The river, and water in general, is a frequent symbol in George Eliot's The Mill on the Floss. In the novel, water takes on a symbolic Yin and Yang relationship. That's not so weird. Star Wars does the same thing with The Force. It just calls it "light" and "dark." Let me give you an example of how water can be both good and bad. First, people need water to live. Hydration is super important. People die without it. Of course people can die from drinking too much water too. That's called water toxicity poisoning. Water can be used for cleaning. Water can be used for fun, like swimming or water skiing, but that same body of water can cause drowning as well.
Eliot uses water in the same manner in The Mill on the Floss. Take for example this brief description of flooding:
"Nature repairs her ravages – repairs them with her sunshine and with human labour. The desolation wrought by that flood, had left little visible trace on the face of the earth, five years after."
Flooding is quite destructive. It can ruin buildings, crops, and even kill people, but it is also cleansing to the land. The flood isn't a death sentence forever, and nature uses the flood to repair its own damages.
The river is symbolic for Maggie as well. For her, it is a place of peace. She frequently spends time by the river, and it is even a place for a peaceful, romantic boat ride with Stephen. That motif isn't unique to this novel. That peaceful/romantic river idea is probably why Venice is so revered. The river as peace idea is probably why so many people love fishing. But the river isn't all wonderful and good for Maggie. It does kill her.