Crappie are more likely to spawn in the backwaters or creeks I would think. They need warm water and the proper river bottom soil to lay their eggs in. The males will pick the best spots and go on the beds to get them ready for the females.

A river Being a Lotic System is ever changing as Temperature of the water can change with a rain of cold water. And the amount of sunshine and air temperatures can change a lot in the early spring. The water that drains off the land into ditches and then into a creek that runs into the River can change temperature when it rains in the spring.

Find an area out of the river's current. A place like Oxbow lakes is ideal for catching crappie. Inland Marina can also hold fish as they are out of the current mostly. Shallow areas at the head or tail end of an island that have structure such as steep drop offs with shallow water shelfs nearby can work too. The island will slow the current down and allow the fish a place to rest out of the main current.