Rain almost always helps to improve the crappie fishing. Just before a thunderstorm, crappie will turn on and feed very aggressively. During a heavy storm, dams, spillways and flooded timber can be great areas to target crappie. In rivers, a big thunderstorm can cause very heavy currents, which will usually make the fishing much tougher. The fast currents and murky water don’t mix well with crappie. The key is to find flooded backwaters and shorelines that are out of the current. Add some timber and brush and the fishing can actually be very good. In the rest of the river, fishing will typically be terrible until the water recedes.
In lakes, you may not have the option to fish a dam, spillway, backwater or shoreline with flooded timber. A very heavy rain may stain the water and make fishing tough for a day or so. On lakes that have tons of good shallow water cover, a heavy rain may actually help you locate crappie faster, which results in better fishing days.