With more observation it becomes obvious that clouds of the ravenous creatures constantly swarm in and out of several large holes spread throughout the riverbed. The closest of these holes on the eastern side of the swarm to the PCs lies about 80 feet from the swarm cloud's edge. The hole itself is a 5-foot-diameter opening. The swarms don't seem concerned with your presence, but their is no way to pass them down the valley. The holes seem the only likely way forward.