Welcome to the forum, Guy. You have come to the right place to get the information you are looking for. The experts will chime in soon and give their opinion. I can offer a bit of generic advice until they arrive.
First, get on the Pond Boss web site and order Mike Otto's book, Just Add Water. Anyone who is building or modifying a pond should read this book before acting.
Second, with regard to the spring questions, the answer is "It depends". A spring is nothing more than ground water leaking out of the ground. If there is drought and ground water is depleted to a point that it is no longer at the level that it was leaking out, then the spring will go dry and it would no longer feed into the pond. My pond is mostly fed by runoff but also has a spring that varies considerably with regard to the amount of water flowing based on season and amount of rainfall.
Finally, springs can be a double edged sword. Water can flow both ways. Plenty of ponds have been built with a spring outlet near the bottom of the pond, only to have the water run out of the pond back into the the spring. Whether it runs out or not depends on how much pressure is forcing water out through the spring. If that pressure is reduced due to drought or seasonal changes, then the water can run out of the pond and into the ground and the pond level will go down.
Again, welcome to the forum!
