Here's some interesting facts about the cottonwood:
The cottonwood can grow to be one of the continent’s largest trees – up to 80 feet tall, with trunks that can reach five feet in diameter.
Cottonwoods consume a large amount of water in their growth cycle. There are estimates that a mature cottonwood tree will use 200 gallons of water a day.
Cottonwoods live up to 100 years old. Their roughly furrowed, thick bark makes them fire resistant, which helped them survive the regular grass fires on the Great Plains in years past.
Cottonwoods help suppress fires. They soak up a lot of water and can be very valuable in community fire-interface zones.
Estimates of annual seed production for a mature, open-grown cottonwood have been as high as 48 million--the great majority of which are viable and ready to germinate.
Although cotton can be a nuisance, it has no allergenic properties. People with allergy symptoms during June cotton distribution are probably suffering from other wind-borne pollen - from grasses, weeds or other trees.
Here's a great article about the tree.