LATE HOLOCENE ALLUVIAL AND PALEOENVIRONMENTAL HISTORY OF CARRIZO WASH, WEST-CENTRAL NEW MEXICO

The late Holocene alluvial record of Carrizo Wash, a tributary of the Little Colorado, contains evidence of prehistoric valley entrenchment around 4000, 2000, 1000, and 600 RCYBP. These events are roughly synchronous with arroyo cutting episodes documented over much of the American Southwest, suggesting the arroyo cycles are related to climate change.

Langbein-Schumm (1958) sediment yield curves provide the foundation for a geomorphic process-response model developed for the study area. Modern climate conditions suggest that sediment yields are presently at or near their maximum. The model assumes that middle and late Holocene conditions were generally not significantly wetter or cooler than the past 65 years and that vegetation was never dense enough during wet periods to significantly diminish sediment loads. The model predicts that increased effective moisture would result in higher energy runoff and increased sediment yields leading to elevated groundwater tables, increased perennial streamflow, and a rapidly aggrading, wide, sparsely vegetated, sandy floodplain. Conversely, increased aridity would result in lower energy runoff and decreased sediment yield, lowered water tables, ephemeral streamflow, and relatively slow deposition of clayey alluvium on a narrower, more vegetated and stable floodplain. A depressed water table (base level of erosion) would set the stage for arroyo cutting triggered by flooding at the beginning of the next wet period, as suggested by Waters and Haynes (2001). During valley entrenchment, soils would form on the stable floodplain until aggrading alluvium overtopped the entrenched channels and buried them.

When the geomorphic model is applied to the Carrizo Wash alluvial record, the inferred environmental changes are generally consistent with both the regional paleoenvironmental record and the local phytolith record. Incorporation of dendroclimatic data from the El Malpais volcanic field and the San Francisco Peaks resulted in more detailed reconstructions for the last 2000 years. The reconstructed environmental history of the study area has important implications for prehistoric land use (including agriculture) and settlement.