From Will Ryan in his book "Smallmouth Strategies for the Fly Rod":
"With the coming of spring, smallmouths rediscover their appetites. Early spring presents a good opportunity to catch a trophy, as prespawn females often can be located on the outside edges of of spawning areas. As the water temperatures reach the upper 50's, the bass move on to gravel spawning areas".
He also talked about reading about someone catching a smallmouth in 42 degree water and was skeptical about the article in the magazine. My guess is 50 degrees though I won't know for sure for 2 to 4 weeks. We have some warm weather in the forecast in a few days. When the water reaches 50 degrees here in town, I'll be downstream on Little River in a canoe stripping streamers.