The bath in question, properly called a flume, is a water-filled receptacle 3 metres by 1.5 metres and 50cm deep, across which carefully crafted trains of ripples can pass.
The rain was falling, it felt like forever, it just kept coming and coming and coming which then was melting or snow melt, so together it just made this huge flume of water overnight.
Long will it be ere these marks are effaced, though Nature is doing what she can, replanting, gardening, sweeping away old dams and flumes, leveling gravel and boulder piles, patiently trying to heal every raw scar.