GEOMORPHOLOGY AND ALLUVIAL HISTORY OF LATE HOLOCENE VALLEY-FILL ALLUVIUM, SOUTHERN COLORADO PLATEAU, SOUTHWEST USA

Bounded by bedrock, most valleys of the southern Colorado Plateau contain fine-grained Holocene alluvium of variable thickness. The alluvium typically extends far into low order drainage basins reaching the base of steep bedrock hillslopes, a likely sediment source. Alluvial-valley geomorphology consists of a single terrace, or less commonly a flight of two or more terraces. The active channel and modern terrace are several to tens of meters below the surface of the valley-fill alluvium. Late Holocene alluvium in high order basins typically has two main stratigraphic units that are bounded by disconformities in the form of paleoarroyos and/or buried soil horizons. These units are approximate temporal equivalents of the Tsegi and Naha alluviums originally defined by Hack (1942). The Tsegi is prehistoric; in most places it was deposited from ca. 2500 B.C. until A.D. 700 to 1200, if not the late A.D. 1200s. A paleoarroyo and (or) a thin weathered horizon that formed between ca. A.D. 1200 to 1400 truncates the Tsegi. The Naha, which locally covers prehistoric Puebloan remains, was deposited between ca. 1400 to 1880. Deposition was ended by historic arroyo cutting that began between 1861 and the early 1900s, although in the Southwest the beginning of incision is considered 1880. The Naha and Tsegi are mappable stratigraphic units that have been traced over large distances within and among drainage basins. In both cases, the spatially separate alluvial systems were probably responding to climatically controlled changes in discharge and sediment load, which in turn were likely affected by changes in the frequency of large, destructive floods. Arroyo cutting prevailed during episodes of frequent large floods, whereas alluviation was favored during times of infrequent flooding. Historic arroyo cutting was related to an increase in the frequency of intense and warm ENSO activity from 1870 to the early 1940s, a condition that probably affected prehistoric erosion as well.