Very-closely related to sister-species
balsam poplar Populus balsamifera.
So close, that some sources call it a subspecies of this plant, Populus balsamifera subsp. trichocarpa.
Both are part of
aspen/cottonwood/poplar genus Populus
in
willow family Salicaceae.
Native to western 🇨🇦 Canada and 🇺🇸 USA, from Alaska and British Columbia, to the Pacific Northwest, Rocky Mountains and California.
🌎︎ Map (Western North America). 
In the North and East of this range, black cottonwood overlaps with a very-closely related sister-species
balsam poplar Populus balsamifera,
which then extends further North, West and East, into Alaska, the Great Lakes and East. Some maps confuse or combine these two sister-species.
Uses by native peoples
(Ethnobotany database)
Populus hosts caterpillars of 367 species
of butterflies and moths, in some areas.
This plant is also known to be a host for (in areas where invasive)
🐝︎ spotted lanternfly (SLF) Lycorma delicatula.
Often grows in clonal colonies
[1]
—look around for other stems!
Propagation protocol.
USFS propagation protocol.