Essentially this is saying that despite life's ups and downs, we're all in this together. A waterfall is a metaphor for periods of flat, peacefulness (on the river) in contrast with periods of dark times (the fall). When the poem says that we're one in the river, it means that we stick together during the time of contentment and happiness. When the poem says that we're one again after the fall, it means that we're there for each other, ready to pick each other back up and survive the "fall". Take this pandemic, for instance. We were basically "one" before it happened, but now, it's the "fall" of the waterfall, and everyone in the world is trying their best to cheer each other up and make things better.
