Man-Made reservoirs usually have very little aquatic vegetation, especially if the water levels fluctuate a lot. Even stable reservoirs can take over 50 years to develop aquatic plant communities. These lakes are built over previous uplands and they do not have the seed stock of aquatic plants to populate the lake with vegetation. Black crappie rely on aquatic insects for a larger portion of their diet than white crappie do. Aquatic vegetation is essential to producing substantial numbers of aquatic insects. Aquatic vegetation can produce 100x the insects than bare bottom. While I don't manage Wappapello, I would be surprised if this isn't the reason there. Hopefully Dave Knuth will comment soon.