I just read this thread but agree with others , SEAL THE BOTTOM ,
Use Masking Tape.
Now for additional tips:
Color the envirotex with food color, try a few drops and stir it in . It will look as though it will not mix but keep mixing and check the color , if you need or want to add more or another color ie: blue and add green for murky water , do so.
Now pour the lake or river and after puoring have some cotton balls handy . When you get the resin to the desired depth save the remainder and set asside. Now shread the cotton ball into small long pieces and insert them into the wet resin and poke them into it with a popsicle stick untill soaked and adjust it to form rapids in the water and repeat as nesissary.
Once dry , drybrush the tops of the rapids with white acrylics.
For pictures of the finished project see PROGRESS ON THE ACT RR in the N and Z section.
Mike