This paper reports new data on qocha ponds from the Rio Pucara-Azángaro interfluvial zone, northern Lake Titicaca Basin, Peru. Qocha are a little known form of Andean agriculture that developed around 800-500 B.C. and remain in use today. Prior estimates suggested that in the study area, there were more than 25,000 qocha. While most Andean sunken beds are excavated to reach groundwater, qocha are rain-fed ponds. How these rain-fed ponds functioned has been an open question, but one that is answered in part by research presented in this paper. We suggest that a thick impermeable stratum of clay that was possibly deposited by paleolake "Minchin" created a perched water table that makes rain-fed qocha reservoir agriculture possible. Field geology shows that within the study area, this stratum only exists under Terrace E. Based on this model, we hypothesized that persistently used qocha should only be found on Terrace E. To test this hypothesis we used remotely sensed data to inventory qocha and to determine their distribution by each terrace present. We identified 11,737 qocha. By area 93.77% and by count 94.33% of the qocha are located on Terrace E. These results strongly supported our hypothesis. This case study illustrates that the long term viability of this form of agriculture is made possible by a physical context that is beyond human control. © 2011 Elsevier Ltd.