GF/LIW/RS and East Medicine Lake Hydration Rates: Preliminary obsidian hydration rates have been proposed for obsidian sources located in the Medicine Lake Highlands, the GF/LIW/RS and East Medicine Lake geochemical groups. Johnson (1969) established a regional obsidian hydration rate for the Klamath Basin at the nearby Nightfire Island Site, northern California, of 3.5µ²/1000 ¹⁴C years, although he made no attempt to control for the chemical composition of the glass. As later trace element studies of Nightfire Island obsidian by Hughes (1985, 1986:123-179) suggest, Johnson’s collection likely included obsidian from the sources identified in the current study. Assuming a similar rate of hydration for the Spodue Mountain*, Silver Lake/Sycan Marsh*, and GF/LIW/RS sources, a proposition originally advanced by Pettigrew and Lebow (1987), Connolly et al. (1994) proposed a "project vicinity" (Medicine Lake Volcano, Spodue Mountain, and Silver Lake/Sycan Marsh sources) hydration rate of 4.1 µm²/1000 years.

Separate hydration rates for two different geographic localities are also reported by Basgall and Hildebrant (1989) for obsidian from the GF/LIW/RS chemical group. The hydration rate for these sources has also been discussed by Skinner (1995b) and Ozbun et al. (1996). It must be noted, however, that in all cases these provisional obsidian hydration rates are constructed from relatively young radiocarbon dates. Because of this, it should be expected that the potential error associated with age estimates for artifacts with larger rims will be greater than for smaller hydration rims.

* Obsidian sources located in southern Oregon