Lithological and structural controls on river profiles and networks in the northern Sierra Nevada (California, USA)

Abstract

In this study, the strong lithological heterogeneity of the northern Sierra Nevada (California, USA) is exploited to elucidate the role of lithology on river profiles and patterns at the mountain-range scale. The analyses indicate that plutonic, metavolcanic, and quartzite bedrock generally host the steepest river reaches, whereas gentle reaches flow across non-quartzite metasedimentary rocks and fault zones. In addition, the largest immobile boulders are often in the steepest reaches, suggesting that wide joint spacing plays a role in creating steep channels, and a positive relationship between boulder size and hillslope angle highlights the coupling of the hillslope and fluvial systems. With respect to river network configurations, dendritic patterns dominate in the plutonic bedrock, with channels aligned down the slope of the range; in contrast, river reaches in the metamorphic belts are mainly longitudinal and parallel to the structural grain. River profiles and patterns in the northern Sierra Nevada, therefore, bear a strong lithological imprint related to differential erosion. These observations indicate that attempts to infer uplift and tilting of the range based on the gradients and orientations of paleochannel remnants should first account for the effect of bedrock erodibility. Indeed, the differences in gradients of Tertiary paleochannel remnants used to argue for late Cenozoic uplift of the range can be wholly explained by differences in lithology.

Geomorphic signatures of the transient fluvial response to tilting

“…Within this forum and in a subsequent solicited review, Gabet argues that the knickpoint we identify as a migrating knickpoint of tectonic origin in the Middle Fork American River is instead a stationary knickpoint that reflects lithologic control (Gabet, 2019a). We agree that lithology has a significant impact on river profiles in the northern Sierra Nevada, but we find that the observed patterns in incision into basement rock below Cenozoic deposits and the relationship between longitudinal profile form and lithologic boundaries in the Sierra Nevada are more consistent with the models presented herein in which the transient response to a punctuated tilt is modulated by heterogeneous lithology (Beeson and McCoy, 2019b), rather than reflecting a response to heterogeneous lithology with no tectonic forcing (Gabet, 2019b).…”

“…Knowledge of the transient response of bedrock rivers to different perturbations thus comprises an important geomorphic tool to characterize the history of rock uplift rates, climate, or changes in river network topology from disequilibrium landscape form (e.g., Beeson et al, 2017;Ferrier et al, 2013;Kirby and Whipple, 2012;Lease and Ehlers, 2013;Tucker and Whipple, 2002;Whittaker et al, 2008;Willett et al, 2014;Wobus et al, 2006). Such histories are critical for testing geodynamic models of orogenesis and quantifying the relative importance of external forcing, such as climate and tectonics, versus internal complex system response, on the evolution of mountainous landscapes (e.g., Beeson et al, 2017;Clark et al, 2005;Gallen, 2018;Kirby and Whipple, 2012;Whipple et al, 2017;Willett et al, 2018;Yang et al, 2015).…”

Comment on Beeson and McCoy

“…Rivers in the American River watershed flow across a wide variety of rock types with a range of erodibilities. In Gabet (2019), I demonstrate that lithology is the primary control on the channel steepness index; therefore, any attempt to extract tectonic information from channel profiles must first account for bedrock erodibility. I calculated the normalized channel steepness for 36 different rock units in the northern Sierra; I also determined an average Ksn for the four main lithological categories.…”

“…A more appropriate estimate for an incision rate can be determined with data from Gabet and Miggins (in review) in which we dated a volcanic deposit that is just 70 m above of the South Fork of the American River. At this site, the river is cutting through granitic rock, which is similar in erodibility to C4 quartzite and metavolcanics (Gabet, 2019;Sklar and Dietrich, 2001). The age of the deposit is 6.5 Ma, yielding an incision rate of 0.01 mm/y over the past several million years; unfortunately, the data for this calculation are in a paper that is under review and, therefore, can't be cited until it is published but it highlights the issue with the rate used in the present ms.…”

“…Both Gabet (2014) and Cassel (2012b) demonstrated that his reconstructions were fundamentally flawed, either because they imply that water flows uphill or because he was linking channel segments that were unrelated in time and space. Moreover, Gabet (2019) demonstrates that, even if Lindgren had correctly reconstructed the channels, the differences in their gradients that he attributed to tilting can be wholly explained by differences in bedrock erodibility. (2004) based their tilt estimates on Lindgren's faulty reconstructions (see above) and are, therefore, inconclusive.…”