Hello,

The skeletonized leaves show damage consistent with that caused by the Elm Leaf Beetle.  https://extension.colostate.edu/docs/pubs/insect/05521.pdf.  Both the adults and larvae cause feeding damage to the leaves.  

Insecticides can be employed in three ways to manage elm leaf beetle: 1) use of soil drenches/soil injections of systemic insecticides that move to kill insects on foliage; 2) use of sprays covering leaves to directly contact and kill larvae and adults (professional applicator required); and 3) trunk
 banding with insecticides to kill larvae as they move to the base of the tree to pupate.

• Soil applied systemic insecticides containing Imidacloprid (Bayer Advanced Tree & Shrub Protect & Feed Concentrate II, Bonide Annual Tree & Shrub Control, ferti-lome Tree & Shrub Systemic Insect Drench; Ortho Bug B Gon Year-Long Tree & Shrub Insect Control, Merit, Mallet, Zenith, Criterion, others) or Chlothianidin (Bayer Advanced Tree & Shrub Protect & Feed Concentrate II, Arena) can be applied by homeowners.  
• Pyrethroid insecticides, such as bifenthrin, permethrin, and cyfluthrin, have shown effective persistence when used as trunk bands.  Read and follow label instructions exactly.

The dead branches in the second photo could be the starting point for the beetle or could be due to another cause.  I do see frost cracks or injuries on the trunk that could be related.  Consider bringing in a sample to the Jeffco Plant Diagnostic Clinic for confirmation ($7 fee for Jeffco residents).

Reference:
Elm Leaf Beetles  https://extension.colostate.edu/docs/pubs/insect/05521.pdf

Let me know what follow up questions you have.  Best regards,