"The reason the Holes are there is that the waterfall has washed and eroded them out over time." The key sentence for some streams.
I have seen boulders the size of a VW moved hundreds of feet in a single spring flood on a small stream in the Catskills NY, that barely runs ten gallons per minute in the middle of summer. I have found car parts and culverts and all sorts of junk in the stream. One big pool has filled and emptied of stones the size of suitcases.
What you really need to find is a place where the water "slows down". THAT is where the items will be deposited. In a narrow canyon with spring flood waters rushing through, the items will be flushed along as if they were in a fire hose, even through the pools below the rapids or falls. Remember it is the action of the water and tumbling rocks that cut out those pools, the same action that can move and shift large rocks will either grind to dust smaller items or propel them downstream until the pressure drops and the heavier items can drop out of the flow. Areas behind big rocks too heavy to be easily moved by a steams spring flood will provide a "calm water" area just down stream of them where some items may drop out of suspension, gold hunters often use this trick. The other thing to look for is bedrock cracks that may "catch" items and prevent the water's force from dislodging them.