Rivers, such as New Mexico’s Rio Grande, contain an amazing record of evolving landscapes. Geological activity, such as faulting, can cause the river to shift course, as can volcanic eruptions when lava flows make dams across rivers. Rainfall variations caused by climate change can also impact a river's evolution. Determining whether tectonics or climate are the primary drivers of river changes can be better understood by studying the age of river terraces (sand and gravel deposits) that mark the river's position on the landscape through time. Terraces are notoriously difficult to date because the sediment comes from many different rocks sources, all of different ages. However a new dating method, being developed at the NMBG&MR, uses our state-of-the-art geochronology laboratory, funded by NSF and NM Tech, to determine the age of detrital sanidine (tiny volcanic minerals) from the deposits. The terrace can be no older than the age of the youngest sanidine crystals in the sediment. Also, if lava from a nearby volcano flows over the terrace, this provides another age constraint because the terrace must be older than the lava, which can also be dated in our lab. Thus, the combined methods serve to bracket the age of the river deposit. Our recent work on the Rio Grande and other major rivers (Colorado, Rio Chama, San Juan) show that during the last 5 million years, the paths of rivers, and the landscapes they shape, have been principally influenced by tectonic processes that uplift the land surface, and by numerous volcanic eruptions. As rivers adjust to this tectonic surface uplift or eruption of lavas into their paths, they cuts deep canyons such as the Taos Gorge or carve mesas that are iconic to New Mexico’s Land of Enchantment.