PALEOHYDROLOGY AND PALEOCLIMATOLOGY OF THE SKUNK CREEK BASIN DURING HOLOCENE TIME (ARIZONA)

Skunk Creek drains a 325 sg km arid to semi-arid basin in central Arizona. Presently it has ephemeral flow and a broad entrenched channel. Hohokam Indian settlement along this stream approximately 1000 BP and late nineteenth century surveys indicate that the stream formerly had a much smaller channel and seasonal flow. To resolve this disparity the author evaluated sedimentary, geomorphic, climatic, hydrologic, historic, and palynological evidence from the basin. This evidence and outside the basin historic and surrogate climatic data permitted a reconstruction of the basin Holocene hydrology and climatology.; The basin sediments indicate that from late Pleistocene through middle Holocene (Altithermal) time the stream had a high energy regime with similar tractive force values as the modern stream. From about 5000 to 80 BP the stream had 0.15 as much tractive force. The modern channel is sized to the 25 year event; the pre-entrenchment channel had 0.15 the cross-sectional area with an overbank frequency of two years. The smaller channel permitted frequent overbank fine-grained deposition and produced 3 m of deposition during late Holocene time. Fine-grained sediments underlying the pre-entrenchment channel reduced transmission losses to 0.03 their modern values and permitted seasonal streamflow.; Since 1900 annual precipitation has had a mean variability of 44 percent; decadal variability has range from plus 27 to minus 10 percent. During the Altithermal (8000 to 5000 BP) the basin had a mean annual temperature 1 C warmer and a mean annual precipitation 70 mm greater than present. During post-Altithermal time the long term temperature has varied from 0.3(DEGREES)C warmer to 1.0(DEGREES)C cooler than present; long term mean annual precipitation has varied about the modern value by approximately 25 mm. Pollen analysis suggests shifts in the seasonal precipitation and extension of riparian vegetation.; Historic records indicate that the stream entrenched its channel between 1894 and 1922, a time of intense sheep and cattle grazing. The largest storm during this period was a 100 year event. Runoff modelling showed that overgrazing doubled flood discharges and increased the frequency of high energy/discharge flows. These changes altered the stream regime so that its hydrology mimics that produced by the high energy/summer precipitation Altithermal climate. Other factors had minor effects. This change was the most significant hydrogeomorphic event to affect the basin during the last 5000 years.