There is a hatch of Little Yellow Stones on the creek.  Most enter the main creek from feeder creeks...  It is one of the few times during the year you can actually get fish to hit a pretty big dry fly on Putah .   There is also a pretty good hatch of caddis in the riffles.  Here is a link to the little yellow stone.

http://www.west-fly-fishing.com/entomology/stonefly/little-yellow.shtml