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(1963, 1965) Post-Piney Creek alluvium, which ages to a period from about 4500 to as late as 2800 rcybp (Miller in prep.). The last fill sequence is Lightning equivalent alluvium (also after Leopold and Miller 1954) which post-dates 1000 rcybp (Miller 1992, in prep). The Moorecroft equivalent cut terrace (after Leopold and Miller 1954) was formed between 6500 and 4500 (Miller in prep.) and incised into Kaycee equivalent alluvium. The fill sequences are related to periods of long term drought. The greatest part of the fill in the valleys (i.e., Kaycee equivalent alluvium) is a result of severe drought accompanying the onset of the Holocene climatic envelope and is the sediment formerly stored on slopes in the Late Pleistocene soil. An alternate theory indicates that fill is a consequence of cooler, wetter conditions, but the ages of Kaycee equivalent deposition correspond almost exactly with the formation of all the major dune fields that formed in the Rocky Mountain basins and western Plains areas (Miller 1992, in prep.), so the association of alluvial deposition with cool, wet climates is erroneous. Deposition in warm, arid intervals is a consequence of lost capacity and competence in the alluvial system (where capacity describes the quantity of sediment a drainage can move, and competence is the largest sized particle a drainage can move). Essentially, the drainages became choked with sediment during deposition of the Kaycee equivalent alluvium. This was an event that could not be repeated because the end of the Pleistocene was the only time such a large volume of sediment was available at one time for transport. Climates ameliorated after 6500 rcybp. With the return of cooler, concomitantly moister conditions, incision began; the Moorecroft-equivalent cut terrace formed somewhere during this period. After formation of the Moorecroft equivalent cut-terrace, however, incision was initiated once more and the present day depth of incision is relict of the interval. Regional and even global evidence mark the middle Holocene as one of the coolest, moistest episodes since the end of the Pleistocene (Miller 1992, in prep.). In-filling (i.e. post-Piney Creek alluvium equivalent) began again by about 4500 rcybp when climates apparently warmed and dried and the streams again experienced diminished power (i.e. loss of capacity and competence). Renewed deposition (post-Piney Creek equivalent) continued until at least 2800 rcybp, when another cycle of erosion ensued. This cycle of erosion likely deepened the incision and culminated in the final depth as seen in the present day. It is notable, however, that the equivalent of Late Glacial gravels, usually exposed by middle Holocene erosion, are not exposed in the area. This attests to the exceptionally large volume of sediment that became available for transport at the end of the Pleistocene glaciation and the inability of the trunk drainages to move it further downstream and out of the local alluvial system. Volume of fill, as mentioned above, is considerably more than exposed in the arroyo walls – again, probably less than half of the total depth of fill is viewable in the present day incisions. After 1000 rcybp, climate became warmer and drier again, and Lightning equivalent alluvium was deposited in the channel bottoms. When climate ameliorated once again during the Little Ice Age, incision followed. 10