Kachess Lake is a beautiful place to paddle in May and June, before storage control. In the late spring of a good water year, this huge sheet of clear green water, surrounded by old evergreens, with views of steep mountain peaks is amazing!
Assuming that Kachess is completely full, you should be able to paddle right up to this cliff face on the east side, reach three feet up, and grab the soda tube strapped to the small maple tree. If it's at least above about 200,000 acre-feet, landing and climbing to the cache should be relatively easy.
Check storage here 
After storage control (that's when demand for irrigation water exceeds the runoff into the Yakima River), the reservoir level starts dropping rapidly, and the bank becomes dusty brown rocks. Algae starts growing in the warm water, and the lake loses its springtime clarity. Late in the summer, the water level may be 60 feet down (or more). You may have more trouble getting to the cache, at that time!
My advice is: High Water Only! It's prettier then, too!