Making a marsh into a pond

We recently moved into a house that has about 3 acres of land. The land on the right side of the creek holds water even when it hasn't rained. About 200 feet to the right of the creek the land slopes up into a hill about 5-6 feet tall. There are two small streams that comes from a spring at the edge of our property. On the of the creek there is a area that is surrounded by a hill forming a edge for the pond. The problem that I have is the creek flows underground and goes underneath the property of a church. We thought how we are going to keep fish in it and decided that a grate wouldn't work due to plants possibly clogging it up. We aren't sure what we can use to prevent it from backing up.

Here's a video about how to make a pond specifically for fish: http://geofflawton.com/videos/perfect-permaculture-fish-pond/

About sealing a pond with ducks: http://geofflawton.com/videos/fixing-a-leaky-pond-with-ducks/

I was looking for a video which shows how the overflow pipe for the pond works, but I'm not sure which one it is..