Rivers flow downhill to other rivers, lakes, or the ocean, but sometimes they dry up first. Other times they fall into a hole in the ground to become underground rivers. They seldom take the shortest route to their destination but simply go downhill, cutting through the softest material. Wide rivers can have a floodplain, which is where all the water goes when a river floods its banks. A floodplain can be many miles wide and have multiple settlements in it.
The age of a river can determine some of its characteristics because the erosion deepens over time. Young rivers like the Brazos River, Trinity River, and Ebro River tend to be steep and fast, with few tributaries but deep rather than broad erosion. Mature rivers like the Mississippi River, Ohio River, and Thames River are less steep, flow more slowly, and have more tributaries, with wider channels. Old rivers are slow, don’t erode much, and have floodplains; examples include the Tigris River, Euphrates River, and Indus River. When adding rivers, we should note their age in our files and consider how to draw them: a nearly straight line suggests a young river while a winding river suggests an old one.
The impact of rivers is mostly felt on settlements, but wide and long rivers have often been the boundary between nations. Keep this in mind when laying them out, for it’s an easy way to decide where one kingdom ends and another begins. There will be many more rivers than the ones we put on the map, so when it comes to drawing, we’re talking about the big ones, like the Mississippi River. Waterways can also impact travel, as we’ll discuss later in the “Travel by Water” chapter.
Lakes are found in natural depressions and along the courses of mature rivers, which are moving more slowly than young rivers. Most of them are also in higher latitudes. When deciding a lake exists somewhere, we need no explanation other than that a river flows into it. Sometimes two lakes that are near each other are connected by a strait. Most lakes on Earth are freshwater, but some are not, such as the Dead Sea and Great Salt Lake.
Lakes have natural outflows in the form of rivers or streams, which serve to keep the lake level consistent (excess water drains off). But some lakes don’t and maintain their level via evaporation or underground drainage; we can have a river end in a lake and not continue out the other side. Lakes sometimes vanish quickly, even in a few minutes, if something like an earthquake opens up a hole through which the lake drains into ground water. This is another scenario we can use in our writing if our inhabitants attribute this to something supernatural. Incidentally, Baikal Lake in Russia is the deepest lake on Earth and is over 5,300 feet (1615 meters)—deep enough for a monster, certainly.
There’s no real difference between lakes and ponds and in fact there’s argument about what the difference is, the usual determining factor being size. That said, ponds far outnumber lakes. Geologically speaking, all lakes are temporary because they’ll eventually fill with sediment, a fact which only becomes relevant if we have time-traveling characters who go far into the future and wonder why a lake is gone.