I second the suggestion of a flashboard riser system. Using that you will still have full flow through the culvert pipe in case of a large rain event, but you can adjust the level of the pond as you wish.

Submerged version to use where beavers are present:
https://www.researchgate.net/figure...forested-welands-or-other_fig3_235985094

For where beavers aren't a problem, these are precast:
https://www.briggsconcrete.com/collections/concrete-flashboard-risers

Using the precast images, you probably can drain the pond down a bit, form one up on-site and pour in place. Use tongue and grooved boards for the "boards" but to span the 5' width, they might want to bow inward. If it was mine, I'd do a "pour on site" style, utilizing a brace in the center of reinforced concrete, so you have two boards side by side, each covering a 2 1/2' wide opening. You could use some thin foam on the bottom of the groove to help seal it and also along the back (downstream) side of the boards. Put a double eye hook (upstream and downstream) at the top center of each board that will give you a handle to put a hook in to pull them up to let water out if needed.

www.hoosierpondpros.com


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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).