There could be several reasons they may be declining from moisture, age, disease, and or insects. When cottonwoods are in poor health beetles can be an issue and the best methods to help the tree is to improve overall health. Watering the root system which may be very large if they are older and adding additional water if it is a dry winter.
There are products out there to help treat for beetles, but if the health of the tree is not good the tree will continue to decline and treatments for the beetles generally need to be done each year if they are the main issue.

I have added a few information sheets that will provide you with more insight and options to chose from. Please let me know if you have further questions after you read through them.

Here is information on common beetles:
https://extension.colostate.edu/topic-areas/insects/shade-tree-borers-5-530/

 https://agrilifeextension.tamu.edu/library/landscaping/wood-boring-insects-of-trees-and-shrubs/   

Watering information:
  https://extension.usu.edu/archive/tips-for-thirsty-trees    https://extension.colostate.edu/topic-areas/yard-garden/fall-and-winter-watering-7-211/