As long as the spring flows more water than what is leaking out, you will keep the pond full. Think of a bathtub. Turn the faucet on 1/2 flow and pull the plug, the water level drops. Turn the faucet on all the way and the water level stays the same or rises.

Remember that it's not just one layer of compacted clay, it's best to do 3-4 different 6"-8" thick layers, compact each layer and then add another on top, compacting it.