FLUVIAL GEOMORPHIC RESPONSE TO HOLOCENE CLIMATE CHANGE ON THE MOGOLLON RIM, ARIZONA

The arid and semi-arid regions of the Colorado Plateau have played host to extensive research focusing on fluvial response to climate change, but no published work has focused on the geomorphology of the more mesic regions on extreme southern edge of the Colorado Plateau. Here, the Mogollon Rim in central Arizona has experienced recent deep incision of low order drainages. A survey of the regional incision and analysis of alluvial stratigraphy from two drainages suggests that the recent incision is unprecedented within at least the last 6000 years. Alluvial stratigraphy exposed in 2-3m deep arroyos in Houston Draw and Dick Hart Draw was mapped, described, and radiocarbon dated revealing possible correlations between alluviation, degradation, and climate.

A dated cut-and-fill sequence in Houston Draw may correlate to the prehistoric arroyo-cutting period (1200-1400 AD) recorded in the arid and semi-arid regions of southern Utah and northeastern Arizona. A calibrated radiocarbon age of 400 + 100 yr B.P. from the base of the unit filling the cut places the aggradation in the Little Ice Age (600-250 yr B.P.), a period of alluviation linked to a decrease in frequency of large floods in the drainages of southern Utah and northeastern Arizona.

Maximum ages of alluvium in Houston Draw, Dick Hart Draw and neighboring Clover Springs suggest that a regionally synchronous event flushed out valley fill some time before 6000 yr B.P. Relatively uniform aggradation appears to have occurred through the early part of the preserved sequences changing at 2250 + 120 yr B.P. in Dick Hart and more recently in Houston Draw. However, aggradation is continuous throughout the sequence at Clover Springs. It appears that a regional response occurred prior to 6000 yr B.P. The transition to the altithermal in the region at ~7000 yr B.P. may have generated a disturbance suitable for the regional response. Shifts in local conditions may account for the lack in stratigraphic continuity between sites after 2250 yr B.P. Intrinsic thresholds responding to weaker climatic fluctuations and/or changes in forest fire frequency could be accountable. Understanding the dynamics of these systems provides a perspective for deciphering causes of the historic regional incision, ultimately allowing land managers to make confident decisions on restoration issues.