Hi Paul, thank you for a very detailed and informative reply, you have clarified a lot.  The shoreline scenario you describe is, I see now, scarily probable, since the river silt is indeed very fine and muddy.  Meantime, major update:  this time of year happens to coincide with some of the lowest tides, and in spite of the boat having been moved to deeper water, obviously not deep enough.  The boat again temporarily grounded (in fine silt) so keel in the mud and about 18″ of hull exposed below the waterline.   This has revealed that there is a gash in the hull about 2-3″ long, as opposed to a lengthy separation line, and it doesn’t look like it lines up with keel/hull attachment seam.  I suspect that at some point, something sharp either rubbed a hole or pierced it, so it looks like just beaching at high tide, supporting on the keel, and then grinding away at the gash to get to clean material and then patching might solve the problem. See photos to view.

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