On one of my spring fed ponds, I used a backhoe to core the areas with springs up to the full-pool level of the pond. Only when the pressure in the ground water allows the spring to flow/seep over this core, will it fill the pond, and it prevents the water from leaving the pond. In essence, it is a one-way valve. So far it has worked very well, although in last year's drought, that pond did drop two feet. My newest (incomplete) pond also has springs and I am not planning to core in front of them. I may regret this later...
