Could someone identify this shrub?

I found a shrub growing on the shore of a pond/lake (not very big) in northwest Montana. The fruit is a shiny black, almost looking like huckleberries. The leaves are shiny green, but were turning a brilliant yellow. The shrub was about 4 1/2 feet high with grey/brown bark on the twigs. It is deciduous. I think it is an Alder Leaf Hawthorn, but I cannot find any reference to this shrub name, nor can I find any photos that remotely look like this. Based on what I have read, black fruited Hawthorn grow in Europe, not the US as a native species, so this could have just "wound up" there, but it was fairly remote. It is not a high bush huckleberry because the leaves are far too big. The leaves are about 2 1/2 to 3 inches long and 1 to 1 1/2 inches wide in the center. Thanks.