I had a chance to try out my copy of Dos Rios this evening, and sat down to a 2-player game. The BGG notes that the game is actually best with 2, with much more chaos with players.
After a quick rules explanation, details of placement, control, movement + chasing and building houses and dams, we scattered our guys around the field and set off. For the first couple of turns the rivers were mostly left alone and people scrambled into place to collect harvest money.
One all the figures were onto the board, we started with phase-one aggression. Lots of running around, leaping out from forests on down from hills to chase people off. At the start, neither player had enough money to build casas or haciendas to stop this. The mountain by the town was a favourite place for launching ambushes.
The dam building slowly started next, red had collected more money and black more wood so a frenzy of dam building started up to try to direct the river over as many fields as possible. There was a little disruption of the river to take it away from its initial sources, but often a second dam would bring it back. Red also managed to build and occupy a casa on the east side which obstructed Black trying to move guys into brown river spaces.
In the end it was a race between Black and Red to get all the buildings down in time, in a race that was actually fairly close. In the final move Black was about to aggressively move the river away from Red's pieces and buildings when it was noticed that a single new building gave him the winning condition. Huzzah!
The end game board looked a little like this (a river piece and red's hacienda have snuck onto the board in the bottom left). Black one with four buildings (including a hacienda) built along the river, Red had enough money to build the 5th building (their hacienda) on the next turn.
We managed to get through the game quite quickly and only one one instance of bandits on each river. This was a bit of a let-down, but we likely didn't spend enough time aggressively moving the river around to avoid handing out money to trigger them more. We certainly didn't take advantage of the fact that the river could be dammed all the way to the top of the map.
I think the game would definitely be worth trying with more players, simply to see how crowded the board could get and what sort of interactions you'd see. I do think that there would likely be a lot more chaos and it would be near impossible to plan. I'd also expect a the rivers to flap a lot more.
The game also seems a little sensitive to initial setup conditions. Getting practically all the mountains just across the top of the map meant that the bulks of the fields in the middle were fairly flat and uniform, so there weren't that many constraints on where the river would flow.
I'm definitely going to get the game out again with two players and see how it goes. It'll stay on my list as something to try with a larger group sometime as well.
It has been quite a while since I played, but I remember a race to the headwaters of the rivers being a big part of the early game. Damming the river up high is really powerful because it allows you to change its course so dramatically.
Thanks for the report. I really should play this again.