Maize agriculture appears to have been introduced into the semiarid southern part of the North American Southwest ca. 2200-2000 B.C., at a time when valley bottoms were aggrading. Although small-scale, high-risk agriculture might have been practiced in many landscape positions, valley-bottom locations having particular geomorphic and/or hydrologic conditions would have provided the places most risk-free for farming. These places, associated with what are here called reach boundaries, have conditions that cause elevated water tables or enhance stream discharge, and have floodplain areas suitable for farming. Reach boundaries are easily identified, are relatively predictable in location and occurrence, and are early settled and/or long occupied. Deposition has been the dominant process in most valley bottoms in the southern Southwest for the last ca. 6000 years. Many streams across the region, however, experienced approximately synchronous periods of erosion when deeply incised channels (arroyos) formed in valley bottoms. Each of these periods of erosion, except for that which began in the late 1800s, was followed by a longer period of arroyo-filling and floodplain aggradation, completing a socalled alluvial cycle. Valley-bottom conditions related to alluvial cycles would have greatly influenced the location, water availability, water management technology, and yield of agricultural ventures. Here we identify more than 280 reach boundaries in parts of the middle and upper Gila River and Mimbres River watersheds in Arizona and New Mexico. Spatial analysis shows a strong 4000-year-long correspondence between reach boundaries and agricultural sites where data are available. Identification and characterization of these locations may prove useful for locating yet unknown, deeply buried early agricultural sites (ca. 2200 B.C.-A.D. 200), in addition to providing explanations for prehistoric settlement patterns through time.

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A number of archeological features, including in-filled irrigation canals of uncertain prehistoric age, occur within the Holocene floodplain of the Salt River at Phoenix, Arizona. In the first attempt to date irrigation-canal sediments using luminescence methods, we obtained age estimates of 1640 Ϯ 190 yr B.P. (1) Thus from photonic dating we can resolve the first and last phases of canal use at this Phoenix site: initiation at ca. 1600 years ago and final use at ca. 800 years ago. These results demonstrate the power of SAR luminescence sediment dating to enhance our understanding of prehistoric irrigation-canal development and usage here and elsewhere in the world. ᭧

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