Here's my 1 cents worth. I have had wetlands reclamation work done and had a large number of small trees (2-10") removed with a forestry chipper. This is a different machine than a stump grinder but it also takes the tree down below ground level. I was impressed with the amount of wood chips (mulch) it produced. I suspect a stump grinder would create a similar type of chip, albeit, in less volume than the chipper; with a hundred stumps to grind in your basin, there will be a considerable volume of chips produced. These chips do not go away quickly on their own. I would be concerned with the amount of organic debris that might end up in your dam that could cause leaks or problems in the future. In order to grind the stumps, you will have to cut down and move the trees to a burn pile anyway. I agree with others above....take them out with a dozer, save a couple of root balls for structure and skip the stump grinder.
