Completely defoliated now. The local poplar trees are not far behind.
Trees often shed leaves as a survival mechanism during extended periods of drought. They are not dying, just going dormant. Our Black Walnut trees in our woodland are very old and very beautiful, but they will shed foliage and look dead during long drought periods. The following spring they leaf out and are fine. Now, I'm not saying yours aren't dead...because even if they are not dead yet and merely are dormant, they still could die, but I am pointing out that they are not necessarily dead.
Once trees go dormant and drop their leaves, they are telling you that they are extremely stressed and are fighting to survive. Whether they survive or not depends on what happens after they drop their leaves. If they go on for many months with no appreciable rainfall, they may very well die. Sometimes they'll even leaf out the following spring and seem fine, and then die within a few months to a year. Other times, they'll leaf out and even bloom in the fall when rainfall returns, but they slowly die over the course of the next year. At other times, they'll survive but be significantly weaker the following year. When they survive but in a weakened state, they can be very vulnerable to pests or diseases. Sometimes they'll leaf out as normal the following year and will be just fine and show no visible issues as a result of the previous year's drought.
I never automatically assume a broadleaf deciduous tree is dead when it drops leaves in mid- to late-summer during a prolonged drought, because generally it isn't. Conifers that brown out more often than not are dying and cannot be saved once they are that far gone.
So, don't give up on your black walnuts, unless for some reason they seem to pose a threat to your home or people and you feel they must be removed. (There's a drought-related issue called sudden limb drop where entire limbs---often very large ones---suddenly drop to the ground without warning and, there again, the tree isn't necessarily dying when that happens either.)
We have been through several very severe droughts since moving here and often have had deciduous trees drop foliage and, to our great delight, have had them come back just fine the following year.
I do need to add that for many of us this is the worst drought our areas have experienced since weather records have been kept, so we ultimately may find we lose trees to death that we'd assumed were only dormant. Time will tell.
I'm linking an article that discusses how trees respond to drought. I think dormancy is discussed in about the fifth paragraph.
Here is a link that might be useful: Trees and Drought