Hi John,

It's tough to say for sure without a photo, but chances are that you are dealing with Bacterial Wetwood, also known as Slime Flux.  

It's a common problem in a wide variety of tree species, including cottonwoods.  Wetwood is a chronic condition that can contribute to general decline of older trees, but many infected trees get along just fine with a minor infection, other than the ooze.  Wetwood can't be treated with chemicals; severely infected trees or limbs may need to be removed.  

I hope this is helpful.

Cordially,

John