with leaf scars=:
narrow raised;  terminal buds=hairy;  buds have hairy inner-scale between 2 outer-scales  or
crescent-shaped, with 3 bundle scars, centered above is side bud scale :

Leaf scars=crescent-shaped, with 3 bundle scars, centered above is side bud scale

terminal buds       > 25 mm (>1 in)  or
terminal buds about 12 mm (½ in) :

terminal buds about 12 mm (½ in)

terminal buds egg-shaped;  twigs=gray-to-brown
swamp cottonwood  Populus heterophylla
terminal buds ♦ taper at both ends;  twigs=yellow-to-yellow-brown
eastern cottonwood  Populus deltoides