Guys this would be a more strategic monster. It could require more communication differentiating good teams from great teams. In order to get dragon or baron you would have to eliminate this monster and kill dragon or baron on the respawn timer.This would help prevent the level 4 solo junglers taking dragon giving an advantage to whoever can communicate better.
This monster could give buffs focused on a location and utility buff. Ideas that come to mind can be full vision of river for a set ammount of time, another idea that comes to mind is increased gold gain and experience gain, my final idea is a toxic water that damages enemy champions in the river dealing 0.5% damage per second.
I feel that this will be a monster that is sought out every time its up. especially early game where gold and expierience are hard to come by so as long as a team has good communication this monster should add teamwork to the game because right now people just ping a cave and say "dragon" or "baron", to desperate teams it may involve their amumu tank to tank baron and their warwick jungler to attack the river monster. This can reward well communicated teams with gold, expierience, half of the map dominance and likely a large sum of gold from dragon and maybe a baron buff.