I don't know a lot about hydroelectric power, just some physics knowledge, but I think what you really want to harness with a hydroelectric turbine is the pressure behind the water, not the velocity from it falling. The water pressure is going to be highest at the bottom of the dam, where you can harness every bit of water pressure from the entire reservoir. Water pressure increases with depth, as you have the weight of all the water above pushing other water through the turbines. Then you could have turbines in a series, but if hydroelectric turbines are anything like steam turbines, there's going to be a drop off in pressure that is better utilized by progressively smaller and smaller blades, so they do try to get as much out of the water pressure as they can. It just makes more sense to put additional turbines in parallel, rather than in series, so that they all get maximum "bottom of the dam" water pressure.