TDUBYA, after thinking about it, the situation in your pictures does not look that different than the small seasonal stream running through our pasture. My wife wanted to dam that up till I explained to her that the cost of the overflow structure alone would likely exceed our net worth and that the cost to permit after consulting the Corps of Engineers and getting EPA approval (so they don't come back later and say we destroyed something important to them) was much more than I wanted to tackle.
What we did instead is a made a nice watershed pond with the back of the dam following the creek. It looks like you have plenty of options other than that creek to make a nice pond adjacent to it rather than trying to dam it up. You would get away from all the problems of trash fish entering via the stream and by limiting the watershed not have near the engineering challenges with so much water flow.
But it is your project. That is just the way I would look at it if it were my property.
