If it's a manmade lake it should have some sort of an old creek or river channel below, but not always. Crappie can hold on very subtle changes that seem almost insignificant to us.
I know on the lake that I fish here in Arkansas, when the water temp is in the low 50's, I rarely have to go deeper than 15 feet. But our lake is pretty dingy most of the time.
Crappie will definately key in on any available structure that they can find at the depth that they want to locate at.