Yeah... that's a river... hahahahaha Rivers around here tend to be quite big, and this one is on the top 5 list (or top 2... I don't think there is any rivers larger than Rio Negro and Rio Amazonas)...
At this point, it's about 4km from one side to the other... and it's about 35 to 50 meters deep... but it can go wider and deeper than that in other places through its course...
and the dark water is from organic matter in suspension... (not sure if that's how it goes in English, but that's the idea)...