Once a pond has been constructed, the beavers simply patrol the perimeter listening for any flow of water. If they hear any, they simply push mud from the bottom of the pond up ahead of them to stop the flow. By continuing this process, the pond continues to grow as long as there is enough water being supplied to the pond to make up for leakage and evaporation. I don't believe they understand all that; they simply have this built-in directive to "STOP EVERY LEAK" and then take advantage of the results. The constant growth in size of the pond is fortunate for them, because they are constantly killing all the trees that are anywhere close to the edge of the pond for the tooth-shortening and food supply reasons mentioned above. They need this constantly growing pond so they can get to the farther-away trees without ever being too far from the safety of their pond.