As a coal geologist, with a lot of work and study in both environmental, and engineering also, and has someone who has worked at coal mines for years, and dealt with water storage and release on a regular basis: Enjoy your dead river systems. Coal is far from dirty black stuff that releases gas. Improperly stored and released you get all the other nasty inclusions too. Lead, mercury, arsenic, tin, antimony are all things that come to mind. These are bio accumulants, ie they stay in the body, and in the food chain. To put it simply - One fish eats 1 mercury. The river bird eats 4 fish. The river bird now has 4 mercury. The river bird is now dead. The fish will eventually be dead. The riverbank vegetation will eventually be dead. Other induistries along that river will die. Bank erosion will be a thing, now. And low lying properties along the rivers will start to see more frequent flooding. The coastal end of the river will suffer due to increased siltation, heavy metals, etc.

The long story short is that pumping water between pits is a very costly exercise and involves huge pumps, fuel costs, pipe, employees, etc. It would be very much easier, but disastrous, to release water straight from the treatment plant, and from the active pit straight to the river.

TL:DR: Water storage in coal mines has very sound science and engineering behind it's management. Managed properly, it's no issue. This will fuck up your rivers like you wouldn't believe.