For sediment, the other idea besides a smaller pond upstream that can be dredged periodically is to pull a side stream off the stream. Let the stream go past the pond not directly into it. Dam up the creek and Pull a 4inch pipe of water perpendicularly from the side of the creek upstream of this dam and run this to your pond. You still get fresh water but most of the stream and silt goes right past your pond.
If not and you want to do the sluice idea through bottom of the dam to periodically drain the pond and let it flush silt out, buy a flash board riser overflow. Available commercially and allows height of water to be adjusted by adding boards or remove all of them to drain pond.
