River Geomorphology in the Sierra Nevada

Does river geomorphology in the Sierra Nevada record late Cenozoic tectonic forcing?

North Fork American River from the Shirttail Canyon Bridge.

Read the paper published in GSA Bulletin here.

Our goal in this project was to constrain the timing, rates, and spatial patterns of uplift in the Sierra Nevada, eastern California, USA, which have been the subject of long, vigorous debate due to their importance in providing constraints to contrasting geodynamic models.

In order to elucidate drivers of late Cenozoic landscape evolution in the Sierra Nevada, we analyzed the modern geomorphology of all major river basins draining the western slope of the Sierra and compared the results to simple 1- and 2-D numerical simulations of perturbations that move a river network away from equilibrium.

Our findings suggest that the majority of the range experienced a period (~1-3 Myr) of rapid rigid-block style tilting to the west in the late Cenozoic, of increasing magnitude to the south, that slowed considerably ca. 3 Ma. The period of rapid tilting is recorded by knickpoints in mainstem rivers, distinct tributary knickzones that increase in height with distance from the mountain front up to the mainstem knickpoint, and canyon incision that also increases linearly with distance from the mountain front up to the mainstem knickpoint.

In the northern Sierra, these signatures of tilt appear to be modulated by heterogeneous lithology, with some mainstem knickpoints stalled on less erodible rock types and deeply incised sections upstream that correspond with the western metamorphic belt. These observations are consistent with model simulations of tilting with bands of variable erodibility in which knickpoints form both at the outlet and anywhere more erodible rock occurs upstream of less erodible rock. This mechanism can explain the observed variability in canyon incision both within and among river basins in the northern Sierra.

On top of late Cenozoic volcanics on Rowton Peak near Donner Summit in the northern Sierra, overlooking the deeply incised Royal Gorge region of the upper North Fork American River.

In addition to the signatures of tilting evident in river networks, we documented a dramatic difference in river geomorphology north and south of the Tuolumne River Basin. We suggest that the northern Sierra is in a transient state of network incision and reorganization due to reestablishment of a fluvial network after Mio-Pliocene volcanism filled and smoothed preexisting topography. In contrast, the river network in the southern Sierra is in geometric equilibrium and appears to pre-date the current westward dip. Southern Sierra rivers are responding to late-Cenozoic tilting by incising into bedrock, with concordant knickzones progressing up mainstems and tributaries, whereas, in the northern Sierra, river response to late-Cenozoic tilting is much more complex owing to disequilibrium in river basin geometry. The dramatically different response to tilting illustrated in northern and southern Sierra rivers comprises a natural experiment demonstrating the importance of initial conditions.