Riffe Lake is a man made lake that rests behind Mossyrock Dam in Southwest Washington. According to the information I read, the dam is the highest in Washington State (over 600'), so the lake is very deep and in the summer it is very clear water.
In my opinion, Riffe Lake is at least as beautiful as any lake in the state, or for that matter, anywhere. I don't know the exact length, or width of the lake, but I would guess that it is about 12 miles long--maybe more and maybe up to 3 miles wide is some places. There are coves and inlets all along the shores and even a few islands.
The main body of the lake is exposed to wind at times and can be challenging to paddlers, but there are plenty of protected bays to explore also. When the lake was formed, some trees were left here and there. The trunks still reach up from the depths and depending on the level of the water, these spires might lurk just below the surface, so it's a good idea to keep an eye out. This caution applies to bays and coves and generally not to the main body.
A round trip day paddle from end to end of the lake might be doable for sea kayakers and maybe robust canoers, but plan on some wind and waves.
There is a great park on the west end of the lake right near the dam. It is only a couple of miles past the town of Mossyrock. The park provides two boat launches and lots of picnic area that are free for day use. There is also extensive camp areas, which go for a nominal fee.