That of course depends on your season:
Spring: Spring is a glorious time of year in Idaho. Wildflowers are in full bloom, and day time temperatures are typically warm and in the 70s. However, the weather is quite variable and can range from cold and rainy with highs in the low 50s, to hot and dry with highs in the 90s. You should be prepared for cold, rainy days and hope for hot sunny days! Obviously, the later in the spring, the better your chances are for good weather. Bring layers!
Summer: Summer is typically hot and dry with day time highs averaging in the 90s and lows in the 60s at night. Temperatures can reach 110F or more during the hottest days of the summer. Fortunately, we are usually near refreshingly cool water to escape the heat. Sunscreen, a hat and protective clothing are necessities.
Fall: September is spectacular here with warm days and cool nights and mostly nice weather. Rain is a possibility though, so be prepared with cool and wet weather gear. During the fall/winter fishing season, expect temperatures in the 30s to 50s during the day with rain and wind distinct possibilities.
Please note: Weather at higher elevations above the river (hiking, biking, horseback riding) can be significantly cooler even in the middle of the summer. Please bring appropriate attire if you plan to recreate at higher elevations.