The waterfall has been saved from immediate destruction. I removed half of the spillway and found that (as I suspected) the weight of the waterfall has settled the spillway so that although the top still drains toward the pond, the area directly beneath the highly porous concrete and rock spillway, drains back toward the shop, and this is aided by the corrugated tin base that the spillway was poured on top of. Anyway, Bill showed up in the middle my destruction and now sees the problem and that it is fixable. He said that he really wants to save the structure as a waterfall so we are going to cut out the tin that I have exposed, build up the area with dirt, and re-pour the spillway. It will always crack and water will always be getting under it, but this will minimize what runs back into the shop. I've been this sure before about other solutions, which failed miserably, for sure this time.
Buddy was so excited about finding the turtle that he searched the yard all afternoon. I thought it might be an ornate box turtle, but it really seemed exotic. The fluting on the back of the shell is not something I remember from the regular turtles here.