Welcome NADam!

Placing a dam in a river is a serious engineering project. Every single flood will try its best to destroy your dam.

If you read posts on Pond Boss to get more background information, most of them concern much smaller dam projects to collect surface rainwater that is partially funneled into the pond site by the topography of the land. Even then, some dams get blown out just by heavy rains on the 20 acres of land upstream of a 1 acre pond.

It might be a $1,000,000 (U.S.) project to build a lasting dam for the size and location you are describing. Will you be getting a benefit of more than a million dollars?

If not, my advice would be to scale back the project to more appropriately match your exact needs. Is irrigation your main benefit? If so, will you lose valuable crops if the river recedes during your growing season? In that case, you might calculate your annual water needs and design a pond to match.

Do you have a much smaller waterway on your land that has a narrow constriction between two hills? That might make a better pond site. Especially if you could place that dam a little higher than the flood elevation of your river. (You don't want the floodwaters gnawing at the backside of your dam, but there are measures you can take if your best site for a smaller pond does fall within your floodplain.)

Even if your pond does not quite capture enough surface water for your annual irrigation needs, you could probably pump water from the river and top off your pond at any time prior to summer drying up your river.

Of course, you have LOTS of other options! You could just dig out some deep pools adjacent to the river course during the dry season. That would be an inexpensive option, but you could capture a lot of water. Not great for your to raise your own fish, since they will travel during the next flood. However, if the river has ample fish, then they should collect in your pools whenever the water level in the river recedes.

What exactly are you describing as your "boreholes"? Are those your irrigation wells? (That term can mean a lot of different things in the U.S.)

I am NOT a dam expert, but I am a geologist. If you start getting some samples of your subsoils on your property for a dam site, I can at least give you some decent advice in that area.

Good luck on your big lake project!