The beaver enters the den from underwater and then travels up into the higher portion of the den where it can breath air. Below is a drawing of a den's struture.
Before I noticed the den I had noticed the effects of a beaver dam on the stream leading into the lake. What is usually a small stream that has just little flowing water in it is now full of standing water. This is a sure sign of a dam.
The area has the signs of the beaver chewing down small trees in the area.