I want to build a dam to raise the water level of an existing pond. Currently the pond is 30' x 50' and the dam will be on the long side, and would raise the water level another foot or so. However I don't have enough clay on hand to build the dam, so I was wondering If I could make a mix of the clay I have, bentonite, and the dirt I dug out of the pond. My plan would be to dig down below current water level, and build the dam up in lifts compacting as I went up. Would that work? If not does anybody have a better idea?

A couple of thoughts/considerations and you may already know these things. The top layer of the stuff you dig from the pond will likely, depending on age, be silt which is mostly useless. It has to be set aside. The added soils have to be knitted into the top with the existing soils. If not done, it will cause a seam that will leak.

Bentonite usage can be tricky. It might be less expensive to buy some loan of about 60 to 65% clay to knit into the existing top. Then get some grass growing on the top asap. Unlike my area of Texas, Va gets dependable rain.

Do you know of a particular method of knitting the levels of soil together to prevent leakage at the seam?

Mix it with a dozer or another piece of equipment. The point is that you don't want the seam. An additional consideration with bentonite or other clay is that it cracks when dry. That's why it needs to be mixed with dirt/loam.

I use a 4'x4' pull-type sheep's foot roller that I bought used (probably cut down from an 8' wide one). My first project with it was a 1/4 acre wetland on a hill with only 1/4 acre of watershed (built last year). It held water better in this year's drought than any of the other "professionally" built wetlands on my place (four others). The two built by INDOT are dry (but still good wildlife habitat). We will see how the new pond holds water after it fills... I pull the sheep's foot with a small dozer.

So would a bentonite mix retain water comparably to a dug clay in this scenario? Or would It be a waste of time, effort, and money? Actual construction of the dam is not my greatest concern at this point of planning, just whether or not a bentonite mix will hold water as a dam.

I would think so. I am no expert, but it appears to be one of the best clays to use.

"Bentonite is a fine-textured colloidal clay that will absorb several times its own weight of water. When Bentonite is mixed with coarse grained materials, then thoroughly compacted and saturated, it will fill pores in the material and make it virtually impervious."

https://www.michigan.gov/documents/deq/deq-wb-nps-ps_250894_7.pdf

Yes. Use a sheepsfoot roller, on layers that are 6"-8" thick and of the correct moisture content to pack well. Compact it until the sheepsfoot starts to "walk out" of the ground, add another layer and repeat.

If the layers are thicker than that, then you won't get the compaction.