I had a wonderful day wading the Yakima yesterday; but this is not a report. I have not fished the Yakima much in the Spring and need an assist in identification of a hatch. Hopefully I can get some working opinions from anyone experienced to assist me next time out. Before the sun set I had crossed over a very wide (whole width of the river) and shallow riffles area to get to an island in the southern part of the 'tree farm' area. The top of the riffles began at the edge of a very large flat (75 yards x 50 yards) and very shallow (2 ft max), with rounded, flat, fist size stones and smaller. While wading over it had seemed like a barren desert, with no holding areas, no boulders, etc., and no fish. About 6:15 p.m., past sun down and getting well into dusk (but I could still see the shallow bottom), I was wading back to the shore when I kicked up a huge trout right at the head of the riffle in about 18" of water. I start walking more carefully, trying to observe the lay the fish was in and any other details, when fish start rising all around me in the flat; probably 8 fish within a 45 ft half circle of me. Must have been a rise every 5 seconds, of very large fish and lots of them, throughout the flat. The rise appeared to be of fish taking emergers, i.e., most fish were not coming out of the water (a few were), but more of a roll or porpoise. My guess is that they were eating something a couple of inches below the surface. It was getting too dark to tie on a fly, so I tried the size 16 parachute Adams I had used earlier with some success, with no takers at all. No flying insects were in the air to indicate what might be hatching, and I saw nothing in the surface film. Hopefully some of you more experienced than I can give me an idea of what would likely be coming off at that time of the evening, and, of course, a hint as to what fly type and size might best do a passable imitation. It was a lot of fun watching the hatch going on, and hard to leave it, but I had a bit of a walk to meet up with my partner and get back to the car. I would like to be ready for it next time. Your help is appreciated.