Beavers always live in underground cavities that are not filled with water. The purpose of the dam is to raise the level of water enough to give them ingress and egress under water with safety from predators. They use deadwood and mud to make dams. Rivers with high enough water they don't dam, they just dig bank dens under the normal water level. They cut trees for food, not to make dams. I have see beavers set up summer residence near corn and move on when it's gone. That's what they've done in your situation. They have the safety of the sinkhole cavities to live in with easy access to the corn. No need for a dam. When the corns gone they will move on.
