I guided Friday (twice), Saturday, and floated (with my son and nephew) Sunday (today) this weekend. I have been carrying a thermometer with me since June 1 this season, since the water temperature and flows have been a concern. I took readings each day, and at different times of day. My thermometer is no cheapo, it's actually a digital thermometer calibrated to +/- 0.1 degree F that we use in the winery.
Friday, 9AM Umtanum, slow section on river right - 59.8 degrees F
Friday, 6PM, just below Lmuma Creek in a slow inside bend: 63.8 degrees F
Saturday 11:30 AM, Just above Bighorn, quick outside bend - 60.8 degrees F
Saturday 2:15PM, Reds takeout, slower water - 63.1 degrees F
Sunday 1:30PM, Just below Red's, shallow riffle - 62.6 degrees F
Sunday 5:30PM, Mahres takeout, slow moving sun exposed water - 65.1 degrees F
All of these readings were taken about 1 foot deep, so it is probable the temps are actually cooler than that, as the surface area is most likely warmer than the water closer to the bottom of the river.
I think that temperature gauge at Umtanum is not functioning correctly. Not only are the temps high, but there is very little fluctuation in the temps. The water is not that warm, and if you look at the Ellensburg temp gauge for the Yakima the temps are fluctuating at least 6 degrees per day (and getting about 63 degrees F max). The temps from the link above are barely moving within a day, which seems a little fishy (pun intended).
I carry the thermometer because I agree, we should not be fishing if the temps are getting up near that 70 degree mark, and I want to make sure for myself that they are not, whether guiding or fishing for fun. I think that everyone that cares for this resource can agree with that. That 65.1 reading I took today is the highest I have taken all year, and it was in an area where the water was barely moving.
I think we are well within the fishable temperature range in the canyon. Additionally, it's OK to fish when the temps are in the lower 60's, just a couple degrees above the ideal temperature range of Rainbows. Use heavier tippet, fight them fast, try not to touch them, and don't take them out of the water. If you are having trouble getting the hook out, cut the line close to the fly. It will come out quick enough after the fish swims away, and it will reduce the time the fish is in the net and/or that you are "handling" them.
On a brighter note, fishing was good today. My son (6) and nephew (8) had multiple rises to their dry flies, and I was able to bring one to the net. Fly popped out while it was in the net, I turned the net over, and we all happily watched him swim away. We have had a nice little cool down from our 14 straight 100+ degree days, so that should help as well. It topped out at 81 in the canyon today.
James