Just look at the calendar.

A lot of trees have had enough growth with the long season and they're just closing down for the season. Note that the winter buds are now forming in your first picture.

This is the time of year when roots elongate up to 60% of their yearly growth. All the way until winter cold stops that. So don't let them get too dry if no rain but their water requirements are really quite low compared to what they needed during the summer months.

Your trees look very healthy and will grow even better next year. :-)

Early yellowing of leaves is sometimes (like this year) caused by heat and dryness, depending on the species of course. If your leaves are turning yellow, not brown, then the tree has slowly extracted the nutrients out of it and "giving up" on it -- callousing it off. Seems like it's prb'ly not a serious issue, as Bill says.

A gallon or two of water a week for a plant is not enough when it is hot and dry out. You need to soak the soil around the root ball. Can you turn a hose on slowly at the edge of the root ball and let it saturate the area?

If the leaves were dark green a week ago and now are yellowing, this time of year, the tree is mostly done growing on top for the season. It will harden off basically.

I agree cottonwood can take a lot of water, but they do grow in some of the dryest areas of the country that I've been to.

If you really want to know how to grow trees up to their full potential, you should have a large diameter, grassless area around each tree with a 2" deep, wood mulch covering the soil to protect the roots from heat of the sun.

The way it is now, if you water a large area around the trees shown in the pictures, you will be mostly watering the grass, without much water reaching the root zone. Grass uses more water and soil nutrients than you might think and will restrict growth during a hot dry season.

That said, at this time of year, I wouldn't overdo the watering. Moist soil is good for root development, sopping wet is not.