Earlier workers postulated that this spillover event occurred between 690 and 300 ka, likely 670 ka, based on the argument that the San Luis Valley did not drain southward into the Rio Grande until deposition of the youngest Alamosa Formation sediments (Wells et al., 1987; Rogers et al., 1992).
Regardless of timing, the overflow of ancient Lake Alamosa likely fully integrated the Rio Grande River from its current upper reaches in Colorado to its earlier headwaters in New Mexico.
He surface-exposure ages (1-sigma) of multiple samples from each terrace indicate Qt6 was likely abandoned at 69.0 8.4/−9.2 ka, Qt5 at 36.7 13.4/−9.0 ka, Qt4 at 26.9 5.5/−4.2 ka, Qt3 at 25.3 3.1/−3.2 ka, and Qt2 at 24.3 7.6/−6.7 ka.
The integration event had profound effects on fluvial dynamics, including but not limited to substantial base-level lowering, pronounced incision, and in particular, knickpoint formation and migration (Machette et al., 2013).
Due to its considerable length, proximity to active structures, and sensitivity to climatic fluctuations, among other factors, the geomorphology of the river changes dramatically along its length, from occupying deeply incised canyons in northern New Mexico to more broad and shallow floodplains farther south.
Within the larger Rio Grande system, the Rio Grande in northern New Mexico is especially unique and enigmatic for a number of reasons.
He is particularly useful in geologic studies (e.g., Marchetti and Cerling, 2005; Foeken et al., 2009) because it is a stable nuclide that has the highest production rate of all TCNs, as well as a low detection limit on a noble gas mass spectrometer.
He is produced primarily via spallation reactions on O, Mg, Si, Ca, Fe, and Al within olivine, pyroxene, hornblende, and garnet crystals.
The most prominent terrace surface (Qt4) falls within MIS 2 and appears to closely track incision associated with Pinedale ice retreat.