Only Sambucus racemosa is present in Alberta: https://anpc.ab.ca/wp-content/uploads/2019/04/Misc-Small-Dicot-Families-2019-04.pdf (Adoxaceae)
It looks pretty flat to me from the photo. Anyways, it resembles me much more this: Sambucus Nigra (Elder, Elderberries) - Trees Please than this Sambucus racemosa - Wikispecies But photos can be deceptive, so I agree, it is possibly S. racemosa. Elderberries are relatively easily spreading into the wild from parks and yards, so meeting of non native species in the forests is not very rare occasion.
If there was significant spread from a park or yard into the wild, it would have been included in the linked PDF above. Lake Louise is well within a national park, it is doubtful that there'd be much spread there. That said, there are species that do--but they are mentioned in the keys, if so (e.g., Sorbus aucuparia in the Rosaceae key: https://anpc.ab.ca/wp-content/uploads/2019/04/Rosaceae-2019-04.pdf )
According to this: Burke Herbarium Image Collection S. cerulea aka S. nigra ssp. cerulea is present in Alberta.
Eh? It doesn't say that. From that page: "Widely distributed on both sides of the Cascades crest in Washington; southern British Columbia to California, east to the Rocky Mountains." Also from that page, a link to collections of Sambucus cerulea made in the Pacific Northwest. True, there is one that seems to be in Alberta, until you click on the record name and read the collection locality description which actually puts it within 200km of the Pacific Ocean near Vancouver. If you need further information (and this site would includes naturalized specimens within its data): list of Sambucus in Alberta. It notes two species in Alberta compared to the Alberta Native Plant Council's one, but that's because it uses the treatment of Sambucus from the as-yet-unpublished Flora of North America which splits out the red-fruited and black-fruited morphs of Sambucus racemosa into Sambucus racemosa and Sambucus melanocarpa. It absolutely does not include Sambucus cerulea.
