This is actually a pretty amazing piece of engineering. It's a buttress dam - a simple inclined slab of reinforced concrete along the water face supported by a lattice of buttresses on the downstream side. They haven't constructed these in decades mostly because of safety. The main advantage of slab and buttress system is increased material savings. 116 years ago, concrete and steel were inherently more expensive than the labor needed to construct elaborate formwork, so labor intensive designs like this were more prevalent. That this structure is still standing and (arguably) functioning are testament to the quality of the design and construction, and to the inherent stability of the dam site. It will be kind of a shame to see it demolished and replaced, but a more modern and safe structure is definitely needed here. If you want even more info on this design type, look up "Ambursen dam" - a Norwegian immigrant engineer who popularized this type of dam construction.