Southeastern PA

I'd call this an "estuary or mudflat" as it is alongside/near the Susquehanna River in southern PA between York and Harrisburg. Easy to reach from I-83.
Really having trouble with this pointer on the map, so do NOT go by these coordinates. Contact me if you want directions.

Several good areas with dark brown mud, some clay, some with vegetation. No vast mudflats, but nice patches, a pretty scenic, wooded area.

Knee deep in many places, but that's about as deep as it gets. You proceed along a road near the power plant and a short hike takes you right to the good ares. Fairly secluded right there, not a place where people are hiking through. Everything around it however is busy, so discretion is key when coming/going. Some areas are visible from the road, others are completely hidden. Even in the driest of summers I've found good mud here. Cleanup is right in the stream, so you can get muddy, rinse, and repeat. The area floods after a heavy rain, but drains off, and some areas that are normally dry have good clay slop just after.