The Preceramic archaeological record of highland Mesoamerica is biased toward rockshelter sites. We advocate more fieldwork in streamside settings, where open-air sites are likely to be found for reasons related both to the systemic context of hunter-gatherer lifeways and to the geoarchaeological context of site burial and preservation. Predicting site location requires attention to the peculiar nature and behavior of incised ephemeral streams (barrancas) and to the complex alluvial stratigraphic sequences that they leave behind. Four case studies—from the Mexican states ofTlaxcala, México, Morelos, and Oaxaca—reconstruct the geometry and age structure of late Quaternary alluvium from exposures in cutbanks, brickyards, and purposefully dug trenches. We identify deeply buried locales with the remains of extinct megafauna, intentionally set fires, and lithic debitage. We distinguish between geographical areas, stream reaches, and time intervals that do or do not hold much promise for further research. The fragmentary nature of the alluvial record and the paucity of sites can be explained by changes in stream behavior wrought by agricultural land use and are conditioned by the intensity and antiquity of agriculture in any given area. Deposits and sites of Paleoindian age may be more commonly preserved than those of Archaic age.