FLOOD GEOMORPHOLOGY OF ARTHURS-ROCK GULCH, COLORADO - PALEOFLOOD HISTORY

Episodic late Quaternary flooding is recorded by bouldery deposits and slackwater sediments along Arthurs Rock Gulch, an ephemeral stream we st of Fort Collins, Colorado. Flood deposits consist of individual gra nodiorite and pegmatite boulders, boulder bars, and coarse overbank se diment that rest on erosional terrace segments along the channel. We i dentified evidence for at least five floods in the lower two thirds of the 1.84 km(2) drainage basin. Flood deposits are differentiated by t heir position above the active channel, weathering characteristics, de gree of boulder burial by colluvium, amount of lichen cover, and posit ion with respect to terrace and colluvial deposits. Age estimates for the flood deposits are based on radiocarbon dating, tree-ring analyses , and relative-age criteria from four sites in the basin. At least two floods occurred in the last 300 years; a third flood is at least 5000 years old, but likely younger than 10,000 yr BP; and the two oldest f loods occurred at least 40,000 years BP.