes up the Sierra Nevada microplate, which is a semi-rigid crustal capture the landscape response to delamination of mantle lithosphere be- block measuring ~600 km long (north-south) and 250 km wide (east-west). neath the southern Sierra Nevada. While tributaries of the upper Kern River The Sierra Nevada is an exhumed Mesozoic batholith emplaced within (upstream of Isabella Lake) primarily drain the high-elevation, low-relief OPEN ACCESS largely Neoproterozoic–Mesozoic metavolcanic and metasedimentary pen- landscape of the Kern Plateau, tributaries of the lower Kern River are con- dants and wall rock (Saleeby et al., 2008; Chapman et al., 2012; e.g., Saleeby fined to the western flank of the range (Fig. 1). Downstream from Isabella et al., 2008; Chapman et al., 2012). The mean peak elevations gradually Lake, the lower Kern River continually steepens as it flows through the nar- increase from north to south to Mount Whitney (4421 m) and then rapidly row Kern River gorge, an inner gorge of the Kern Canyon, and traverses the decrease southward (Wakabayashi and Sawyer, 2001). Internal deformation southern Kern arch region as it enters the San Joaquin Basin (Fig. 1). The within the Sierra Nevada microplate is largely concentrated between 35°N Kern arch is a topographic promontory located at the transition between This paper is published under the terms of the and 36.5°N latitude (Mahéo et al., 2009). This structural and topographic the western Sierra Nevada and San Joaquin Basin that has been interpreted CC‑BY-NC license. complexity is coincident with greater amounts of exhumation and higher to reflect late Quaternary epeirogenic uplift (e.g., Saleeby et al., 2012, 2013).

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1101 Research Paper

r O

a w

r e e N v n i e s R v V s a a g d n l i a l e K f y r o f a n t u a l t l

f z a o u n l t e z o n e

r Figure 1. Map of the southern Sierra Ne-

v i vada and San Joaquin Basin, California,

n showing major late Cenozoic faults as Sierra Nevad r

er K well as important structural and geomor- Riv eah Kaw phic features. The location of the Kern River watershed is outlined on the inset map of California and overlaid in white Area of Fig. 1 on the 30 m shaded relief of the regional map. The mouth of the Kern River (white a square) is located at the transition between the southern Sierra Nevada and the San Joaquin Basin. Prominent knick-

iver K points along the Kern River are shown as ule R e r

T r e

n white circles. The location of Breckenridge v

i C

R a Kern Plateau Mountain (BkM), the Greenhorn Moun-

Tulare sub-basin n e n c y r

o tains, and the Piute Mountains are shown. a e tr n K

e f The Kern arch region (tan), low-relief up- a g k n u r i l o land surface (green), and exhumed early t

h z F

n o Tertiary nonconformity surface (green o h i n t t a e in u patterned) are after Saleeby et al. (2016). m o a S el Major fault systems that bound the south- D ern Sierra Nevada are modified from the San Joaquin Basi U.S. Geological Survey Quaternary faults database (U.S. Geological Survey and Cali fornia Geological Survey, 2006). Faults of the southern Sierra Nevada fault system Greenhorn Mtns Isabella Lake are after Mahéo et al. (2009) and Saleeby

P et al. (2016). The solid black lines represent o so C Kern arch W faults with certain locations while dashed re e e s t k t l black lines represent faults with estimated B r u e a n f or inferred locations. The thick red line de- K ck Piute Mtns e e e n BkM rn r g notes the present position of the delami- id d G i o ge r n nation hinge trace that has been migrat- rg fa e u e fa lt k ing northward since <1 Ma (after Saleeby ult c e r et al., 2012, 2013). B er Riv rn Bakersfield Ke

e on t z ul fa ck S lo a ar n G A Maricopa sub-basin n d re a s f au lt zo ne

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1102 Research Paper

The proximity of this major river network to the Kern arch means that the tonic history (e.g., Clark et al., 2005; Mahéo et al., 2009; Chapman et al., topographic signal of relative base-level fall and increased rock uplift po- 2012). Some of the exhumation, however, is attributed to erosion driven tentially associated with lithospheric delamination should be recorded. In by transient epeirogenic uplift and subsidence as well as reactivation of this study, we conduct stream profile analysis on the lower Kern River and Late Cretaceous structures during the Cenozoic (Stock et al., 2004; Clark its tributaries to: (1) quantitatively constrain the shape and steepness of et al., 2005; Mahéo et al., 2009; Cecil et al., 2014). This Cenozoic activity is longitudinal stream profiles, (2) identify topographic signatures associated interpreted to reflect the landscape response to progressive delamination with relict landscapes, and (3) locate knickpoints that may reflect transient of a lithospheric root beneath the southern Sierra Nevada and San Joaquin signals associated with changes in rock uplift. These analyses allow us to Basin (e.g., Saleeby et al., 2012, 2013; Cecil et al., 2014). place new constraints on the incision history of the lower Kern River and evaluate the landscape response to proposed mechanisms of rock and sur- Lithospheric Root Delamination face uplift in the southern Sierra Nevada. Constraints provided for Neogene–Quaternary faulting, subsidence, GEOLOGY OF THE SOUTHERN SIERRA NEVADA and uplift within the southern San Joaquin Basin and Sierra Nevada have pointed to the delamination of a lithospheric root into the mantle. Volcanism, The Sierra Nevada Microplate geophysical data, and sedimentary and topographic records suggest that a colder and denser lithospheric root is being replaced by hotter, more buoy- The Sierra Nevada and Great Valley make up an extensive coupled ant mantle as the delamination hinge migrates toward the northwest (Jones mountain-basin system known as the Sierra Nevada microplate. The Sierra et al., 1994, 2004; Zandt et al., 2004; Saleeby et al., 2012, 2013; Cecil et al., Nevada microplate primarily consists of Late Cretaceous granitic-tonalitic 2014). Beneath the Tulare sub-basin is the Isabella anomaly, a vertical fea- plutonic rocks in the Sierra Nevada batholith and Cretaceous–Quaternary ture with high-velocity seismic structure and an anomalous gravity signa- sedimentary rocks and deposits in the Great Valley (e.g., Saleeby et al., ture that is interpreted as the still-attached lithospheric root (Jones et al., 2008; Chapman et al., 2012). The microplate is a generally westward-tilted 1994; Zandt et al., 2004). Adjacent to the Isabella anomaly is the Kern arch. block, which has experienced both counterclockwise (ca. 36–24 Ma) and The Kern arch is a prominent dome-like structure along the mountain-basin clockwise (24 Ma to present) rotation due to the interaction between the Pa- transition that is interpreted to be a region experiencing uplift and exhu- cific and North American plates (Unruh, 1991; Unruh et al., 2003; McQuarrie mation caused by mantle upwelling to replace the recently detached litho- and Wernicke, 2005). It is bound to the east by the Sierra Nevada frontal spheric root. The delamination hinge trace shown in Figure 1 corresponds to fault zone, a normal fault system that has generated a high-relief eastern the surface trace of the hinge about which mantle lithosphere delamination escarpment. The Owens Valley fault system and Walker Lane are located has progressed to the present. Currently the hinge trace lies in the transition immediately to the east and accommodate up to 25% of the relative Pacific– zone between the Tulare sub-basin and (1) the Kern arch to the southeast North American plate motion (e.g., Dixon et al., 2000; Bennett et al., 2003; and (2) the Sierran foothills to the east and northeast (Saleeby et al., 2012). Frankel et al., 2007; Lee et al., 2009). The rest of the relative plate motion Ample stratigraphic data from drilling operations in the San Joaquin is taken up along the western margin of the microplate by the right-lateral Basin have been used to help constrain the geodynamic history of the San Andreas fault system, which includes the San Andreas, Hayward-Cala- southern Sierra Nevada region. Thermochronometric analyses of detrital veras, Rinconada, and San Gregorio–Hosgri faults (e.g., Dixon et al., 2000; apatite recovered from well core samples of Miocene sediments provide Argus and Gordon, 2001; Bennett et al., 2003). The southern boundary of constraints on the subsidence and exhumation history of the Kern arch. the Sierra Nevada microplate is defined by the left-lateral Garlock fault. Partially to completely reset (U-Th)/He ages and inverse thermokinematic Internal deformation within the southern Sierra Nevada is more com- modeling suggest that subsidence and sedimentation occurred between plex than in the rest of the microplate. South of ~36.0°N latitude, the Sierra 6 Ma and 1 Ma and was immediately followed by rapid exhumation (Cecil Nevada has a southward-sloping topographic gradient and local topo- et al., 2014). Focused exhumation within the Kern arch resulted in partition- graphic relief caused by Cenozoic displacement along faults of the southern ing of sedimentation in the southern San Joaquin Basin, where erosional Sierra Nevada and Kern Canyon fault systems (e.g., Wakabayashi and Saw- unroofing redistributed sediment to the west, south, and north (Saleeby yer, 2001; Clark et al., 2005; Mahéo et al., 2009). The development of this et al., 2012, 2013, 2016). North of the Kern arch, thermochronometric, geo- modern landscape is thought to have been preconditioned by gravitational morphologic, and geodetic constraints indicate that the Tulare sub-basin collapse of the southern Sierra Nevada during the Late Cretaceous (Chap- has experienced continuous subsidence to present (Stock et al., 2005; man et al., 2012). Structural, topographic, thermobarometric, and thermo- Mahéo et al., 2009; Saleeby et al., 2013, 2016; Cecil et al., 2014). The cause of chronometric data indicate a profound southward increase in tectonic and this subsidence is interpreted to be the result of a still-attached lithospheric erosional exhumation that is largely attributed to this Late Cretaceous tec- root (e.g., Saleeby et al., 2012, 2013).

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Southern Sierra Nevada Fault System STREAM PROFILES AND LANDSCAPE EVOLUTION

Internal deformation within the Sierra Nevada microplate is unique to Bedrock rivers function as a first-order control on how the landscape the southern Sierra Nevada fault system (35.0°N to 36.5°N latitude), a Neo- responds to tectonic and climatic processes, where the rate of channel inci- gene–Quaternary fault system comprising the N-S–trending, west-side-up sion into bedrock controls adjacent hillslope processes and thus total mass Breckenridge–Kern Canyon fault zone and NW-trending, west-side-down removal from a drainage basin (Whipple and Tucker, 1999; Snyder et al., normal faults that propagate into the basin (Fig. 1) (Mahéo et al., 2009). The 2000, 2003; Wobus et al., 2006; Kirby and Whipple, 2012). An inverse power- Breckenridge and Kern Canyon faults display evidence for normal displace- law relationship between drainage area and channel slope is expected for ment within the early Neogene and Quaternary and play an important role rivers in steady state (i.e., fully adjusted to the present tectonic and climatic in contributing to the westward tilt of the southern Sierra Nevada region conditions), resulting in a smooth concave-up profile (Hack, 1957; Whipple (Mahéo et al., 2009; Nadin and Saleeby, 2010). The NW-trending normal and Tucker, 1999; Snyder et al., 2000; Wobus et al., 2006). When a land- faults within the southern Sierra Nevada and Kern arch are interpreted to scape is in steady state, it is assumed that bedrock channel incision rates be in large part remobilized from Late Cretaceous basement shear zones, are in sync with rock uplift rates. If this is the case, channel lowering with having been reactivated with west-side-down normal displacement in re- respect to time is zero (dz/dt = 0) and is directly related to the competition sponse to the passage of the slab window and ensuing delamination of the between rock uplift (U) and erosion (E) rates (Whipple and Tucker, 1999; lithospheric root (Mahéo et al., 2009). Snyder et al., 2000): Prominent faults within the study area include the Breckenridge–Kern dz Canyon fault system and the West Breckenridge and Kern Gorge faults. The =−UE=−UKASm n, (1) dt Breckenridge–Kern Canyon fault system is a west-side-up normal fault that has controlled the position of the upper Kern River and ponded sediments where K is the erosion coefficient,A is drainage area, S is channel slope, and m to form the Isabella and Walker intermontane basins (Mahéo et al., 2009; and n are positive constants related to basin hydrology, hydraulic geometry, Nadin and Saleeby, 2010; Amos et al., 2010). The west-side-down West and erosion process. Breckenridge and Kern Gorge normal faults are interpreted to have accom- The erosion parameters in Equation 1 can be solved for the equilib- modated extensional deformation in response to the passage of the slab rium slope, S, and simplified to represent the power-law function of local window (Mahéo et al., 2009). channel slope and contributing drainage area (Snyder et al., 2000; Wobus et al., 2006):

−θ Geomorphology of the Southern Sierra Nevada Sk= sA ; (2)

1n The Kern River watershed traverses much of the southern Sierra Nevada. kUs = ()K ; (3) The headwaters are proximal to the tallest peaks of the Sierra Nevada and flow southward, draining the high-elevation, low-relief landscape of the θ=m n, (4) Kern Plateau. Downstream from Isabella Lake, the lower Kern River crosses

the Kern Canyon fault and switches to a southwest flow direction; similar to where ks is the steepness index and θ is the concavity index which describes the other major Sierran rivers to the north. Channel gradient along the lower the rate of change of slope as drainage area increases. The intrinsic concavity Kern River increases with increasing drainage area, contrary to what we index m/n can be considered equivalent to θ (Equation 4) only when chan- would expect from a channel in equilibrium with uplift rates (Whipple and nels are in equilibrium and encounter spatially uniform rock uplift, climate, Tucker, 1999). The lower Kern River continues to steepen as it flows through and bedrock properties (e.g., Snyder et al., 2000). For streams in equilibrium,

a narrow inner gorge (Kern River gorge) and traverses the Kern arch region Equation 2 typically yields a smooth concave-up channel profile. Bothθ and ks before rapidly shallowing as it debouches into the San Joaquin Basin. Trib- can be directly estimated by regression of slope and area data. utaries along the lower Kern River drain two prominent, perched, low-relief While this power-law relationship explains a channel system in steady topographic domains: (1) a high-elevation domain within the Breckenridge, state, deviations from a smooth concave-up form can be approached with Greenhorn, and Piute Mountains, and (2) a lower-elevation domain near the the aim to extract signals of changing boundary changes (Wobus et al., mountain front–basin transition. Because it traverses both the Kern arch and 2006). The form of channel adjustment to tectonic perturbation (i.e., an in- western flank of the southern Sierra Nevada, trunk and tributary longitudinal crease in uplift rate) is through channel steepening, and thus a change in channel profiles of the lower Kern River are uniquely suited to record land- concavity, and can give insight into transient versus adjusted landscape scape evolution potentially driven by lithospheric delamination. within a drainage network. For instance, a channel profile that displays two

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distinct reaches with differing ks and θ values can be interpreted as the fill pits in the DEM and generate flow direction and flow accumulation ras- river displaying a relict, steady-state form within the upstream reach and a ters for the study area. Pour points were selected for the Kern River trunk transient form along the downstream reach. The point that separates them, channel and tributary channel junctions that had flow accumulation values termed a knickpoint, migrates up the drainage network as the channel ad- of at least 20,000 cells (2 × 106 m2 drainage area). A model builder tool justs to the new set of boundary conditions (Snyder et al., 2000; Wobus was then developed in ArcGIS to generate watershed-specific folders, de- et al., 2006; Kirby and Whipple, 2012). lineate watershed boundaries, clip DEM and flow accumulation rasters for Channel adjustment through trunk channel steepening and resultant each watershed, and convert from raster to ASCII format. The resulting .txt erosional lowering establishes a knickpoint which then migrates upstream files were then converted to .mat format for stream profile analysis using (Crosby and Whipple, 2006; Wobus et al., 2006). Channel lowering on the MATLAB software. trunk channel serves as a local base-level fall for tributary channels. This produces a knickpoint that is initiated at the tributary junction and is translated upstream throughout the tributary watershed (Wobus et al., 2006). Stream Profile Analysis Although knickpoint migration rates are largely controlled by contributing drainage area, the vertical rate of knickpoint migration is constant, and A stream profiler tool (http://geomorphtools.geology.isu.edu/) was uti- therefore knickpoints located at similar elevations should reflect the same lized for stream profile analysis. The stream profiler tool integrates ArcGIS

transient topographic signature (Wobus et al., 2006). The position of knick- and MATLAB software to obtain θ, ks, and normalized steepness index ksn points within a fluvial network therefore allows for identification of relict values along river channel profiles through regression of slope-area data topography, remnants of landscapes that have not yet adjusted to the new extracted from a DEM (e.g., Wobus et al., 2006; Whipple et al., 2007). This

boundary conditions (Clark et al., 2005). In this study, we analyze the lower analysis requires specification of a reference concavityθ ref. While the cho-

Kern River trunk channel and its tributaries to constrain the evolution of the sen θref value can drastically change the absolute values of ksn within an

southern Sierra Nevada landscape during the Quaternary. individual watershed, the relative value of ksn between watersheds remains

consistent as long as the same θref is used. All analyses in this study uti-

lized a θref of 0.45 to allow for inter-watershed comparison and to facilitate

METHODOLOGY comparison with other studies (Kirby and Whipple, 2012). The units for ksn values (m0.9) originate from dimensional analysis of the slope versus drain-

Stream profile analyses of trunk and tributary channels of the lower Kern age area relationship and are tied to the value of θref (Wobus et al., 2006; River were conducted to help characterize the potential landscape response Ouimet et al., 2009). to delamination of a lithospheric root beneath the southern Sierra Nevada The stream profiler tool was run in two separate modes for the Kern region. Tributary channels were analyzed to constrain pulses of trunk chan- River and each of the lower Kern River tributary watersheds. The automatic

nel incision along the lower Kern River. These tributary channels allow us to ksn mode (batch profiler code) automatically calculatesk sn values through spatially and quantitatively identify how adjustment of the lower Kern River regression of slope-area data within a window that moves along the chan- is translated throughout the drainage basin. nel network. These calculations are performed along all channels within the channel network that drain a user-specified minimum critical area. In this study, we used a minimum critical area of 1 × 106 m2 for analysis of Data Collection and Preparation the entire Kern River watershed and each of the lower Kern River tributary

watersheds. Maximum ksn and watershed averaged ksn values were deter- We used digital elevation model (DEM) data obtained from the U.S. Geo- mined for each of the watersheds analyzed.

logical Survey (USGS) 3D Elevation Program (3DEP) (www.nationalmap. The manual ksn mode calculates ksn and θ values through regression of gov). Multiple National Elevation Dataset (NED) 1 × 1 degree tiles with a slope-area data between user-specified regression bounds along individ- resolution of 1/3 arc-second were mosaicked to create a DEM that covered ual channels. This mode also allows for the identification and marking of the entire Kern River watershed. This raster data set was then projected knickpoint locations for later importation into ArcGIS. Regression bounds using a North American Datum of 1983 geographic coordinate system and and knickpoint locations were selected based on visual inspection of the a Universal Transverse Mercator Zone 11S projected coordinate system. channel longitudinal profile and plots of channel gradient versus drainage The resulting raster was resampled to produce an equidimensional DEM area and channel gradient versus distance from the mouth. To identify the with a resolution of 10 m. The DEM was then prepared for stream profile presence of relict topography, we chose to focus on prominent knickpoints analysis following the procedures outlined below and detailed by Whipple that separate channel reaches with markedly different characteristics. et al. (2007). The hydrology toolbox within ArcGIS software was used to These knickpoint locations were marked, and the bounds of the upstream

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reach were selected for regression. This analysis was performed on the The knickpoint located between the Kern Canyon and upper gorge reaches

trunk channel of the Kern River watershed and each of the lower Kern River marks the upstream extent of consistently high ksn values obtained along tributary watersheds. the lower Kern River (Figs. 3 and 4). The results from manual regression of slope-area data along each section of the Kern River are presented in Table 2. These results were used to reconstruct channel longitudinal profiles and de- Channel Profile Reconstructions termine projected channel elevations at the mouth of the Kern River water

shed. All errors for θ, ksn, and projected channel elevations are presented as Trunk channel profiles of the Kern River and lower Kern River tributaries ±2σ. We were unable to obtain a reasonable fit to the longitudinal profile

were reconstructed using methods similar to those employed by Schoen- of the upper Kern River using the ksn value obtained along the High Sierra bohm et al. (2004) and Clark et al. (2005). We used the relationship between reach. Regression of slope-area data along the Kern Canyon reach resulted 0.9 contributing drainage area and distance from the mouth of the current in a θ of 0.54 ± 0.20 and a ksn of 147.5 ± 3.7 m . These ksn data produced a

channel profile, along with aθ ref of 0.45 and the ksn value obtained for the channel profile reconstruction that was a good fit for the Kern Canyon reach channel reach immediately upstream of each prominent knickpoint, to cal- and resulted in a projected channel elevation at the mouth of the Kern River culate the shape of the reconstructed channel profile. The reconstructed watershed of 442 ± 3 m. This reconstructed channel profile also provided a profile was then adjusted to match the elevation of the prominent knick- more reasonable fit to the High Sierra for the upstream Kern Canyon reach.

point. This method allowed us to assess the fit of the reconstructed profile Manual ksn analysis of the upper gorge and lower gorge reaches resulted in 0.9 0.9 along the entirety of the current channel profile and was used to determine ksn values of 349.4 ± 36.0 m and 574.7 ± 42.9 m respectively. The recon- the elevation of the reconstructed trunk channel profile at the mouth of its structed channel profile for the upper gorge reach has an elevation of 285 ± respective watershed. The resulting difference in elevation of the recon- 12 m at the mouth of the Kern River watershed. structed profile and modern profile represents the minimum total incision of the Kern River associated with each knickpoint (Clark et al., 2005; Kirby and Whipple, 2012). Lower Kern River Tributaries

Forty-eight (48) tributary watersheds that feed the lower Kern River RESULTS were analyzed to elucidate the dynamic response of the landscape to the downstream channel steepening observed between Isabella Lake and the Kern River southern San Joaquin Basin (Table 1; Fig. 4). These tributary watersheds drain the western half of the southern Sierra Nevada with watershed relief 2 The Kern River watershed drains an area of 5957 km with elevations ranging between 306 m and 1853 m. Automatic ksn analyses yielded min- 0.9 0.9 between 210 m and 4412 m. Automatic ksn analysis of the entire Kern River imum and maximum ksn values that range from 0 m to 591 m , respec- 0.9 watershed yielded a maximum ksn value of 3125 m with an average ksn tively. Watershed-averaged ksn values for individual tributary watersheds 0.9 0.9 0.9 value of 93 ± 1 m (2σx̅ ; twice the standard error of the mean). Analyzed on range from 12 ± 1 m (2σx̅ ) to 142 ± 8 m (2σx̅ ). Manual ksn analyses of

its own, the trunk channel of the Kern River yielded a maximum ksn value of tributary trunk channels resulted in the identification of 72 prominent knick- 0.9 0.9 1062 m with an average ksn of 185 ± 9 m (2σx̅ ) (Table 1). High ksn values points (Table 2; Fig. 4). All errors for θ, ksn, and projected tributary channel within the lower Kern River watershed are concentrated both along the trunk elevations are presented as ±2σ. Eight tributary trunk channels had no dis- channel and on tributary channels located near the mouth of the Kern River cernible knickpoints. Tributary knickpoint elevations range between 641 m

gorge (Fig. 2). Tributary channels that drain the high Sierra also have high ksn and 2094 m, with the lower-elevation tributary knickpoints located along values upstream of their junctions with the upper Kern River (Fig. 2). the lower gorge and upper gorge reaches. Higher-elevation tributary knick- The longitudinal channel profile of the Kern River displays a con- points are primarily located along the Kern Canyon reach. The wide range

cave-up profile upstream of Isabella Lake, where the river switches from a of θ and ksn values obtained along tributary channel segments immediately south-trending to a southwest-trending flow direction. This switch in flow upstream of each tributary knickpoint are presented in Table 2. Channel direction is concomitant with channel steepening that yields a convex-up profile reconstructions, based on these data, were created and used to de-

channel profile downstream of Isabella Lake (Figs. 2 and 3). Manualk sn termine the elevation of each reconstructed profile at the modern channel’s analysis of the Kern River trunk channel resulted in the identification of three confluence with the lower Kern River. This information was then used to prominent knickpoints at elevations of 1891 m, 556 m, and 404 m (Table 2). estimate the magnitude of river incision that is associated with each tribu- These knickpoints were used to separate the Kern River into the High Sierra, tary knickpoint (Table 2). Our estimates of minimum total incision along the Kern Canyon, upper gorge, and lower gorge reaches respectively (Fig. 3). lower Kern River range between 27 ± 1 m and 1298 ± 17 m.

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TABLE 1. WATERSHED LOCATION, INFORMATION, AND RESULTS OF AUTOMATIC NORMALIZED STEEPNESS INDEX (ksn) ANALYSIS, KERN RIVER WATERSHED, SOUTHERN SIERRA NEVADA, CALIFORNIA

Watershed pour point location Watershed informationResults of automatic ksn analysis* Distance from Maximum Minimum Mean elevation Watershed Maximum Minimum Mean

mouth Easting Northing Elevation Area elevation elevation ± 2σx̅ relief ksn ksn ksn ± 2σx̅ Watershed ID (m) (m) (m) (m) (km2) (m) (m) (m)† (m) (m0.9) (m0.9) (m0.9)† Kern River 0 337021 3923365 211 5957.2 4412 2112109 ± 0.2 4201 1062 0185 ± 9 Tr ibutary watersheds KR_003342 3342 338841 3925645 288 1.7 878288 734 ± 2 590256 16 116 ± 14 KR_004396 4396 339811 3925695 311 2.7 1076 311891 ± 1 765378 15 117 ± 14 KR_005807 5807 340801 3926435 361 1.0 968361 780 ± 2 608308 23 87 ± 11 KR_006250 6250 341151 3926645 374 5.3 1089 374920 ± 1 716488 087 ± 12 KR_009478 9478 342781 3927655 422 5.0 1261 422848 ± 1 839194 17 77 ± 3 KR_010826 10,826 343491 3927235 434 23.3 1925 4341094 ± 1 1491 3781375 ± 2 KR_012153 12,153 344531 3926875 447 3.5 1311 447963 ± 2 864278 094 ± 8 KR_013582 13,582 344741 3928055 469 23.2 2295 4691592 ± 2 1826 3970120 ± 5 KR_014626 14,626 344891 3928965 491 2.9 1367 491999 ± 2 876236 0119 ± 9 KR_016699 16,699 346361 3929875 530 8.8 1856 5301416 ± 2 1327 3730142 ± 8 KR_017260 17,260 346581 3930345 533 1.4 1420 5331038 ± 3 887228 0104 ± 9 KR_017270 17,270 346571 3930365 534 2.0 1366 534975 ± 3 832166 28 96 ± 6 KR_018194 18,194 346671 3931225 551 2.1 1449 5511109 ± 3 898263 17 104 ± 9 KR_020192 20,192 348051 3932295 568 2.1 1614 5681102 ± 3 1046 199099 ± 8 KR_020529 20,529 348121 3932595 573 4.2 1492 5731095 ± 2 919208 23 91 ± 4 KR_021309 21,309 348601 3933045 580 1.1 1367 5801006 ± 3 787137 32 83 ± 4 KR_021552 21,552 348821 3933025 582 2.4 1673 5821130 ± 4 1090 16628108 ± 4 KR_021771 21,771 348981 3933105 581 3.4 1491 5811119 ± 2 910182 14 97 ± 5 KR_022838 22,838 349831 3933335 587 5.0 1575 5871125 ± 2 988184 30 102 ± 2 KR_026057 26,057 351371 3933665 601 4.7 1823 6011217 ± 3 1222 16844121 ± 3 KR_027245 27,245 352301 3934035 610 4.9 1601 6101095 ± 2 991125 25 89 ± 2 KR_027585 27,585 352611 3934095 613 1.5 1430 613979 ± 4 817179 27 74 ± 5 KR_028380 28,380 353191 3934535 615 36.4 2308 6151476 ± 1 1693 2880121 ± 2 KR_029503 29,503 353621 3935395 620 4.9 1665 6201065 ± 2 1045 1312686 ± 2 KR_030711 30,711 354241 3935985 625 9.2 1711 6251202 ± 2 1086 23332107 ± 4 KR_032382 32,382 355661 3935595 633 1.0 1395 6331026 ± 3 762170 43 97 ± 6 KR_033175 33,175 356331 3935775 641 1.6 1918 6411358 ± 5 1277 23037134 ± 7 KR_033371 33,371 356481 3935905 640 3.3 1944 6401352 ± 3 1304 20030135 ± 4 KR_034567 34,567 356521 3936975 647 22.3 1991 6471562 ± 1 1344 591084 ± 4 KR_034807 34,807 356631 3937155 649 2.3 1588 6491178 ± 3 938173 22 99 ± 5 KR_035932 35,932 357601 3937015 659 1.3 1698 6591221 ± 4 1039 19233113 ± 5 KR_037206 37,206 358391 3937775 666 2.2 1589 6661198 ± 3 923257 27 109 ± 9 KR_037607 37,607 358701 3937985 667 1.9 1501 6671027 ± 3 834167 27 81 ± 3 KR_037968 37,968 358901 3938255 670 13.2 1981 6701388 ± 2 1310 3460108 ± 4 KR_041465 41,465 361431 3937845 685 124.3 2538 6851495 ± 1 1853 3591493 ± 1 KR_041815 41,815 361691 3937915 688 1.3 1359 6881034 ± 3 67195075 ± 3 KR_044363 44,363 361371 3939735 697 1.7 1233 697885 ± 2 536752849 ± 2 KR_044668 44,668 361561 3939915 699 17.4 1964 6991234 ± 1 1265 2461387 ± 2 KR_045312 45,312 362101 3940005 702 1.1 1176 702886 ± 2 47468045 ± 2 KR_046449 46,449 362451 3940815 707 1.5 1100 707881 ± 1 39351033 ± 1 KR_048637 48,637 363931 3940555 719 1.5 1282 719915 ± 2 563813147 ± 2 KR_049313 49,313 364411 3940635 721 42.6 2389 7211429 ± 1 1669 2491694 ± 1 KR_049703 49,703 364521 3940965 723 1.1 1028 723798 ± 1 30622012 ± 1 (continued)

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1107 Research Paper

TABLE 1. WATERSHED LOCATION, INFORMATION, AND RESULTS OF AUTOMATIC NORMALIZED STEEPNESS INDEX (ksn) ANALYSIS, KERN RIVER WATERSHED, SOUTHERN SIERRA NEVADA, CALIFORNIA (continued)

Watershed pour point location Watershed informationResults of automatic ksn analysis* Distance from Maximum Minimum Mean elevation Watershed Maximum Minimum Mean

mouth Easting Northing Elevation Area elevation elevation ± 2σx̅ relief ksn ksn ksn ± 2σx̅ Watershed ID (m) (m) (m) (m) (km2) (m) (m) (m)† (m) (m0.9) (m0.9) (m0.9)†

Tr ibutary watersheds (continued) KR_050912 50,912 365011 3941875 727 92.4 2539 7271622 ± 1 1812 385092 ± 1 KR_051494 51,494 365161 3942395 730 5.6 1615 730958 ± 2 884109 050 ± 3 KR_052141 52,141 365011 3942955 732 1.1 1061 732856 ± 1 329991731 ± 3 KR_053394 53,394 364971 3944065 744 10.2 1779 7411181 ± 2 1038 283084 ± 3 KR_055409 55,409 366051 3945615 794 47.5 2164 7531507 ± 1 1410 5281473 ± 2 Note: All location information is based on a North American Datum of 1983 geographic coordinate system and a Universal Transverse Mercator Zone 11S projected coordinate system. The normalized steepness index 0.9 (ksn) is a dimensional coefficient with units that depend on the reference concavity (θref) (Wobus et al., 2006). A reference concavity (θref) of 0.45 results in normalized steepness index (ksn) values with units of m . *A reference concavity (θref) of 0.45 was used for all automatic ksn analyses. † Uncertainties for mean elevation and mean ksn values are presented as twice the standard error of the mean (± 2σx̅).

DISCUSSION River. Numerous studies interpret this extensive low-relief surface to be relict topography resulting from a period of slow exhumation that followed We identify two domains of low-relief topography that have been incised the end of Sierran arc magmatism in the Late Cretaceous (e.g., Clark et al., by the lower Kern River and its tributaries. These domains are interpreted 2005; Cecil et al., 2006; Pelletier, 2007; Mahéo et al., 2009). We consider the as relict landscapes that formed under previous steady-state conditions Breckenridge, Piute, and Greenhorn Mountains to contain remnants of the between rock uplift and erosion. These domains function as markers from Kern Plateau relict landscape that have been dissected and differentially which we constrain the magnitude and spatial distribution of incision along tilted by the Breckenridge and Kern Canyon normal faults (Mahéo et al., the lower Kern River. We compare our estimates of incision with existing 2009). These remnants have since been incised by the lower Kern River and models of surface uplift and exhumation within the southern Sierra Nevada. its tributaries.

Relict Landscape of the Kern Plateau Relict Landscape of the Kern River Gorge

High-elevation, low-relief topography is observed to the north and south A second domain of low-relief topography is identified along the west- of the Kern River in the Breckenridge, Piute, and Greenhorn Mountains. ern flank of the southern Sierra Nevada (Figs. 4 and 6). Tributary watersheds Maximum relief within this domain is 1264 m with a mean elevation of located near the mouth of the Kern River gorge drain this surface and have 1700 m. A concentration of tributary knickpoints located at an elevation elevations that range between 288 m and 1089 m. Stream profile analyses

of ~1275 m separates this landscape from the steep slopes found within the on tributaries that drain only this surface yielded ksn values that range from 0.9 0.9 0.9 Kern Canyon (Fig. 5). The elevations of all tributary knickpoints within this 19.1 ± 0.8 m to 70.8 ± 1.2 m with a mean value of 32.4 ± 14.3 m (2σx̅ ).

domain range between 1275 m and 2094 m with most knickpoints located The average θ value obtained along these tributaries is 0.36 ± 0.11 (2σx̅ ). more than ~650 m above the present position of the Kern River. Higher- This low-relief domain is spatially correlated with the steepest reaches of elevation tributary knickpoints are typically located farther from the river the Kern River. Knickpoints within tributary watersheds downstream of the (Fig. 4). Stream profile analyses on tributary reaches immediately above upper gorge knickpoint mark the abrupt transition between this low-relief 0.9 each tributary knickpoint yielded ksn values that range from 16.2 ± 4.5 m landscape and the steep channels and hillslopes within the Kern River 0.9 0.9 to 135.2 ± 3.1 m with a mean value of 47.0 ± 9.9 m (2σx̅ ). Concavity index gorge. Tributary knickpoint elevations decrease from 891 m to 676 m over a (θ) values determined along these reaches have a mean value of 0.53 ± 0.14 distance of 12.9 km upstream toward the upper gorge knickpoint (Fig. 5). An

(2σx̅ ). Clark et al. (2005) identified the Kern Plateau as a similar high-ele- additional two prominent tributary knickpoints were observed at elevations vation low-relief relict landscape drained by tributaries of the upper Kern below the low-relief landscape and within the walls of the Kern River gorge.

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1108 Research Paper

i e r r a O N w e v e a n d s

a V

Lower elevation limit fr a o l l n e Normalized steepness index (k ) of major glaciers ta y sn l f fa a u u l < 75 lt t

z z o o n 75–150 e n e

150–225 θref = 0.45 225–300 > 300 Kern River mouth Kern River knickpoints Trunk channel Kern River Tributary channels Figure 2. Map showing the results of auto matic normalized steepness index (ksn) analysis of the entire Kern River watershed as well as major late Cenozoic faults. The minimum critical area chosen for this analysis was 1 × 106 m2. The reference con-

cavity (θref) for all analyses was 0.45. Low

values of ksn are represented by cool colors

(blues), while high ksn values are repre-

sented by warm color (reds). The trunk

e channel of the Kern River is represented by

r n

a slightly thicker line weight to highlight

C a

n ksn values along it. Prominent knickpoints y

o on the trunk channel are marked with

f a white dots (see text for discussion). The

major late Cenozoic faults are described in

n Figure 1. The position of the Isabella Dam

e and Democrat Dam are marked for reference. The lower elevation limit of major glaciers is interpreted from Moore and Mack (2008) and Moore and Moring (2013).

Area of Figure 4

Isabella Dam Isabella Lake Democrat Dam

e t s l t B u a re f K c e ke e rn n g r d G id i o g r r e n ge fa e f ul k au t c lt e r

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1109 Research Paper

A 4000 Kern Canyon High Sierra

3000 Lower gorge Upper gorge

Figs 4–7 Lower elevation limit 2000 of major glaciers Democrat Dam Isabella Dam Figure 3. Results of manual normalized Elevation (m)

steepness index (ksn) analysis along the 1000 Kern River knickpoints trunk channel of the Kern River. (A) Longi- tudinal channel profile (purple line) showing the location of prominent knickpoints (white circles) that are used to delineate 0 separate reaches along the Kern River. 050100 150 200 250 The blue lines represent the reconstructed Distance from mouth (km) channel profile that corresponds to the best fit of slope-area data shown in B. B The red box highlights the region that is the focus of most of this study. (B) Plot 100 of gradient versus drainage area. The fits of manually selected regression analysis θ = 0.74 ± 0.14 are represented by the blue lines with the k = 186.3 ± 6.7 sn θ = 0.54 ± 0.2 corresponding concavity index (θ) and -1 k = 147.5 ± 3.7 values located immediately above. All 10 sn ksn analyses were conducted using a refer-

ence concavity (θref) of 0.45.

10-2 Gradient

-3 10 Kern River knickpoints

θref = 0.45 10-4 103 104 105 106 107 108 109 1010 1011 Drainage area (m2)

These tributary knickpoints decrease in elevation from 704 m to 641 m preted to have stripped off and redistributed upwards of 1000–1800 m over an upstream distance of 2.5 km toward the lower gorge knickpoint. of Tertiary sediments from this surface (Cecil et al., 2014). Mahéo et al. We interpret this low-relief topographic surface as a remnant of a relict (2009) interpreted the Eocene nonconformity surface as a continua- landscape related to the high-elevation low-relief relict landscape of the tion of the high-elevation low-relief relict landscape. Differences in the Kern Plateau. Several studies have interpreted this surface to be an ex- present-day elevation of these surfaces are explained by late Cenozoic humed early Tertiary nonconformity that extends from the foothills of offset along normal faults of the southern Sierra Nevada fault system the Sierra Nevada into the subsurface in the San Joaquin Basin (e.g., (Mahéo et al., 2009; Saleeby et al., 2016). The Kern Gorge, West Brecken- Saleeby et al., 2013, and references therein; Cecil et al., 2014; Sousa ridge, and Breckenridge faults offset the relict surface within the study et al., 2017). In the Kern arch region, surface uplift and erosion are inter- area (Mahéo et al., 2009).

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1110 Research Paper

TABLE 2. RESULTS OF KNICKPOINT IDENTIFICATION AND CHANNEL RECONSTRUCTION, KERN RIVER WATERSHED, SOUTHERN SIERRA NEVADA, CALIFORNIA Knickpoint identification Channel reconstructions* Distance from Elevation above Elevation at Estimated † § § mouth Easting Northing Elevation mouth ksn ± 2σ mouth ± 2σ incision ± 2σ Watershed ID (m) (m) (m) (m) (m) θ ± 2σ† (m0.9) (m) (m)

Kern River knickpoints KR_Lower gorge reach 0 337021 3923365 2110134.38 ± 361.94 574.7 ± 42.9no datano data KR_Upper gorge reach 8509 341571 3927915 404192 48.62 ± 52.88 349.4 ± 36.0285 ± 1274 ± 12 KR_Kern Canyon reach 19,235 346651 3931205 556345 0.54 ± 0.20 147.5 ± 3.7 442 ± 3 231 ± 3 KR_High Sierra reach 159,363 373791 4018365 1891 1680 0.74 ± 0.14 186.3 ± 6.7 no datano data Lower gorge tributary knickpoints KR_003342 1174 338671 3926725 704415 0.26 ± 0.09 21.9 ± 1.6654 ± 4 366 ± 4 KR_005807 713 340271 3926845 641280 0.38 ± 0.16 44.4 ± 1.3577 ± 2 216 ± 2 Upper gorge tributary knickpoints KR_004396 1292 340101 3924585 797486 1.22 ± 1.05 47.7 ± 5.7713 ± 10402 ± 10 KR_004396 2510 340951 3923845 891580 0.35 ± 0.11 19.1 ± 0.8817 ± 3 506 ± 3 KR_005807 2406 340101 3928225 860500 0.33 ± 0.08 21.2 ± 2.1715 ± 14354 ± 14 KR_006250 1587 341941 3925775 833459 0.41 ± 0.13 23.5 ± 1.1796 ± 2 422 ± 2 KR_009478 2611 341741 3929115 779358 0.16 ± 0.03 25.9 ± 2.2689 ± 8 267 ± 8 KR_010826 2852 344401 3925665 766332 0.65 ± 0.20 70.8 ± 1.2665 ± 2 231 ± 2 KR_012153 962 345041 3926375 705259 0.29 ± 0.64 123.2 ± 1.8 566 ± 2 120 ± 2 KR_016699 386 346651 3929665 676147 11.09 ± 6.34301.2 ± 12.8589 ± 4 59 ± 4 KR_017260 668 347151 3930465 795262 1.49 ± 5.57 155.6 ± 5.0 599 ± 6 66 ± 6 KR_017270 627 346131 3930705 700166 1.11 ± 1.06 128.6 ± 8.1 573 ± 8 39 ± 8 Kern Canyon tributary knickpoints KR_012153 2537 346371 3926015 937490 0.35 ± 0.11 36.6 ± 1.3810 ± 5 364 ± 5 KR_014626 2227 344161 3930655 1103 6120.28 ± 0.07 28.9 ± 1.8980 ± 8 489 ± 8 KR_016699 1451 347581 3929475 932402 8.91 ± 3.15 193.6 ± 30.2716 ± 34186 ± 34 KR_016699 2565 348551 3929385 1153 6232.19 ± 0.92 187.8 ± 12.8764 ± 27235 ± 27 KR_017260 1248 347651 3930605 952419 0.75 ± 0.09 57.3 ± 2.6810 ± 7 276 ± 7 KR_017270 1252 345621 3930855 870336 0.41 ± 0.22 86.6 ± 1.3679 ± 3 145 ± 3 KR_017270 2137 345191 3931485 1049 5150.63 ± 0.09 36.1 ± 1.5886 ± 7 352 ± 7 KR_018194 1213 345721 3931565 975424 2.44 ± 1.62 68.9 ± 3.8844 ± 7 293 ± 7 KR_018194 2168 345091 3932035 1133 5821.11 ± 0.60 45.1 ± 3.3950 ± 14399 ± 14 KR_020192 466 348411 3932185 682114 no datano datano datano data KR_020192 1305 348551 3931525 936369 0.80 ± 0.14 68.4 ± 5.1777 ± 12209 ± 12 KR_020192 1892 348981 3931215 1101 5330.59 ± 0.10 56.4 ± 1.3835 ± 6 267 ± 6 KR_020529 441 347761 3932775 654810.34 ± 0.03 115.0 ± 2.0 600 ± 1 27 ± 1 KR_020529 3007 345981 3933665 1275 7020.23 ± 0.08 30.8 ± 1.81074 ± 12501 ± 12 KR_021309 1366 347661 3933825 921341 0.27 ± 0.07 54.5 ± 2.1759 ± 6 179 ± 6 KR_021771 2638 347511 3934905 1053 4720.44 ± 0.15 77.0 ± 1.7 767 ± 6 186 ± 6 KR_022838 2768 348871 3935475 1006 4190.09 ± 0.04 77.3 ± 3.9 739 ± 13151 ± 13 KR_027585 2187 353201 3932395 1049 4360.82 ± 0.15 54.5 ± 2.7808 ± 12194 ± 12 KR_030711 2107 353901 3937585 873248 1.32 ± 0.20 125.4 ± 9.2 663 ± 1638 ± 16 KR_032382 547 355691 3935105 807174 1.34 ± 0.43 70.9 ± 5.8721 ± 7 88 ± 7 KR_033175 1085 356881 3934975 1029 3890.68 ± 0.25 117.1 ± 2.0 813 ± 4 172 ± 4 KR_034567 1811 355251 3937745 1026 3795.88 ± 7.32 230.6 ± 15.0818 ± 14171 ± 14 KR_034807 1595 355811 3938315 1077 4283.97 ± 1.84 101.7 ± 11.9780 ± 35131 ± 35 KR_035932 882 357721 3936235 942283 2.77 ± 3.09 128.3 ± 10.8731 ± 1871 ± 18 (continued)

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1111 Research Paper

TABLE 2. RESULTS OF KNICKPOINT IDENTIFICATION AND CHANNEL RECONSTRUCTION, KERN RIVER WATERSHED, SOUTHERN SIERRA NEVADA, CALIFORNIA (continued) Knickpoint identification Channel reconstructions* Distance from Elevation above Elevation at Estimated † § § mouth Easting Northing Elevation mouth ksn ± 2σ mouth ± 2σ incision ± 2σ Watershed ID (m) (m) (m) (m) (m) θ ± 2σ† (m0.9) (m) (m)

Kern Canyon tributary knickpoints (continued) KR_035932 1495 358071 3935825 1098 4391.81 ± 0.40 90.9 ± 4.6829 ± 14169 ± 14 KR_037206 749 358701 3937185 930264 0.72 ± 0.11 84.9 ± 2.8839 ± 3 173 ± 3 KR_037607 2119 357081 3939005 1086 4190.22 ± 0.14 70.8 ± 3.5771 ± 16104 ± 16 KR_037968 2810 358751 3940545 1180 5103.25 ± 0.74 115.0 ± 17.0907 ± 40236 ± 40 KR_045312 1691 363041 3939175 1005 3030.59 ± 0.19 29.4 ± 1.6849 ± 9 147 ± 9 KR_046449 1108 361981 3941475 805980.52 ± 0.17 28.0 ± 0.9746 ± 2 39 ± 2 KR_046449 2703 362891 3942325 966259 0.09 ± 0.20 12.9 ± 3.7868 ± 28161 ± 28 KR_048637 2182 363651 3938735 975257 no datano datano datano data KR_052141 367 364681 3942885 789570.74 ± 0.23 19.9 ± 2.5775 ± 2 42 ± 2 Kern Plateau tributary knickpoints KR_013582 9126 352661 3927355 1674 1205 0.57 ± 0.09 55.1 ± 2.71388 ± 14919 ± 14 KR_013582 13,188 355531 3925535 2094 1625 0.39 ± 0.22 26.6 ± 1.41767 ± 171298 ± 17 KR_016699 4519 350021 3929585 1485 9550.45 ± 0.13 42.4 ± 1.31314 ± 5 785 ± 5 KR_018194 3217 344611 3932805 1291 7400.78 ± 0.27 18.5 ± 3.11153 ± 23602 ± 23 KR_020529 3007 345981 3933665 1275 7020.23 ± 0.08 30.8 ± 1.81074 ± 12501 ± 12 KR_021552 2969 350411 3931345 1436 8540.58 ± 0.73 47.9 ± 2.41069 ± 19486 ± 19 KR_021771 4031 346991 3935895 1392 8110.33 ± 0.29 15.8 ± 0.51247 ± 4 666 ± 4 KR_027585 3182 353281 3931525 1317 7040.14 ± 0.06 22.6 ± 3.31110 ± 31496 ± 31 KR_028380 11,754 356431 3926705 1966 1351 0.65 ± 0.31 39.4 ± 3.41658 ± 271043 ± 27 KR_030711 4727 352471 3939455 1328 7030.31 ± 0.07 44.5 ± 1.71109 ± 8 484 ± 8 KR_033175 2087 357461 3934325 1307 6660.29 ± 0.06 86.8 ± 2.6950 ± 11309 ± 11 KR_034567 5061 354501 3940035 1384 7370.48 ± 1.10 62.1 ± 3.81219 ± 10572 ± 10 KR_034567 9203 355851 3942285 1625 9780.62 ± 0.39 36.5 ± 2.21410 ± 13764 ± 13 KR_034567 12,474 355451 3944975 1824 1177 0.67 ± 0.22 16.2 ± 4.51659 ± 451013 ± 45 KR_034807 2379 355801 3939005 1285 6361.91 ± 0.68 55.1 ± 8.71015 ± 43365 ± 43 KR_035932 2437 358781 3935345 1370 7110.59 ± 0.48 58.4 ± 1.91031 ± 11372 ± 11 KR_037607 2929 356441 3939355 1366 6990.20 ± 0.08 24.0 ± 2.51156 ± 22489 ± 22 KR_037968 5866 357541 3942625 1606 9360.95 ± 0.32 45.3 ± 4.11345 ± 23674 ± 23 KR_037968 8211 356681 3944475 1815 1145 0.31 ± 0.19 31.0 ± 2.21507 ± 22837 ± 22 KR_041465 17,050 358921 3927915 1432 7470.64 ± 0.15 69.1 ± 1.8879 ± 15194 ± 15 KR_044668 6063 358611 3943885 1418 7190.77 ± 0.22 71.5 ± 5.01031 ± 27332 ± 27 KR_044668 7230 357851 3944565 1693 9940.31 ± 0.27 51.5 ± 2.81239 ± 24540 ± 24 KR_049313 9418 369731 3935115 1439 7180.47 ± 0.17 135.2 ± 3.1 798 ± 1577 ± 15 KR_049313 13,781 371831 3932095 2048 1327 0.69 ± 0.06 26.5 ± 1.81791 ± 181070 ± 18 KR_050912 21,888 374971 3928875 1960 1233 0.26 ± 0.15 70.8 ± 2.31373 ± 19646 ± 19 KR_053394 4178 362211 3945395 1337 5930.03 ± 0.15 31.1 ± 3.11185 ± 15441 ± 15 KR_055409 6781 362361 3947835 1333 5390.68 ± 0.05 55.1 ± 2.91192 ± 8 398 ± 8 Note: All location information is based on a North American Datum of 1983 geographic coordinate system and a Universal Transverse Mercator Zone 11S projected coordinate system.

*Data for channel profile reconstructions were determined using a reference concavity (θref) of 0.45. † Values of concavity index (θ) and normalized steepness index (ksn) were obtained through manual regression of slope-area data along the channel reach located immediately above the respective knickpoint. The normalized steepness index (ksn) is a dimensional coefficient with units that depend on the reference concavity (θref) (Wobus et al., 2006). A reference concavity (θref) of 0.45 results in 0.9 normalized steepness index (ksn) values with units of m . § Errors on reconstructed profile elevations and incision estimates are based on the propagation of uncertainty in the ksn values (± 2σ) used to project the reconstructed channels to the tributary junction.

GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1112 Research Paper

e n o z

Normalized steepness index (ksn) lt Isabella Dam u a f < 75 n o y 75–150 n Figure 4. Map showing stream profile a C analysis results obtained along the lower θ = 0.45 150–225 ref n Isabella Lake r Kern River and its tributary watersheds. e 225–300 K Automatic normalized steepness index (ksn) > 300 analyses are shown for each of the tributary watersheds. A reference concavity Kern River knickpoints (θref) of 0.45 was used for all analyses. The Tributary knickpoints location of tributary knickpoints identified

Kern Plateau during manual ksn analysis is shown. Knick- Kern Canyon points that define the boundary between the relict landscape of the Kern Plateau Upper gorge and the Kern Canyon are shown as green Lower gorge circles; knickpoints that are within the Kern Canyon domain are shown as black circles; W e knickpoints that define the Kern Gorge s t B Democrat Dam domain are shown as yellow circles, while re c k those located within the walls of the gorge e n are shown as orange circles. Knickpoints r t id l g u on the Kern River are shown as white cir- e a f fa u e cles. The low-relief upland surface (green) lt g d and exhumed early Tertiary nonconformity i r n surface (green patterned) are after Saleeby e

k c et al. (2016). Select faults of the southern

e r Sierra Nevada fault system (thick black B lines) are after Mahéo et al. (2009) and Saleeby et al. (2016).

K er n Go rge fa ult

Incision of the Kern River minimum total incision determined by Clark et al. (2005) along the upper Kern River. Our data correspond to a maximum long-term incision rate Incision estimates derived from the difference in elevation of recon- of ~0.07 mm/yr. This is similar to the results of thermochronometric age- structed tributary channel profiles and their confluence with the lower elevation measurements that yielded long-term erosion rates of 0.04–0.06 Kern River reveal several discrete periods of incision. The first period of mm/yr from 73 to 32 Ma in the Kern and Kings River low-relief regions incision is related to departure from the slow and steady erosional degra (Clark et al., 2005). Cosmogenic radionuclide–derived basin-averaged ero- dation of the high-elevation relict landscape of the Kern Plateau (e.g., Clark sion rates of the northern Sierra low-relief surfaces also range from 0.015 et al., 2005; Cecil et al., 2006; Pelletier, 2007). This period is loosely con- to 0.075 mm/yr (Riebe et al., 2001). If we consider only incision that oc- strained to have initiated ca. 20 Ma in response to activation of the south- curred prior to formation of the present-day Kern River Canyon, the lower ern Sierra Nevada fault system (Mahéo et al., 2009; Saleeby et al., 2016). Kern River is likely to have incised ≥1000 m into the relict landscape of Reconstructed channel profiles from the relict landscape yielded minimum the Kern Plateau during this period (Fig. 7). This estimate, however, does total incision estimates that range between 77 ± 15 m and 1298 ± 17 m not account for any Tertiary sediments that may have blanketed the relict (Fig. 7). These incision estimates are similar to the 360 m to 1360 m of landscape (Mahéo et al., 2009; Saleeby et al., 2016).

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2500 Kern River knickpoints Tributary knickpoints Kern Plateau 2000 Kern Canyon Figure 5. Identification of tributary knick- Upper gorge points in the lower Kern River watershed. Lower gorge (A) Longitudinal profile (solid black line) of the lower Kern River that shows the dis- 1500 tribution of tributary knickpoints within identified landscape domains. Red dashed

st Breckenridge fault lines show the reconstructed channel profiles using the results of manual normal- We

1000 ized steepness index (ksn) analysis of the Elevation (m) Kern River. A sub-horizontal black dashed line reflects the approximate boundary between the high-elevation low-relief land- 500 scape associated with the Kern Plateau Isabella Dam Breckenridge - Kern Canyon fault zone relict domain (above the line) and regions Democrat Dam primarily affected by late Cenozoic incision 0 along the lower Kern River. (B) Graph that 0210 030405060 shows the height of tributary knickpoints above the trunk channel of the lower Kern Distance from mouth (km) River. In both plots, tributary knickpoints B are shown at the location of the confluence between their respective tributary watershed and the lower Kern River. ) Green circles represent knickpoints of the 2000 Kern River knickpoints Tributary knickpoints Kern Plateau relict domain, black circles Kern Plateau t are knickpoints located within the Kern Kern Canyon Canyon, yellow circles are knickpoints that 1500 Upper gorge demarcate the Kern Gorge, orange circles Lower gorge represent knickpoints within the walls of the gorge, and white circles are promi- 1000 nent knickpoints on the trunk channel of the Kern River. The position of the West Breckenridge fault and Breckenridge–Kern est Breckenridge faul Canyon fault zone is shown as dashed ver- W 500 tical gray lines. Breckenridge - Kern Canyon fault zone

Height above modern channel (m 0 0210 030405060 Distance from mouth (km)

We are unable to resolve the position of any knickpoints on the upper 2013). Upstream of this knickpoint, automatic ksn analyses show tributary

Kern River that are associated with the initiation of this period of incision channels with anomalously high ksn values that enter a section of the upper

along the Kern River (Fig. 3). The uppermost knickpoint identified on the Kern River with low ksn values (Fig. 2). This pattern is suggestive of a hang- Kern River occurs at an elevation of 1891 m and is a prime candidate. A ing-valley setting where the channel form has been extensively impacted poor match between the longitudinal channel profile and the reconstructed by glacial processes. Other factors may also complicate both the identi-

channel profile based on measuredk sn values, however, precludes us from fication of a single knickpoint and the reconstruction of a fluvial bedrock this interpretation. This uppermost knickpoint roughly corresponds with the channel profile along this reach. The upper Kern River, above Isabella Lake, lower-elevation limit of major glaciers in the Kern River watershed during roughly follows the strike of the Kern Canyon fault zone and closely paral- the Tahoe glaciation (Fig. 3) (Moore and Mack, 2008; Moore and Moring, lels the eastern flank of the Breckenridge-Greenhorn horst (Mahéo et al.,

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2009; Amos et al., 2010; Nadin and Saleeby, 2010). The presence of highly fractured and fault-damaged rocks within this region is likely to cause sig- A nificant variation in the erodibility of the channel bed (e.g., Stock et al., 2004; Crosby and Whipple, 2006; Wobus et al., 2006). Fracture spacing may have also influenced the overall shape of the valley during glaciations as a result of focused plucking and quarrying (Dühnforth et al., 2010). These complicating factors may have caused the upper Kern River to deviate from the power-law slope-versus-area relationship expected for equilibrium channel profiles in fluvial systems. Our analysis also does not consider the likely role that older, pre-Neogene paleochannels may have had on the incision history of the Kern Plateau (e.g., Wakabayashi and Sawyer, 2001; Wakabayashi, 2013; Saleeby et al., 2016). Because of these difficulties, we ~2 m focus our attention on the remaining periods of incision interpreted along the lower Kern River. Incision of the lower Kern River has been punctuated by at least two waves of rapid incision. The first wave of rapid incision in the study area is associated with the formation of the Kern River Canyon. Tributary knickpoints that separate the high-elevation relict topography from the steeper Tributary channel above knickpoint channels and hillslopes along the lower Kern River reveal an upstream decrease in incision along the Kern Canyon reach. Incision estimates range θref = 0.45; ksn = 23.5 ; θ = 0.41 from 501 ± 12 m immediately upstream of the upper gorge knickpoint to 77 ± 15 m near the Isabella Dam (Fig. 7). This decrease in incision occurs B over a stream distance of 28.8 km and suggests that incision of the Kern knickpoint River has progressed through upstream knickpoint migration. The current location of this knickpoint appears to be at the Isabella Dam, in very close proximity to the Breckenridge–Kern Canyon fault zone. The presence of the dam, low channel gradients, and the confluence of the South Fork Kern River make it difficult observe a clearly defined knickpoint in the longitudinal profile and slope-versus-area or gradient-versus-distance plots at this location. Regardless, this period of rapid incision along the lower Kern River appears to have initiated near the western range front and propa- ~2 m gated upstream. The West Breckenridge normal fault defined the western range front of the southern Sierra Nevada throughout the Miocene (Mahéo et al., 2009; Saleeby et al., 2016). Subsidence and sedimentation histories in the adjacent southern San Joaquin Basin indicate mean subsidence rates of ~0.055 mm/yr between 40 Ma and 6 Ma (Cecil et al., 2014). Observed early Neogene variation in the timing and rate of subsidence from wells within the Kern arch may reflect localized fault control along the southern Sierra fault sys- Tributary channel below knickpoint tem (Mahéo et al., 2009; Saleeby et al., 2016). Subsidence rates across the θ = 0.45 ; k = 469 ; θ = –4.6 ref sn Kern arch significantly increase to 0.2–0.5 mm/yr between 6 Ma and 1 Ma (Cecil et al., 2014). This increase is interpreted to reflect transient epeiro- Figure 6. Photographs that show a tributary channel above (A) and below (B) a prominent knickpoint (indicated in B) that defines the boundary of the Kern Gorge. The results of manual genic uplift and subsidence driven by flexural-isostatic responses to delam-

normalized steepness index (ksn) analysis for the respective channel reach (KR_006250) are pre- ination that may also involve local remobilization of preexisting faults due sented along with the concavity index (θ) determined through manual regression of slope-area to flexural forces (Saleeby et al., 2012, 2013; Cecil et al., 2014). Epeirogenic data. The reference concavity (θref) used for this analysis was 0.45. uplift and subsidence are likely to have driven increased incision across the transition between the Sierra Nevada and San Joaquin Basin. We there-

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1500 Kern River knickpoints Tributary channels Kern Plateau Kern Canyon 1000 Upper gorge Lower gorge

500 Incision (m) est Breckenridge fault W Breckenridge - Kern Canyon 0 fault zone 0210 030405060 Distance from mouth (km)

Figure 7. Results of estimated incision along the lower Kern River. Incision estimates were determined from the difference in elevation of reconstructed channel profiles and the modern channel of the lower Kern River at the confluence. Green squares represent incision estimates associated with knickpoints of the Kern Plateau relict domain, black squares are incision estimates from Kern Canyon knickpoints, yellow squares show incision determined from knickpoints that demarcate the Kern Gorge, and orange squares represent incision within the walls of the gorge. White circles show the location of prominent knickpoints on the trunk channel of the Kern River. Black dashed lines highlight the upstream decrease in incision determined from knickpoints that delineate the boundary of each landscape domain. Red dashed lines show the difference in elevation between each of the reconstructed trunk channel profiles and the modern channel. The position of the West Breckenridge fault and Breckenridge–Kern Canyon fault zones is shown as dashed vertical gray lines.

fore investigate the possibility that the lower Kern River experienced an the upper gorge knickpoint migrated upstream along the Kern River from increase in incision ca. 6 Ma. If this is in fact the case, our incision estimates the range front, rather than being initiated at or modified by displacement correspond to a maximum long-term incision rate of ~0.08 mm/yr and a on the West Breckenridge fault. Cecil et al. (2014) used detrital (U-Th)/He horizontal knickpoint migration rate of >4.8 mm/yr. The observed linear in- thermochronometry and other metrics to reveal a ca. 1 Ma pulse of uplift cision-versus-distance relationship suggests a minimum total incision of and exhumation in the Kern arch region. Since this time, anywhere from 642 m, a long-term incision rate of 0.11 mm/yr, and a horizontal knickpoint 1000 to 1800 m of sedimentary strata was stripped off of the Kern arch. The migration rate of 7.3 mm/yr from the West Breckenridge mountain front. resulting exhumation rates of 1–1.8 mm/yr are consistent with contempo- We contend that this wave of incision on the lower Kern River is similar to rary uplift of the Sierra Nevada determined by GPS and interferometric syn- a period of increased incision observed on major Sierra Nevada rivers to thetic aperture radar (InSAR) measurements (Cecil et al., 2014; Hammond the north ca. 3.5 Ma (Stock et al., 2004, 2005). The extent of this incision et al., 2016). We therefore assert that this period of uplift and exhumation event along the western flank of the Sierra Nevada suggests that regional is fundamentally tied to exposure of the Kern River gorge relict landscape east-to-west delamination of the lithospheric root is the principal driving and formation of the Kern River gorge. Our data, therefore, correspond to mechanism (Saleeby et al., 2012, 2013). Minor differences in the timing and an upper long-term incision rate of 0.51 mm/yr and a horizontal knickpoint magnitude of incision may reflect north-south variations associated with migration rate of 12.9 mm/yr. The observed incision-versus-distance rela- detachment of a megaboudin from the northern segment of the delaminat- tionship suggests a minimum total incision of 575 m, a long-term incision ing lithospheric root ca. 3–4 Ma (Saleeby et al., 2012, 2013). rate of 0.58 mm/yr, and a horizontal knickpoint migration rate of 18.3 mm/yr A second wave of increased incision is revealed by tributary knickpoints from the modern mountain front. Our results highlight north-south differ- within the lower Kern River watershed. This wave of incision is responsible ences in rock uplift and incision along the southern Sierra Nevada at this for formation of the Kern River gorge. Knickpoints on tributaries located time. Incision rates leading to the formation of the Kern River gorge are downstream of the upper gorge knickpoint define the abrupt transition significantly higher than the ~0.3 mm/yr obtained by Stock et al. (2004, between the Kern River gorge relict topography and the steep channels 2005) and attributed to the incision of inner gorges (ca. 3 Ma) along major and hillslopes along the lower Kern River (e.g., Fig. 6). Incision estimates Sierran rivers to the north. Our incision estimates do not account for the re- consistently decrease upstream of over a distance of 12.9 km toward the moval of any sedimentary cover from the relict landscape and are therefore upper gorge knickpoint (Fig. 7). These values range from 506 ± 3 m near minimum values. Stock et al. (2004, 2005) reported a dramatic decrease in the mouth of the Kern River at the Kern Gorge fault to 39 ± 8 m near the incision rates along northern rivers to ~0.02 mm/yr that began ca. 1.5 Ma upper gorge knickpoint. The consistent decrease in incision estimates from and continues to the present. This large difference in incision supports a tributaries located downstream of the West Breckenridge fault suggest that north-south variation in rock uplift associated with northward migration of

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a delamination hinge ca. 1 Ma (Saleeby et al., 2012). This pattern is further to the north. Tributary knickpoints near the mountain front help to define manifest by uplift and exhumation of the Kern arch and anomalous sub the Kern gorge relict landscape. This relict landscape was offset from the sidence in the Tulare sub-basin (Saleeby et al., 2012; Cecil et al., 2014). Kern Plateau by periodic displacement along the West Breckenridge fault We observe two additional tributary knickpoints at elevations below the throughout the last ~20 m.y. Incision estimates from tributary knickpoints Kern River gorge relict landscape and within the walls of the inner gorge on this relict landscape provide evidence for a second pulse of incision (Fig. 4). These prominent tributary knickpoints reflect modern incision along the lower Kern River. This pulse of incision migrated upstream at a that is in the process of moving through the Kern River watershed. Inci- rate of 18.3 mm/yr and initiated a vertical incision rate of ~0.58 mm/yr along sion estimates from these tributary knickpoints decrease from 366 ± 4 m the lower reaches of the river. This incision event formed the Kern River to 216 ± 2 m over an upstream distance of 2.5 km toward the lower gorge gorge and is interpreted to reflect increased uplift and exhumation related knickpoint (Fig. 7). Unfortunately, we are currently unable to constrain the to thermal effects on the crust following northward migration of an actively initiation of this pulse of incision to better than <1 Ma. Based on the ob- delaminating lithospheric root <1 Ma. Tributary knickpoints within the Kern served incision-versus-distance relationship, we estimate upper long-term River gorge loosely constrain a third pulse of incision that occurred within incision rates of >0.37 mm/yr and a horizontal knickpoint migration rate this time frame. These tributary knickpoints are tied to the steepest reach of >2.5 mm/yr. The correlation with the lower gorge knickpoint, steepness of the lower Kern River and suggest an incision rate of ~1.2 mm/yr at the of the lower gorge reach of the Kern River, and the steepness of the in- mouth of the gorge. These results constrain the incision history of the lower cision-versus-distance relationship, however, suggest that these values Kern River and provide geomorphic evidence that corroborates existing should be equivalent to or greater than incision values associated with the models of late Cenozoic tectonics and its impact on the evolution of the initial formation of the Kern River gorge. We interpret these knickpoints to southern Sierra Nevada landscape. reflect a further increase in rock uplift rates tied to the thermal effects of mantle delamination. ACKNOWLEDGEMENTS The authors would like to thank Kristin Koehler and Spencer Schroer for helpful discussions and assistance with using the stream profiler tool. We also thank Jason Saleeby, Greg Stock, and asso- CONCLUSION ciate editor Jeff Lee for reviews and comments that helped to improve the manuscript. This project was funded by National Science Foundation grants HRD-1137774 and HRD-1547784. Additional We identified two prominent relict landscapes that have been deeply support was provided by a student research grant from the Geological Society of America (to Foreshee). incised by the lower Kern River and driven in response to late Cenozoic rock uplift within the southern Sierra Nevada and San Joaquin Basin region. REFERENCES CITED The Breckenridge, Piute, and Greenhorn Mountains are remnants of the Amos, C.B., Kelson, K.I., Rood, D.H., Simpson, D.T., and Rose, R.S., 2010, Late Quaternary slip high-elevation relict landscape of the Kern Plateau. 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GEOSPHERE | Volume 14 | Number 3 Krugh and Foreshee | Geomorphic constraints on the incision history of the lower Kern River 1118