Black pond expanding foam works. Spray, let dry, carve fall. Once done and tested can cover with an oatmeal consistency slush made of Titebond III and shredded coco fiber.
I thought that using wood glue and/or silicone with running water will eventually cause it to fade or completely be washed away until only the foam spray is left? I thought you had to use drylok or hydraulic cement for waterfalls?
“Life is a journey, not a destination.” -Ralph Waldo Emerson