Interesting use of the mask. Thanks for the idea.
I played a bit more with masking the falls area (masking blur), but there are sharp areas such as rocks, ferns, as well as the moving falls around them all. Image above was just small portion of the entire area since people were also in the frame. Some of the falls were falling off some long ferns too. Tougher area than I thought it would be. To try and fix a lot of photos would be a lot of work.
Best is to not use electronic flash near them, and try and use maybe a tungsten fill of some sort. Would have to be a lot of lumens, maybe 12,000 or so. Getting a reflector in there would be hit of miss since the leaves in the forest canopy were dense and just spots of sunlight in areas.
There are some slow-burn flashbulbs made in Ireland that burn for 2 seconds that could work too, just they cost $65 per flashbulb. Some absurd guide number of 5000 at ISO 100 and 1/6 second off them too. Evidently they use them in those high-speed auto crash photos into barriers for safety testing of impact damages where they need a couple of seconds with some high-speed cameras running.
SG