This is a Siberian elm (Ulmus pumila). Also called Chinese elm. Branch dieback is common, probably caused by fungal cankers and insects. They also get bacterial wetwood, which is an oozing that comes out of the trunk caused by bacteria in the trunk. However, they are tough trees and this one doesn't look too bad. All it needs is an arborist to prune out the dead wood. I could be wrong, but it looks to me like you wouldn't lose too much of the canopy. They are very drought tolerant, but with the canal having been put in a pipe, you should irrigate it more.