At about what water temperature will smallies begin moving from main river haunts (referring to non-tidal, "upper" rivers above fall lines) into major creeks or tributaries?

If it helps any, I'm thinking of smallmouth rivers here in northern Virginia west to the shenandoah valley, but I'd assume the temp (not the timing) would be similar in many places. Thanks for any info.

Around here in NY it happens at the same temps as all other prespawn activity, between 45-50 degrees. They'll stack up in the mouth first for a few days and feed heavily before they start trickling up to find suitable spawning areas. Very good fishing.