There is really nothing mysterious about it. The collection is the brain child of couple of private boaters who are married and have been down the canyon about 60 times between the two of them.
The collection started by accident as one of them found a heart shaped rock upstream (carried it with them for a few days) and then found a larger one at the site of the collection. They felt compelled to place the two rocks together. Then as more years past, and more trips, more rocks were found by the couple and placed on the site.