Hi Steve and ENTS,
In response to the large cottonwood Steve mentioned I thought I'd post a monster cottonwood that I saw a few weeks ago on a trip to the Gila National Forest, NM.  Definitely not an eastern tree, but we've had discussions regarding western trees in the past....
 

23' CBH Cottonwood on the middle fork of the Gila River, NM- nothing for scale, but this was an enormous tree in the high desert

not sure of the height

 
~Eli