Ideas to test and develop spring

I believe I have a spring fed pool in the forest of my property. It's sits very low, below all the surrounding ground level.

Evidence it's a spring: never dried out, even in drought. Never froze over, even in sub zero winter. Always reasonably clean, never scums over. Always some water trickling from the outlet.

The pool is roughly 15 feet in diameter and flows out through a 5 foot wide, steep sided creek bed. One bank is 5 feet tall, the other is more like 20 feet tall (diagram attached.)

My question: How would you investigate this further and develop it? At the very least, I'd like to have a swimming hole or at least more of a pond.

I'm wondering how to measure the flow and thinking this might require a temporary dam at the outlet that funnels water in a way I could measure it since the spring itself is underwater.  
Also open to suggestions on how to make a more permanent dam. Not a lot of rocks available here. Couldn't be more than the 5 or so feet high.
I am not worried about legalities since it's so low flow, hidden, and water rights aren't such an issue in NW Missouri. Also not in a place that could cause any downstream damage in the event of dam failure.

Think of the water flow as an underground river, running over a floor of dry clay. Not tampering with the dry clay is important and will determine the depth you dig if you decide to make this hole bigger. At my place they drilled core holes to find the water stream. In my case it was a 1" sand seam that carried the water. Under that was dry clay. They did not determine how deep that clay layer was because if they punched through it,  it would be a big problem (the water would drain out the hole).

On the dam, they dug a trough down to this dry clay layer then filled the trough with clay. Then added clay above ground level, then covered that with soil. The soil moderates the moisture,  keeps clay dam from cracking.

After this, you can theoretically make the pond as long as you want, as wide as the dam permits, and as deep as the dry clay allows.

I hope that all makes sense. Clay is a % of clay. I cant remember the number. As they dug out the pond they did it laboriously, sorting it as they dug it out. Topsoil went here, clay rich soil got piled there. The sorted soils were used for the dam.