There are two cottonwoods in SB, this one is more common in the front country. The two may be distinguished by the shape of the leaf, the edge of the leaf for this species is smooth, while the other has a serrated leaf.
The name “Cotton”wood comes from the fluffy white puffs that attach to the seeds when ripe.
Cottonwoods are deciduous trees with the leaves turning to bright yellow in the fall and then dropping. In 2015 leaf buds had appeared by mid-February when I first noticed the blooms.
Note the smooth edges on the leaf. Compare to the leaf of the Fremont’s Cottonwood.