It could possibly be a fungus and the wet clay soil does not help the situation.  Right now you need to collect all the fallen leaves and dispose of them.  Do not compost.  The fungus will over-winter on the leaves.  You could treat with a fungicide next spring just as buds begin to break.  I will give you the information I have found but it may be best to have a certified arborist examine your trees for an onsite evaluation:  https://www.treesaregood.org/
Here is some of the information I have found:  file:///C:/Users/owner/Downloads/aspen-leaf-spot.pdf

Treating Aspen Blight

Once you see the blemished, blackened leaf on an aspen, it's time to start a
treatment plan to salvage next year's growth. Remove the infected
leaves and reduce the spread of spores by:

Defoliating blackened leaves in the fall.
Pruning back infected branches in the fall.
Raking away and destroying fallen infected leaves to remove spores from the immediate area.
Keeping moisture to a minimum.

Reducing the spore spread is the first step, but if fungal diseases persist the next growing season, you may need to try a more aggressive tactic such as applying a fungicideat bud break. If you're treating an aspen for fungal disease and the weather is wet, repeat fungicide application every 10 to 14 days.

Thank you for the input! I was able to get in touch with an arborist and will have them come out to assess.