A surprise partial skunking on a little mountain stream

I had an interesting experience yesterday evening (July 31st) fishing a very small stream on the east slope of the Cascades above 3,000 feet. When I was there for the first time eight days earlier, I was catching a pretty little westslope cutt (or five) in every likely-looking pool and pocket. Yesterday, fishing adjacent reaches at the exact same time of day, under similar weather conditions, I saw no sign of any trout. Not so much as a fingerling nipped at my fly, and no shadows darted away when I waded into a pool.

I'm new to this area and cutthroat fishing, but I've fished a lot of small streams elsewhere, and they've always been really consistent unless there's some obvious weather or seasonal reason why the fishing would change. To have a piece of water that was teeming with fish a week ago seem totally empty now was really surprising. At first I thought maybe I had gone in too far upstream above some impassible barrier, so I went back down to where I left off catching fish last week, and there was still no sign of fish in several pools that should have been full of them.

Finally, I guessed maybe the fish were making some spawning-related movements even farther into the headwaters, although that seemed unlikely. I should still have seen some immature fish scattered throughout the lower reaches. Nevertheless, I drove up even higher to try another reach, and the stream was back to normal: eager, beautiful fish everywhere I expected them. Fishing was great until dark.

I don't think the fish I caught all moved up from the reaches where I got skunked, because there are just too many barriers to migration at this water level. Temperature doesn't seem to fit as an explanation, either, because the water was plenty cool in this shady headwater throughout both trips. Hatches also don't seem to explain it; they're rarely important in streams of this character, and only tiny midges were abundant.

So I'm kind of out of ideas and wondering if anyone experienced with these fish and small Cascades streams has an explanation for the odd shift in action. Of course, I'm not complaining about briefly not catching fish -- but solving the mystery of some unusual pattern is part of the fun.