Here are the basics, there are a whole lot of things to consider but these are the simplest things:
1. Water flows downhill. It will not flow from the plains up and over a mountain chain to some other plains area.
2. Lots of rivers and streams flow into a lake but there is only one that flows out. That one might have some little delta-like thing branching at the start but they will join up to form one river channel and the area with the softest dirt to cut into or the steepest angle to flow down will eventually win out producing one outflowing channel. The known exceptions are: an endorheic basin which forms below sea level so therefore has no outflow; and a crater lake.
3. Rivers wander all over the place and vary in width...flat = fat (generally).
4. Rivers don't split and stay split except in deltas. They may go around a more dense chunk of rock to form and island but they rejoin rather quickly. The reason that they don't rejoin in deltas is that the area is too flat so they wander around like tweens at a sock-hop and they dump into the ocean before they can get a chance to rejoin. This is the biggest thing that people mess up. They usually make these giant deltas and forget about scale. We try to encourage the analogy of a tree - put the branches in the mountains, put the trunk on the plains, and put some tiny tiny roots near the coast.
5. Rivers do not flow so that they connect two oceans or two separate parts of the same ocean. You could have a man-made canal, though.
6. Don't rely on "magic" to explain things away - that's bad physics and would require so much power that only a god could maintain it.
Here is a link to a good discussion: