We studied the alluvial histories of eight small (<40 km2) watersheds in the uplands of northeastern New Mexico. The data come from radiocarbon-dated sections exposed in the banks of arroyos and permanent-channel streams. Results show that nine periods of valley aggradation separated by incision episodes occurred over the past 12,000 14C yr. These millennial-scale cycles occurred in rough synchrony within different drainages. Parts of some cycles coincided with well-known climatic fluctuations. For example, aggradation occurred during the Younger Dryas chronozone, 11,000–10,000 14C yr B.P., and valley fills persisted in incised states during the early Holocene peak in Milanko-vitch summer insolation. Incision occurred early in the Medieval Warm Period (ca. A. D. 1000–1300), aggradation during the Little Ice Age (A.D. 1300–1880), and incision during the last century. Changes in precipitation tied to the strength of the North American Monsoon system probably drive these cycles of aggradation and incision: when the system is strong, more frequent summer thunderstorms cause increased flooding in these small watersheds. Wetter summers over periods of decades to centuries allow forest vegetation to spread, which reduces sediment input from hillslopes at the same time floods are eroding valley fills. Aggradation of valley fills probably occurs when summer thunderstorms are less frequent, and large floods are correspondingly rare. Prolonged summer drought reduces forest cover, which increases erosion from slopes and causes sediment to accumulate in valleys. In turn, the strength of the North American Monsoon system is probably controlled by fluctuating sea-surface temperatures. The geomorphic cycles we describe may reflect a previously unrecognized, millennial-scale climate oscillation that is important in shaping the landscapes of the southern Great Plains.