Question

How and when the Grand Canyon was formed is a subject of debate among geologists. Two models propose different timelines and mechanisms for its creation:

Model 1 (Ancient Canyon Hypothesis)
This model proposes that the carving of the Grand Canyon began approximately $70\text{ million}$ years ago ($70\text{ Ma}$). Ancestral river systems slowly carved the canyon over tens of millions of years, driven by the gradual tectonic uplift of the Colorado Plateau. According to this model, the modern Colorado River simply adopted this pre-existing, ancient canyon system.

Model 2 (Young Canyon Hypothesis)
This model proposes that the Grand Canyon is a relatively recent feature, with carving beginning only about $6\text{ million}$ years ago ($6\text{ Ma}$). In this view, several smaller, separate paleocanyons were cut by different rivers over time, but these did not become the Grand Canyon until the modern Colorado River carved through the barriers separating them, integrating the system rapidly within the last $6\text{ million}$ years.

Based on Model 1 and Model 2, the two models differ on which of the following aspects of the Grand Canyon?