As a retired Excavator operator, it looks like the guy in the excavator is not building fish habitat, but, rather doing flood control... The large rocks are either being stacked as a berm to prevent erosion from a bank, or they are laid to form pools, so the brook won't wash out as fast. The excavators, do not harm the fish, they move !! I have seen fish swim right past me while working. A 40 ton Excavator, will not do any damage to the rocks, anymore than a small hard rain storm.. Of course, the main item is, the machine cannot have any hydraulic or fuel leaks...