The cottonwood (Populus deltoides var. occidentalis) is a large tree of the poplar family that likes sun but also likes moist soil. A cluster of them out in the countryside often signifies a water source nearby.
It's early June, so 'tis the season when the cottonwood trees release their seeds in cottony tufts that waft around in the air, befuzz your dog's water dish, and in some areas accumulate like an early snowfall on the green grass. This scene is on 9th Street, between Water and Division, earlier today.