May to Early July. Use the Enviroment Canada Guage "Similkameen at Princeton". I havnt done that section yet but Im told over half is flat. Theres also some issues with the new land owners at the take out and now you have to continue on another 15 km making the run over 38.5 km, thats almost 42 Miles. There is another section, the "Below the Falls Section" just up river that looks really fun when its higher water. Im going to try and do a multi day on the Similkameen and combine the 4 runs in the "White Water Southwest British Columbia"
If your in the area I highly recomend hitting up the Ashanola as well, and the falls and short run on Copper Creek.
Here is some vid from Copper Creek and Ashanola River right by the Similkameen.
Later,
Ryan