Derived from the fundamental laws of physics (conservation of energy and momentum), we have the Fundamental Law of Hydrogeography: Water flows downhill.
How we see this on maps:
-- Water follows the path of steepest descent.
-- Natural rivers and streams cross contour lines at right angles.
-- Rivers start as smaller streams that merge together to form larger ones.
-- River outflow from every area must balance river inflow. Outflow may be through a channel that carries water, evaporation, or sinking into the soil, but it must all balance.
-- Rivers erode landforms and produce sediment, which they move along towards their outflow.
-- Rivers can only carry so much sediment. The steeper the river, the more energy it has and the more sediment it can carry. When a river suffers a traumatic loss of energy (such as flowing into a lake or river), it drops some of its sediment load. The formation where a river enters a body of water is called a delta and is how rivers form new land.
-- Rivers in mountains expend their energy cutting down rather than from side to side.
-- Rivers do not split unless there's a very good reason like a river delta that keeps having the channel silted in until the channel gets higher than the surrounding land, forcing it to split and move over. River deltas with channels are typically very small compared to the length of the river and are swampy, icky places.
-- The farther along a river's course you go, the more likely it it to have a flat-bottomed valley because a river can't cut down below the local sea / lake level (that is, simple fluvial erosion doesn't happen underwater) so it expends its energy going side to side instead.
-- A lake will not have more than one outflow. If there are two outflows, one will eventually capture all of the outflow as it cuts its way down. Or another way: them that has, gets.
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