Without too many occasions to watch beavers at work, if I wanted to construct a dam on a stream on my property, which of my two choices would make me dam happy.
Divert the stream and build the dam. Correct the diversion and send the water to the dam.
Put up a temporary dam up stream. Build the dam and then release the temporary dam.
…and are there more choices? I ain’t talking Hoover size, just a four foot deep type.
…and I don’t know why I’m adding this, but, this I have experienced. When a beaver slaps his tail on the water as a sign of danger to other beavers, if you don’t see him do it, you will think someone fired a high powered rifle at you.
Do you have water rights for diversion and capture?
What kind of flow rate are we talking?
What materials were you thinking of using?
It sounds like your two ideas overlap, at any rate. To divert the stream, as in #1, you would need an upstream cofferdam. Or if the flow is low enough you could pump the flow around the construction. There’s also the possibilty of a siphon from an upstream pool down to a position below construction.
How deep and wide is the stream right now and are the banks high enough to help you? If it is fairly shallow, you can build the main part of the dam above the water level and then build the lower part separate so you can slide it down the front of the high part into the water if you follow. The deeper it is, and faster it is moving, the harder it will be.
Don’t underestimate how strong your dam needs to be for “only four feet deep.” A four foot wide by four foot deep dam will have a total pressure of over a ton bearing on it.