I read an article in a fly fishing magazine (dont remember the name of the mag.) about nymphs, streamers and wet flies in still water. The aricle instructed anglers to use bigger flies like streamers early in the late the day. The longer the day goes on the smaller your flies,like nymphs, should get until you reach late afernoon. Then, switch back over to streamers or big wet flies. I'd imagine this has a lot to do with temperatures and maturing insects as they become more widespread.