The San Juan River and its largest tributary, the Animas River, produced extensive alluvial terraces that record at least seven episodes of alternating aggradation and incision during the overall incision-dominated Quaternary history of the region. Terrace deposits are typified by 5 to 20 m-thick poorly sorted pebble- through boulder-dominated gravels with well-sorted silicate-dominated sands in the matrix; these sediments are attributable to depositional environments within channels or braidplains of paleorivers. Recent geological mapping efforts on the Cedar Hill, Aztec, and Bloomfield 7.5’ quadrangles (all of which are along the Animas or San Juan Rivers ~22 to 40 km upstream of their confluence) have uncovered several previously undescribed deposits of gypsum/anhydrite-dominated sands associated with mainstem alluvial terrace deposits. In each instance, these gypsum sands can be geographically linked to a tributary drainage that might have been impounded by aggradation of mainstem gravel alluvium near its mouth, though high-resolution geochronology will be necessary to prove this relationship, The gypsum sands described here are well-sorted, very fine- to medium-grained, plane-bedded and cross-bedded, 60 to 290 cm thick, and contain >50% to ~90% gypsum/anhydrite grains. Primary bedding and krotovina are common within these deposits. Krotovina often are filled with bedded silicate sands that do not match the texture or color of overlying sediments, suggesting that burrowing occurred while the gypsum sand deposits were at the surface and prior to burial. Individual gypsum/anhydrite sand grains in these deposits show a broad range of textural morphology, suggesting that grains were transported via sedimentary processes and not significantly recrystallized insitu. While New Mexico and neighboring Texas and Coahuila are host to the largest and best-known gypsum sand deposits on Earth, all of those deposits occur in closed-drainage basins associated with long-lived playa evaporite systems the likes of which are not known to have existed in the San Juan basin in the Quaternary. At present, the genetic origin of the gypsum sands described here remains unknown. However, given their texture, location on the downwind sides of major rivers, and geographic association with potentially-impounded tributaries, it is possible that some combination of climate, shallow sulfate-rich groundwater, and local incision-influenced topography created microenvironments amenable to the creation and preservation of gypsum sands during incision episodes of the San Juan and Animas Rivers.