Reading the Record of Baselevel Change, River Incision, and Surface Uplift on the Colorado Plateau

Ever since geologist John Wesley Powell led the first daring descent through the canyons of the Colorado Plateau's Green and Colorado Rivers in 1869, their origin has been the subject of intense study (Powell, 1875). In fact, historians of science often assert that debates Powell had about canyon formation with contemporaries William Morris Davis, G.K. Gilbert, and Clarence Dutton were integral to the birth of geomorphology, the first scientific discipline to originate in North America (Oldroyd & Grapes, 2008).

A distinctive feature of many Colorado Plateau rivers is that instead of detouring around locales where tectonic processes have arched and uplifted the rocks (the famous Colorado Plateau anticlines and monoclines), the rivers instead flow directly into the deformed rocks and cut majestic canyons, including the Grand Canyon. Powell and his colleagues understandably focused on the puzzle of how and when the rivers managed to do that, pondering whether they were “antecedent” (with courses established prior to deformation), or “superimposed” (lowered onto the deformed rocks from above, with courses originally set on undeformed rocks that erosion has since removed) (Rabbitt, 1969).

But deformed rocks are the exception, not the rule, on the Colorado Plateau. So, a second puzzle is how and when the Plateau rose to its current 2,000 m average elevation absent major deformation. Despite over 150 years of research, the intertwined puzzles of canyon incision and plateau uplift remain unsolved. Obtaining consensus answers to these questions is important, especially since incision of the Colorado River, the master stream draining 642,000 km² of the American Southwest, controls the tempo of geomorphic change across this vast region. No one study will singlehandedly solve these puzzles, but Tanski et al. (2025) move our understanding forward in important ways by deriving incision histories for the Colorado River in Glen (Figure 1) and Meander Canyons, analyzing longitudinal profiles of the river and its tributaries, and constructing a model to track upstream migration of a wave of rapid incision associated with integration of the modern Colorado River.

We now know the Colorado Plateau anticlines and monoclines formed ∼65 Ma, during the Laramide Orogeny (Davis & Bump, 2009), and the modern course of the Colorado River wasn't established until ∼5.3 Ma, when drainage integrated across the Colorado Plateau-Basin and Range boundary at the Grand Wash Cliffs (R. S. Crow et al., 2021; Dorsey et al., 2007). So, strictly speaking, the Colorado River can't be antecedent, but since many processes cause river reaches to mix and match, debate rages over how integration of the modern Colorado River was accomplished (e.g., Barnett et al., 2024; Blackwelder, 1934; Flowers & Farley, 2012; Hill & Polyak, 2020; Karlstrom et al., 2014; Lucchitta, 1989; Scarborough, 2001; Wernicke, 2011).

The tectonic “surface” uplift (England & Molnar, 1990) required to raise average Colorado Plateau elevation to 2,000 m need not have been contemporaneous with Laramide deformation. Long-wavelength (epeirogenic) surface uplift with minimal accompanying deformation is an isostatic response to a decrease in the average density of a lithospheric column (triggered by changes in lithospheric temperature, thickness, or composition), or it can occur above the rising limb of a mantle convection cell (i.e., dynamic topography) (Becker et al., 2014; Molnar et al., 2015; Moucha et al., 2009). Some epeirogenic process elevated the Colorado Plateau, but what that process was and when it acted is the subject of another vigorous debate (e.g., Flowers, 2010; Humphreys et al., 2003; Jones et al., 2015; Karlstrom et al., 2012; Levander et al., 2011; Levandowski et al., 2018; McKee & McKee, 1972; Roy et al., 2009; van Wijk et al., 2010).

The history of incision since river integration is recorded by the depositional ages of terraces perched at various heights above the modern river. This record also offers clues to the history of Colorado Plateau surface uplift but extracting it is complicated because the incision record is a convolution of the drainage basin's tectonic and climatic histories (Molnar & England, 1990). The task is further complicated by the fact that erosion is isostatically compensated, triggering rock uplift and further river incision that is not directly caused by contemporaneous tectonic or climatic changes (Lazear et al., 2013; Pederson et al., 2002, 2013; Pelletier, 2010).

Integration of the modern Colorado River at the Grand Wash Cliffs involved a large drop in baselevel; numerical modeling (Whipple & Tucker, 1999) demonstrates that baselevel drop must have produced a transient knickzone (an anomalously steep river segment) that migrated progressively upstream. The terrace record inscribes the time of knickzone passage as a fluctuation in incision rate, with an initial increase followed by a decrease, though several factors can produce additional complexity (Cook et al., 2009). Multiple studies have interpreted terrace data or incision proxies as recording knickzone passage through Grand Canyon (e.g., Abbott et al., 2015, 2016; Cook et al., 2009; Darling et al., 2012; Pelletier, 2010; Polyak et al., 2008). By contrast, other studies have concluded that Grand Canyon incision rates remained steady throughout the last several million years, requiring that the area experienced geologically recent mantle-induced surface uplift (e.g., R. Crow et al., 2014, 2015, 2018; Karlstrom et al., 2007, 2008).

Tanski et al. (2025) tested the migrating knickzone versus steady-state hypotheses in two ways. First, they used a combination of luminescence and terrestrial cosmogenic nuclide dating of river terraces to determine when the incision rate changed in Glen and Meander Canyons, upstream of Grand Canyon. Second, they constructed χ (chi) transformed longitudinal profiles (Perron & Royden, 2013) for the Colorado River and its major tributaries. Knickzones manifest on such χ-plots as changes in slope; projection of the gentle slope upstream of the knickzone to the y-axis provides an estimate of river height prior to knickzone-inducing baselevel fall. A channel network adjusting to a common baselevel drop, such as that initiated at the Grand Wash Cliffs during river integration, will possess transient knickzones at similar elevations and χ values on all tributaries.

Tanski et al. (2025) documented slow Early Middle Pleistocene incision followed by discrete Middle or Late Pleistocene episodes of accelerated incision in both canyons (their Figure 3). The χ-plots for the Colorado River and its major tributaries have knickzones with upstream limits at approximately χ = 4,000 m (their Figure 7). Projections of their upstream slopes all intercept the y-axis at ∼1,100–1,300 m, which matches the elevation atop the Grand Wash Cliffs. These data strongly support the conclusion that the transient knickzone that formed at the Grand Wash Cliffs during river integration migrated through Grand Canyon after 5.3 Ma and reached Glen and Meander Canyons in the Middle and Late Pleistocene, respectively.

Next, Tanski et al. (2025) constructed a celerity model to track the rate of upstream knickzone migration (their Figure 9). Despite its limitations, the model provides a useful comparison between theory and observation. It suggests ∼2–4 Myr elapsed before the river-integration knickzone reached the central Colorado Plateau, consistent with the terrace results, and predicts the knickzone lies today between Westwater and Glenwood Canyons, matching the Colorado River χ-plot.

This study further illustrates the complexity of the Colorado River system's incision rate history (Cook et al., 2009) but demonstrates that passage of the transient knickzone produced by river integration can be extracted from that complexity. Furthermore, the celerity model provides a useful framework for evaluation of existing and future incision rate data in the context of transient knickzone migration.

Lastly, the authors pointed to another puzzle worthy of future research. They noted that Early Pleistocene incision in Glen and Meander canyons was exceedingly slow prior to arrival of the river-integration knickzone and hypothesized that blockage of the river by western Grand Canyon lava dams may have interfered with steady upstream propagation of the river-integration knickzone. Alternatively, slow Early Pleistocene incision may represent long-term baselevel stability. They further showed that Pleistocene incision of the central Colorado Plateau associated with river integration is just a few hundred meters, much less than the total late Cenozoic exhumation magnitude of ∼2 km (Bailey et al., 2024; Murray et al., 2016, 2019; Ryb et al., 2021). What process(es) could trigger a pulse of Pliocene erosion followed by baselevel stability prior to arrival of the river integration signal? In other words, river integration was necessary but not sufficient to produce the present iconic landscape; much work remains for geomorphologists, geochronologists, and geodynamicists before we fully understand the Colorado Plateau's surface uplift and exhumation history.