CHANNEL INITIATION IN FORESTED LANDSCAPES OF DIFFERING WATERSHED LITHOLOGY

Identifying where channels begin on the hillslope and determining the physical extent of the channel network is integral to understanding headwater stream systems. However, there are relatively few published field data on channel initiation in Washington State and no known work that compares channel head locations in different watershed lithologies. Several workers have presented the theory that under steady-state conditions, channel head locations are a function of an inverse slope-drainage area relationship. This study compares channel head locations in basalt and sandstone lithologies. It is hypothesized that due to the fractured nature of fine-grained basalt, channel head locations are independent of slope within this rock type. Conversely, channel head locations in sandstone are expected to follow the inverse slope-drainage area relationship seen in other lithologies. Channel heads were mapped using a high resolution Global Positioning System (GPS) in basalt and sandstone lithologies in the Willapa Hills region of western Washington. Contributing area upslope of the channel head was plotted against field measured local slope. There is considerable scatter in the data; however, channel head locations for both sandstone and basalt lithologies appear to follow an inverse slope-area trend. For the same local slopes, contributing areas are generally larger in basalt compared to sandstone, although systematic differences in rainfall amounts may explain much of this difference. To date, these results suggest that headwater channels are dominated by near-surface hydrologic processes rather than deeper groundwater flow in fractures, even in the rock type where such flow paths would be most likely to occur.