Essential river guidelines for mapping

[Midgardsormr]

The issue was raised recently that there is no central source for information on rivers, how they behave, and how they are drawn on a map. The purpose of this thread is to gather advice, guidelines, and styles for river mapping into one place. Comments, corrections, criticism, and contributions are welcome.

Rivers do not generally fork as they travel downstream. Since this is the most common "river violation" we see, it goes at the top. Unless there is some kind of intervention, it is very unlikely that a river will split naturally. It may happen from time to time, but one of the forks will eventually erode low enough below the other that it will become the sole channel. Lakes behave the same way; there will generally be only one outflow from a lake, although there may be numerous inflows.

Rivers always flow downhill. We all know that water always runs downhill and takes the path of least resistance. There are some ramifications to this in mapping. On a contour map, a river will always cross a contour line at right angles. Rivers will usually move away from mountains, and they will very rarely move through a pass between two mountain ranges because that space between the ranges, though lower than either mountain chain, is usually still higher than the land on either side. Rivers will almost always run toward the coastline of an ocean, sea, or lake, since those bodies of water form where the terrain is at its lowest.

Natural rivers never connect one ocean to another. In most worlds, sea level is the same worldwide. Since water flows downhill, there is no force that will cause water to leave an ocean, flow across land, and back into an ocean. Anywhere that appears to happen is not really a river, but an ocean channel. The tides will usually cause the water to reverse its direction of motion through such a channel. Rivers don't reverse their motion in this fashion.

The steeper the slope, the straighter the river. Mountain streams tend to run straight and fast. Rivers moving across relatively level plains will meander and may frequently change their courses over time.

Lakes with no outflow are often salty. If the only way for water to leave a lake is through evaporation, the lake will tend to accumulate salt and other minerals dissolved in the water. If the water seeps into the earth at a significant rate, though, the lake will probably remain fresh.

[RobA]

Lakes naturally (and typically) only have one outflow (river) leading from them. Sometimes you will end up with two, but this is an unstable situation that will eventually favour one, and dry up the other.

-Rob A>

[waldronate]

Lakes with no outflow are salty. If the only way for water to leave a lake is through evaporation, the lake will tend to accumulate salt and other minerals carried by the water. The longer the lake goes with no outflow, the saltier it will get. These kinds of lakes and seas often form below sea level. Erosion will generally create an outflow eventually in a lake above sea level.

The parts about sea level in relation to salty lakes aren't generally true. Basins with internal drainage (lakes or salt pans with no outflow) are characteristic of arid regions and don't have any relation to sea level.
Virtually all of the salt lakes/playas (dried up salt lakes) in the Western US are well above sea level, for example, as are those in the Taklimakan and Alptiplano deserts in China and South America. Some of the more well-known examples of salt lakes such as the Dead Sea and Salton Sea are below sea level, but that's just coincidence. If you're a basin below sea level that's not full of water then it's because you're having internal drainage, not because you're below sea level.

[RobA]

The parts about sea level in relation to salty lakes aren't generally true. Basins with internal drainage (lakes or salt pans with no outflow) are characteristic of arid regions and don't have any relation to sea level.
Virtually all of the salt lakes/playas (dried up salt lakes) in the Western US are well above sea level, for example, as are those in the Taklimakan and Alptiplano deserts in China and South America. Some of the more well-known examples of salt lakes such as the Dead Sea and Salton Sea are below sea level, but that's just coincidence. If you're a basin below sea level that's not full of water then it's because you're having internal drainage, not because you're below sea level.

The real trick is for this to happen the water has to evaporate, not seep into the earth. If the ground is porous, the water (minerals and all) will just drain away. It is only when the ground is non-porous, or when it evaporates faster than it seeps that the minerals build up and forms salt pan or salt flat.

-Rob A>

[Midgardsormr]

Thanks for the correction, Joe. I'll amend my original post to remove the misinformation. I found that discussion I remembered about delta formation, but you just described it much more elegantly, so I'll just post a link to that thread: A brief discussion of Marshland and river deltas.

[waldronate]

Deltas form where rivers deposit their sediment, typically in response to a change in slope and/or entry into a body of water (which is pretty much a change of slope to 0). They usually take the form of a pie-shaped wedge because as the silt is dropped in the river channel, it raises the river bottom, forcing the river to the side, where it creates a a new channel. This process repeates over and over, so the river moves back and forth, depositing new sediment to form the alluvial fan.
Deltas often have more than one distributary because the sediment drop is liable to clog the one channel and split it into multiples. Deltas change relatively rapidly over time, so a map from 100 years ago is unlikely to reflect current realities.

Estuaries are places where a freshwater river meets the tides of a sea. As such, they are usually found on or near deltas. The mixing of fresh and salt water creates a habitat that works well for many plants and animals. They are often marshy (grassy wetlands) areas, but some have more woody plants as well (the woody plants and trees moving them over to the swamp side of the spectrum).

[alaskanflyboy]

Deltas form where rivers deposit their sediment, typically in response to a change in slope and/or entry into a body of water (which is pretty much a change of slope to 0). They usually take the form of a pie-shaped wedge because as the silt is dropped in the river channel, it raises the river bottom, forcing the river to the side, where it creates a a new channel. This process repeates over and over, so the river moves back and forth, depositing new sediment to form the alluvial fan.
Deltas often have more than one distributary because the sediment drop is liable to clog the one channel and split it into multiples. Deltas change relatively rapidly over time, so a map from 100 years ago is unlikely to reflect current realities.

Estuaries are places where a freshwater river meets the tides of a sea. As such, they are usually found on or near deltas. The mixing of fresh and salt water creates a habitat that works well for many plants and animals. They are often marshy (grassy wetlands) areas, but some have more woody plants as well (the woody plants and trees moving them over to the swamp side of the spectrum).

It should also be noted that all rivers have an estuary, but not all have a delta. Deltas form in rivers with heavy sediment which requires a river that runs considerably fast and runs through material that easily erodes. A fast river running through granite does not accumulate much sediment, but one running through sand or loose earth will. A river will also carry sediment for as long as a sufficient velocity is maintained. The faster the river, the larger the sediment it will carry. Any river that is still carrying a lot of sediment by the time it reaches the sea will typically dump it when the tidal forces (which can actually flow upstream) slow the flow enough that it can no longer hold it.

The shrubs and trees growing in the water of estuaries are known as mangroves.

[RobA]

There is a nice wikipedia article on longshore drift of river sediment leading to all sorts of coastal features: http://en.wikipedia.org/wiki/Longshore_drift

here is a nice (public domain) image of delta changes over time:
Rhone_delta_img.jpg


-Rob A>

[RPMiller]

Makes me think Joe knows a little something about geology or something.

[Midgardsormr]

Cross-link to Redrobes' thread on the same topic:

http://www.cartographersguild.com/showthread.php?t=3822