Gretchen:  Thank you for contacting the UVM Extension Master Gardener Helpline with your question regarding the leafing out of black locust trees on your property

You mentioned that these trees are planted along the Lamoille River.  They may have been planted as a riparian area cover along the riverbank for erosion control because of their massive root system.   If this area was flooded during last year's growing season, the flooding may be the cause of this year's problem.   When trees are under water for an excessive length of time, the roots are deprived of oxygen, causing the decline of the tree.

How to manage flood damage to trees | UMN Extension
Management of Floodplain Forests | Natural Resource Stewardship (iastate.edu)

I would suggest watching the trees and see if they eventually recover.  You may also want to consult an arborist in your area, if you are still concerned with the health of the trees.

Find an Arborist (treesaregood.org)

Note:  Black Locust trees are not native to New England, and they are extremely invasive, and, in many States, outlawed.   (See the listing of Vermont Invasives, below from the Vermont Invasives website and the Vt Dept of Forests).  Therefore, if you find that the trees are failing, you may want to consider their removal, and replacing them with other trees used in riparian areas, such as willow shrubs.  Black Locust trees are very difficult to remove because of their excessive root system and their ability to sprout new growth from the root cuttings. 

I am also attaching a link to a University of Maine document suggesting alternatives to planting invasive species.

Black Locust | Vermont Invasives (vtinvasives.org)

https://fpr.vermont.gov/forest/forest-health/invasive-plants

Microsoft Word - Buffer Plant List Final.doc (maine.gov)

I hope you find this information helpful.