Glenn Hhavbert wrote:How deep and how large do you want this pond to be? How high will the dam be above the toe of the slope? What kinds of soil do you have in the immediate area? Have you dug a test hole to the depth of the pond? If you have a reasonable proportion of clay the soil and are not trying to make a pond that is too large for the potential runoff to replenish easily, you may need no liner, just proper grading technique. If there is a perennially wet area, your chances are better, unless there is a layer of gravel below that.
If the aspens are not far downslope from the pond, you don't need to worry about their roots except to keep the suckers mowed/pruned back so the grove doesn't spread farther than you want. It would be big roots in the actual dam that would be a problem, and the farther you spread out the downhill slope, the less you have to worry. Do you have a supply of rock/stone that you can line the downslope spillway with? It will obviously have a greater slope than the current stable flow, so will likely need reinforcement.