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You were correct to identify it in the populus genus, however I am pretty certain this is a cottonwood. The young growth on cottonwoods is pale gray / white in color. On a young tree the bark will look like it does in the picture, with darker, furrowed bark appearing on the trunk as the tree ages. On a mature tree the outer branches will still be white. In the picture we can see mature bark developing around the base of the trunk.
This page has some good pictures of immature and mature cottonwoods. http://www.stevenkharper.com/blackcottonwood.html
Edit: Although this is probably an Eastern Cottonwood. Source- I am a trained forester.