LATE HOLOCENE ALLUVIAL STRATIGRAPHY AND GEOMORPHOLOGY OF THE LITTLE COLORADO RIVER BETWEEN HOLBROOK AND WINSLOW, ARIZONA

Four major terraces mapped along the Little Colorado River between Holbrook and Winslow, Arizona document episodes of aggradation and degradation during the past 3,000 years. The alluvium forming these four terraces are denoted as the Desert Broom, the Tamarisk, the Cottonwood, and the Moenkopi alluvium in order of increasing age. The terraces and their associated deposits were mapped on the basis of their elevation and relative position to the active channel and adjacent units, their surface morphology, and the dominant type of vegetation and relative coverage of that vegetation on the terrace surface. The extent of soil formation, stratigraphic relationships, and other sedimentological properties also helped distinguish the various units from each other.

The chronology for these alluvial units was based on the extent of soil development, 14 radiocarbon ages, and ring counts from 42 tree cores or slabs. The Desert Broom alluvium exhibits no soil development and is covered by sparse or no vegetation. This alluvium appears to have been deposited during the last decade based on the maximum age of vegetation. The Tamarisk alluvium also shows very little or no soil development. Stratigraphic and dendrochronologic data indicate that this is a complex unit deposited between the 1940's and 1970's. The Cottonwood alluvium exhibits a weak to moderately developed soil and, based on numerous tree cores and several radiocarbon ages, was deposited between several hundred years to perhaps 1000 years ago. The Moenkopi alluvium is a complex cut-and-fill sequence that apparently represents time-transgressive deposits. Based on the extent of soil formation, the Moenkopi terrace has been stable for at least 1000 years, but radiocarbon ages indicate that the older parts of the deposit may be more than 3000 years old.

The timing of aggradation and degradation in this reach of the Little Colorado River is similar to the alluvial history reported in nearby areas on the Colorado Plateau. The favored theory is that these episodes are climate-driven with periods of alluviation occurring during dry cycles and periods of degradation occurring during wet cycles. Based on present channel conditions and presence of bedrock in the channel, the river between Holbrook and Winslow currently appears to be in a stable or slightly degrading state.