“…There are two prevailing models for the birth and early evolution of the Colorado River: (a) “early initiation” at ~5.3 Ma, following roughly 1 Myr of marine deposition in the lower river valley (Figure a–d; Buising, ; Dorsey et al, , , ; McDougall, ; McDougall & Martínez, ; O'Connell, Dorsey, & Humphreys, ); or (b) “late initiation” at ~4.8 Ma, following a very short period (<50 Kyr) of lake deposition in the lower river valley (Figure e–h; Bright, Cohen, Dettman, & Pearthree, ; Bright et al, ; Crow et al, ; Gootee, Pearthree, House, Youberg, O'Connell, et al, ; Gootee, Pearthree, House, Youberg, Spencer, et al, ; House et al, ; Pearthree & House, ; Spencer & Patchett, ; Spencer et al, ). Both models propose emergence of the integrated river network after 6.0 Ma and before 4.5 Ma, but with notable differences within that time frame.…”