You may do better than you think with your seasonal trout. I have a 20 x 40 pond in my back yard that has a max depth of 4ft that I am able to keep trout in from October to Late May. I am curious if a spring pond of similar size (or smaller) could hold them through the summer.

What is the GPM of the spring? Not sure. If you dig a hole it fills in short order though.

Do you know what the temp of the water is? How about during the summer?

Water temp is in the low 50's coming out of the ground year 'round.

Can you aerate? No spring is in a wooded area.

Does it matter to you what kind of trout? Doesn't matter what kind although I prefer Rainbow because they are easy to come by.

Do you have a water source that can add to the spring flow if needed? I don't not however the spring has never been dry and there are records back to the civil war era when it was used for a pottery factory.