Riparian tree, I saw this same species at Rattlesnake Creek and Jordan Creek.
In underbrush under deciduous trees in partial shade. Alongside blackberry bushes. Single bush (or sapling?) maybe 1m tall. Alternating leaves. Color of leaves is bright yellow green and veins are darker green. Edge of leaves slightly serrated. Top leaves about 7 inches long, 3.5 inches wide. I only saw one in this location. The color stood out among the other plants.
Populus trichocarpa (black cottonwood; also known as western balsam poplar or California poplar) is a deciduous broadleaf tree species native to western North America. It is used for timber, and is notable as a model organism in plant biology. Its full genome sequence was published in 2006. It is the first tree species to be sequenced.