Google Earth allows you to look through historic satellite imagery too. In Sept 2009 the lake was pretty low but all other imagery from summer into fall had it fairly full. I'm not familiar with the Buckhorn area but I'd imagine it slowly drains throughout the summer and in years with low precip such as this it winds up close to empty. Interestingly in 2009 there was a relatively decent amount of rain in Seattle, maybe the Buckhorn area was heavily rainshadowed that year?
Here is a link to NWS Seattle's recent Twitter post about Seattle rainfall that I referenced.