River Basics

 River behavior can be understood through both a scientific and historical lens. Its most important principle is that water only moves downhill- it can never flow from the ocean or up into a source. This also means that it's nearly impossible for a river to split into two and drain into two separate river sheds, and that there are only a few dozen examples of it in the world only under special geologic conditions. One of these places is the "Parting of the Waters" in Wyoming- where a river splits into two creeks that drain into both the Pacific Ocean and the Gulf of Mexico, and would theoretically be a natural canal across the Americas if the rivers were fully navigable.

 This also means-

It is nearly impossible for a lake to have two or more outlets

It is nearly impossible for rivers to collide directly- below is the single exception

 

It is impossible for a river to flow from ocean to ocean

It is almost impossible for a river to switch directions seasonally

It is very rare that a tributary comes very close to the source of the river it flows into

It is very rare that a river comes very close to a different outlet than where it terminates

Rivers both influence and are influenced by the topography around them. They are easily detected in a topographic map: 

  

The dominant effect is usually decided by the flatness of the terrain. In very flat areas such as Central Illinois, rivers can even change direction on a geological time scale and drain in an opposite direction.

Typically only around a third of rainfall in a certain area ends up in its rivershed. 25% of this is from rainfall, and 7% is from draining groundwater. To find the square meters of water that a river will typically carry, multiply a third of the rainfall of the river and its tributaries at the point measured with the area in thousands of kilometers squared by 1/10. River depth depends on dredging, topological features, and incision (the tendency of a river to drag along sediment under it), and does not have to constantly increase throughout the length of a river.

There are five fundamental types of rivers: Straight rivers, which are relatively straight rivers with few meanders, braided rivers, which contain many small interconnected streams that produce a great number of islands, slightly meandering rivers, heavily meandering rivers, and anastomosing rivers, which are somewhat like braided rivers but tend to collect themselves and split again into a number of river islands.

Braided and anastomosing rivers typically form alluvial plains, or floodplains, on which they carry a different sediment compared to their surroundings. They can becoming incised, or cut into the ground, where they become more established. In these rivers, flow can occur under the floodplains, which makes it impossible to farm or develop the area. Most rivers braid sometimes, but some rivers braid all the time.

Meanders typically at minimum have a 23/130 ratio for radius of the bends to the radius of the meander. However, depending on the geology, they can have any kind of greater meander size. They typically flow faster on their outside to maintain their volume flow rate, so the outside is usually incised, rocky, and occasionally raised up compared to the inside which typically experiences more wetlands, flooding, and low forests. The river can easily cut through the marshy inside of the curve, cutting out part of the old river and forming an oxbow lake. This can happen overnight in a flood as the rush of water seeks a faster path through the river. With high degrees of incision over a geologic period of time, rivers can form canyons or ravines.

The vegetation along rivers and their type of sediment can vastly affect the qualities of the river. Healthy riparian vegetation reduces the chance of flooding greatly, and when trees are cut neighboring rivers it can eventually have disastrous effects that require more human efforts in modification that make the regrowing of vegetation more and more difficult. Rivers can carry anything from small sediment to heavy rocks. Typically the flow rate at the bottom of the river is near zero because of the higher pressure of water weighing down on it (pressure is inversely proportional to velocity) and because of friction with sediment. This means that sediment is often carried through the river, and that a river does not have to look clear to be healthy. Wood in the river from trees surrounding it provides for important animal habitat and is a very healthy sign. Typically river environments should be various- including fast, slow, deep, and shallow regions.

 The navigability of the river is one of the most important human aspects and depends on the number of rapids in its paths and the depth of the river. Rapids need to be bypassed by dam and lock systems with are costly for humans and for the riparian environment. 

Cities have been placed on rivers for millenia, so they are extremely important for human development. The biggest city with no river or comparable source of fresh water is Johannesburg, a relatively new city. Rivers were important for hygiene, fresh water, industrial processes, and navigation, and are an essential part of most of the world's everyday lives.

Thanks for reading,

Jacob