Water is a frequent symbol in "The Mill on the Floss." What is great about the water symbolism is that it is never only bad or only good. It is portrayed as two sides to the same coin. A very Yin and Yang type relationship. For example, all people need water to live. It's important to hydrate. But too much can lead to water toxicity poisoning, which can lead to death. Water can be incredibly cleansing, such as swimming in a river to get rid of grime, but you could also drown.
In "The Mill on the Floss," water and rivers are symbolized in this good and bad motif. For example the river is a place of peace and romance for Maggie and Stephen. They spend a day happily rowing around and enjoying each other's company. But in the end Maggie is killed by drowning in a flood.
The path of many rivers is also not known. Its end is known, but the path it takes is not known. That's true for the characters within the story as well. Their path and story arc is not known and reveals itself "like the course of an unmapped river."