<p>I will attempt at rephrasing your interesting question after these considerations:</p>

<ol>
<li><p>At the continental scale, flowing water has no significant inertia, so water flows following the <strong>maximum slope</strong>. A river is just a path along the continent that follows the maximum local slope at any of the places it passes through.  </p></li>
<li><p>What amazes you (let me interpret) is that continents are mostly drained by rivers that flow <strong>continuously</strong> <em>downslope</em>. When water lacks a downslope path ahead (local topographic minimum), it forms a lake. So, the striking thing is that there are relatively few such topographic minima, lakes or marshes, in our continents. </p></li>
<li><p>If the river ends in the ocean, the fact that it is long is just imposed by the size of the continent and its sinuosity. If it ends in an endorheic (closed, internally-drained, intracontinental) basin, then by definition the river will form a lake at the lowest spot of the basin.</p></li>
<li><p>Because continents are above the level of the surrounding ocean, it is clear that between any location at the continent and the coast, every path will be, on average, downslope. Continental areas below sea level are by definition endorheic (see 3) and therefore they drain to internally-drained lakes. </p></li>
<li><p>Provided some slope, erosion makes the river network gradually entrench the terrain at typical rates of 0.01 to 1 mm/yr. Rivers move laterally more easily at very flat regions, where they loose energy and deposit their sediment load, forming meanders and marshes. Over geological time, the river network, together with other <em>surface transport</em> processes, make the landscape decay towards a flat (peneplain) surface at the level of the ocean. </p></li>
</ol>

<p>So what would be the opposite to a landscape dominated by long rivers? Answer: Having many locations with no downhill slope to follow, topographic minima (basins) where <strong>lakes</strong> form. </p>

<p>Therefore your question reduces to this one: </p>

<p><strong>"Why aren't there more lakes?"</strong></p>

<p>The 117 million lakes on Earth occupy only 4% of the land surface[1]. The rest of the continental surface has a downhill slope for water to flow. Even endorheic drainage basins are drained mostly by rivers, although these drain to close lakes instead of the ocean. </p>

<p>Importantly, large lakes are relatively rare because they are ephemeral: once they form by tectonic and geodynamic changes of the topography they tend to disappear due to two processes:</p>

<p>A) Lake basins tend to fill up with the sediment carried by tributary rivers[2]. </p>

<p>B) Lake basins tend to be lowered in level due to the erosion along their outlet river[3], see my point #5 above. </p>

<p>Consider a landscape formed with a purely random topography. It will contain many lakes because their steepest descent path will not lead to the sea. Processes A and B would tend to fill the lakes and erode their outlets, making this topographic minima disappear and establishing a <em>normal</em> fluvial network with progressively fewer lakes. </p>

<p>On Earth, erosion is a process trying to remove lake basins in timescales of hundreds of thousands to tens of millions of years, depending on the size or elevation of the lake and its drainage basin. If we still have this 4% of the planet surface occupied by lakes it is because river erosion competes with relief-generating mechanisms such as plate tectonics or glaciers. But that's a different story. </p>

<p>[1]: National Geographic: <a href="http://voices.nationalgeographic.com/2014/09/15/117-million-lakes-found-in-latest-world-count/" rel="nofollow noreferrer">117 Milion lakes in the planet</a>.</p>

<p>[2]: Carroll, A.R., and Bohacs, K.M., 1999, Stratigraphic classification of ancient lakes: Balancing tectonic and climatic controls: Geology, v. 27, p. 99–102, doi:10.1130/0091-7613(1999)027&lt;0099:SCOALB>2.3.CO;2.</p>

<p>[3]: Garcia-Castellanos, D., 2006. Long-term evolution of tectonic lakes: Climatic controls on the development of internally drained basins. In: Tectonics, Climate, and Landscape evolution. Eds.: S.D. Willett, N. Hovius, M.T. Brandon &amp; D.M. Fisher. GSA Special Paper 398. 283-294. doi:10.1130/2006.2398(17)</p>

CC BY-SA 3.0
2017-11-07T14:16:50.410Z
you're right, i will change that
DrGC
CC BY-SA 3.0
2017-11-09T20:22:59.840Z
A small lake formed by the 1903 [Frank slide](https://en.wikipedia.org/wiki/Frank_Slide#Aftermath) is almost filled with sediment already.
Keith McClary
CC BY-SA 3.0
2017-02-21T22:56:00.033Z
*"Rivers move laterally only when they reach very flat regions"* - that's not really true. Rivers are constantly carving chunks out of hill sides. They definitely move laterally slower and less extremely in mountainous regions, but they still move.
naught101
CC BY-SA 3.0
2016-01-15T22:46:33.990Z
2017-11-07T23:18:24.067Z
2017-11-07T23:18:24.067Z
DrGC
<p>Well, I can only agree that it is indeed amazing and it doesn't get less amazing when some geology, hydrology and geomorphology is added to the the amazement. </p>

<p>All precipitation that falls on land must flow back to the oceans somehow, and unless it evaporates it will flow in rivers or as groundwater towards a lower hydrostatic level. Water can not flow uphill, so it follows (and crafts) the slope in topography. Water will flow from mountains towards the ocean and due to geometric and morphological reasons, rivers joint to <a href="https://en.wikipedia.org/wiki/Drainage_system_%28geomorphology%29" rel="noreferrer">a drainage pattern</a> due to the <a href="https://en.wikipedia.org/wiki/Fluvial" rel="noreferrer">geology and topography.</a></p>

<p>Mountain ranges are often formed from orogeny, where tectonic plates collide. Rivers starts at high altitude, radiating out in all directions from mountains, but as collisions in the present continental settings are often on the rim of large plates (e.g Andes, Alps, Himalaya), and the <a href="https://en.wikipedia.org/wiki/Drainage_divide" rel="noreferrer">rivers can't cross the range</a>, it will have to travel the whole continental plate to reach ocean level. In the case of Asia, most large rivers starts in Himalaya (or other tectonic active regions, as Altai), in Europe large rivers starts in the Alps. In Africa, they start in the tectonic active rift zone. </p>

<p>This map shows the ocean drainage dividers. The border between the drainage areas are the line where rivers start. </p>

<p><a href="https://i.sstatic.net/XYDww.png" rel="noreferrer"><img src="https://i.sstatic.net/XYDww.png" alt="&quot;Ocean drainage&quot; by Citynoise at English Wikipedia - Transferred from en.wikipedia to Commons.. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Ocean_drainage.png#/media/File:Ocean_drainage.png"></a></p>

<p>Lakes could be understood as temporary stop in the flow, and it will quickly fill up with vegetation and sediments as the velocity of the water decreases. In a geologic context, lakes are never very old, Baikal being the oldest, 25 million years. Drainage patterns are often much older. </p>

<p>So why are some rivers so long? Because of tectonic uplifts and orogenies. The answer to your question 'Did the rivers make the downhill paths themselves?' is yes, as erosion works toward the peneplain (low relief). </p>

CC BY-SA 3.0
2015-07-26T07:29:25.620Z
Nice answer. One thing: what is the "American and Mediterranean Sea" in the map?
Gimelist
CC BY-SA 3.0
2015-07-26T07:32:24.560Z
It's a oceanographic term for Caribbean Sea and the Gulf of Mexico combined. Rather rarely used term, I think.
user2821
CC BY-SA 3.0
2015-07-26T09:11:40.017Z
So I guess they mean "Caribbean", not "Mediterranean"...
Gimelist
CC BY-SA 3.0
2015-07-26T09:15:16.517Z
I think that Gulf of Mexico, technically speaking, is not part of Caribbean, so that's why they use this term. I'm no expert in this naming convention. Wikipedia clarifies a bit: https://en.wikipedia.org/wiki/American_Mediterranean_Sea
user2821
CC BY-SA 3.0
2015-07-26T10:00:40.993Z
The labels in the diagram are a little confusing. The colour of the label means "Land in this colour drains into the sea/ocean with this name, which isn't necessarily where this label is written." I guess "European Mediterranean Sea" means "Mediterranean Sea"; the "Australasian Mediterranean Sea" seems to be all that contentiously named stuff between China, Japan, the Philippines and Indonesia.
David Richerby
CC BY-SA 3.0
2015-07-26T10:47:52.990Z
_Mediterranean sea_ is not only a name of the sea between Africa and Europe, it's also a general term in oceanography. The map is probably made for hydrologists and oceanographers that are familiar with the word. I agree that the color scheme could be clearer.
user2821
CC BY-SA 3.0
2015-07-26T10:48:45.103Z
https://en.wikipedia.org/wiki/Mediterranean_sea_%28oceanography%29
user2821
CC BY-SA 3.0
2015-07-26T12:20:12.410Z
@Tbbe thanks - that was the source of my confusion.
Gimelist
CC BY-SA 3.0
2015-07-27T10:57:13.073Z
If a lay person listens to Arctic scientists who keep talking about how the ice cover of their *mediterranean* sea decreases each year, this person may be quite confused.
gerrit
CC BY-SA 3.0
2015-07-27T11:15:50.200Z
It's indeed one of the more confusing terms. Sorry for bringing it up...
user2821
CC BY-SA 3.0
2015-07-25T16:48:04.147Z
2015-07-25T20:00:49.243Z
2015-07-25T20:00:49.243Z
user2821
<p>I was recently on a long-haul flight over Siberia and it struck me as rather remarkable that something like the <a href="https://en.wikipedia.org/wiki/Lena_River" rel="nofollow noreferrer">Lena River</a> could exist. It seems very surprising that there's a route from some random spot near Lake Baikal over 2,800 miles or so to the Arctic Ocean, which is downhill all the way. Likewise, the Missouri River tells us that there's an even longer downhill-only route from some place in Montana to the Gulf of Mexico.</p>

<p>But these things certainly do exist and it's a pattern repeated all over the world: the great majority of the Earth's surface is drained by rivers that run downhill to the sea. They don't get stuck and they don't seem to form huge numbers of lakes where they have to flood an area to raise the water level over some barrier. I'm aware of <a href="https://en.wikipedia.org/wiki/Endorheic_basin" rel="nofollow noreferrer">endorheic basins</a>, which don't drain to the ocean but they only cover about 18% of the land surface, mostly in central Asia.</p>

<p>Is the existence of long downhill routes more likely than I imagined? (I can't tell if I think it's more likely in mountainous areas than in flatter areas, which suggests I have poor intuition about this) Or is it just a coincidence of our planet? Did the rivers make the downhill paths themselves? </p>

CC BY-SA 3.0
2015-07-25T18:46:32.053Z
If there is a basin somewhere along the flow line, it will (if there is sufficient precipitation) eventually fill (forming a lake), and water will spill over the basin rim at the lowest point and continue downhill from there.  The water will then start eroding a deeper channel, eventually draining the lake.  Sometimes this can happen quite quickly, e.g. https://en.wikipedia.org/wiki/Bonneville_flood  So eventually a "downhill all the way" route is created, if it didn't already exist.
jamesqf
CC BY-SA 3.0
2015-07-25T19:29:45.603Z
@jamesqf I think your comment contains the key idea I was missing (that lots of lakes *do* form but they drain when they burst). Would you be interested in turning it into a full answer?
David Richerby
CC BY-SA 3.0
2015-07-25T20:00:16.767Z
The main concept of how lakes disappears are that they fill up with sediments and vegetation. But erosion can indeed happen quick and there are examples of natural dams that have collapsed. Pluvial (ice dammed) lakes are indeed an interesting case, but not very common nowadays, except at high altitudes.
user2821
CC BY-SA 3.0
2015-07-26T17:03:49.807Z
@Tbbe:  I don't know whether sedimentation or erosion of the outlet would be quicker.  Bonneville of course was the latter (not AFAIK glacial, like Lake Missoula).  For a future instance, consider the way the Niagara River is slowly eroding its way into Lake Erie.
jamesqf
CC BY-SA 3.0
2015-07-26T17:21:34.203Z
I think Lake Bonneville flood was rather a collapsed dam. Lake Agassiz is maybe a more famous example of ice dammed lakes. The main point is that lakes are something temporary, a response to change climate and drainage pattens, post glacial uplift... One can say that rivers are shaping the landscape continuously, but lakes only exits when there is a special event that causes them to be there.
user2821
CC BY-SA 3.0
2015-07-27T10:55:20.963Z
In the north, they *do* form huge numbers of lakes.  Look at a map of Finland...
gerrit
CC BY-SA 3.0
2015-07-29T12:23:35.890Z
Lakes in Finland and Scandinavia are also a response to post glacial uplift. Lakes are turning into bogs and dries up so fast that sometimes place names doesn't keep up  with the pace.
user2821
<hydrology><rivers><geomorphology>
How do long rivers exist?
David Richerby
# How do long rivers exist?

(Asked by: username_0 on 2015-07-25)

I was recently on a long-haul flight over Siberia and it struck me as rather remarkable that something like the [Lena River](https://en.wikipedia.org/wiki/Lena_River) could exist. It seems very surprising that there's a route from some random spot near Lake Baikal over 2,800 miles or so to the Arctic Ocean, which is downhill all the way. Likewise, the Missouri River tells us that there's an even longer downhill-only route from some place in Montana to the Gulf of Mexico.

But these things certainly do exist and it's a pattern repeated all over the world: the great majority of the Earth's surface is drained by rivers that run downhill to the sea. They don't get stuck and they don't seem to form huge numbers of lakes where they have to flood an area to raise the water level over some barrier. I'm aware of [endorheic basins](https://en.wikipedia.org/wiki/Endorheic_basin), which don't drain to the ocean but they only cover about 18% of the land surface, mostly in central Asia.

Is the existence of long downhill routes more likely than I imagined? (I can't tell if I think it's more likely in mountainous areas than in flatter areas, which suggests I have poor intuition about this) Or is it just a coincidence of our planet? Did the rivers make the downhill paths themselves?

**Tags:** hydrology, rivers, geomorphology

> **Comments:**
> **username_1**: If there is a basin somewhere along the flow line, it will (if there is sufficient precipitation) eventually fill (forming a lake), and water will spill over the basin rim at the lowest point and continue downhill from there.  The water will then start eroding a deeper channel, eventually draining the lake.  Sometimes this can happen quite quickly, e.g. https://en.wikipedia.org/wiki/Bonneville_flood  So eventually a "downhill all the way" route is created, if it didn't already exist.  
> **username_0**: @username_1 I think your comment contains the key idea I was missing (that lots of lakes *do* form but they drain when they burst). Would you be interested in turning it into a full answer?  
> **username_2**: The main concept of how lakes disappears are that they fill up with sediments and vegetation. But erosion can indeed happen quick and there are examples of natural dams that have collapsed. Pluvial (ice dammed) lakes are indeed an interesting case, but not very common nowadays, except at high altitudes.  
> **username_1**: @Tbbe:  I don't know whether sedimentation or erosion of the outlet would be quicker.  Bonneville of course was the latter (not AFAIK glacial, like Lake Missoula).  For a future instance, consider the way the Niagara River is slowly eroding its way into Lake Erie.  
> **username_2**: I think Lake Bonneville flood was rather a collapsed dam. Lake Agassiz is maybe a more famous example of ice dammed lakes. The main point is that lakes are something temporary, a response to change climate and drainage pattens, post glacial uplift... One can say that rivers are shaping the landscape continuously, but lakes only exits when there is a special event that causes them to be there.  
> **username_3**: In the north, they *do* form huge numbers of lakes.  Look at a map of Finland...  
> **username_2**: Lakes in Finland and Scandinavia are also a response to post glacial uplift. Lakes are turning into bogs and dries up so fast that sometimes place names doesn't keep up  with the pace.  

---

## 2 answers

### Answer by username_2 on 2015-07-25. Score: 16 | Accepted answer

Well, I can only agree that it is indeed amazing and it doesn't get less amazing when some geology, hydrology and geomorphology is added to the the amazement.

All precipitation that falls on land must flow back to the oceans somehow, and unless it evaporates it will flow in rivers or as groundwater towards a lower hydrostatic level. Water can not flow uphill, so it follows (and crafts) the slope in topography. Water will flow from mountains towards the ocean and due to geometric and morphological reasons, rivers joint to [a drainage pattern](https://en.wikipedia.org/wiki/Drainage_system_%28geomorphology%29) due to the [geology and topography.](https://en.wikipedia.org/wiki/Fluvial)

Mountain ranges are often formed from orogeny, where tectonic plates collide. Rivers starts at high altitude, radiating out in all directions from mountains, but as collisions in the present continental settings are often on the rim of large plates (e.g Andes, Alps, Himalaya), and the [rivers can't cross the range](https://en.wikipedia.org/wiki/Drainage_divide), it will have to travel the whole continental plate to reach ocean level. In the case of Asia, most large rivers starts in Himalaya (or other tectonic active regions, as Altai), in Europe large rivers starts in the Alps. In Africa, they start in the tectonic active rift zone.

This map shows the ocean drainage dividers. The border between the drainage areas are the line where rivers start.

[!["Ocean drainage" by Citynoise at English Wikipedia - Transferred from en.wikipedia to Commons.. Licensed under Public Domain via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Ocean_drainage.png#/media/File:Ocean_drainage.png](https://i.sstatic.net/XYDww.png)](https://i.sstatic.net/XYDww.png)

Lakes could be understood as temporary stop in the flow, and it will quickly fill up with vegetation and sediments as the velocity of the water decreases. In a geologic context, lakes are never very old, Baikal being the oldest, 25 million years. Drainage patterns are often much older.

So why are some rivers so long? Because of tectonic uplifts and orogenies. The answer to your question 'Did the rivers make the downhill paths themselves?' is yes, as erosion works toward the peneplain (low relief).

> **Comments:**
> **username_7**: Nice answer. One thing: what is the "American and Mediterranean Sea" in the map?  
> **username_2**: It's a oceanographic term for Caribbean Sea and the Gulf of Mexico combined. Rather rarely used term, I think.  
> **username_7**: So I guess they mean "Caribbean", not "Mediterranean"...  
> **username_2**: I think that Gulf of Mexico, technically speaking, is not part of Caribbean, so that's why they use this term. I'm no expert in this naming convention. Wikipedia clarifies a bit: https://en.wikipedia.org/wiki/American_Mediterranean_Sea  
> **username_0**: The labels in the diagram are a little confusing. The colour of the label means "Land in this colour drains into the sea/ocean with this name, which isn't necessarily where this label is written." I guess "European Mediterranean Sea" means "Mediterranean Sea"; the "Australasian Mediterranean Sea" seems to be all that contentiously named stuff between China, Japan, the Philippines and Indonesia.  
> **username_2**: _Mediterranean sea_ is not only a name of the sea between Africa and Europe, it's also a general term in oceanography. The map is probably made for hydrologists and oceanographers that are familiar with the word. I agree that the color scheme could be clearer.  
> **username_2**: https://en.wikipedia.org/wiki/Mediterranean_sea_%28oceanography%29  
> **username_7**: @Tbbe thanks - that was the source of my confusion.  
> **username_3**: If a lay person listens to Arctic scientists who keep talking about how the ice cover of their *mediterranean* sea decreases each year, this person may be quite confused.  
> **username_2**: It's indeed one of the more confusing terms. Sorry for bringing it up...  

---

### Answer by username_4 on 2016-01-15. Score: 12

I will attempt at rephrasing your interesting question after these considerations:

1. At the continental scale, flowing water has no significant inertia, so water flows following the **maximum slope**. A river is just a path along the continent that follows the maximum local slope at any of the places it passes through.
2. What amazes you (let me interpret) is that continents are mostly drained by rivers that flow **continuously** *downslope*. When water lacks a downslope path ahead (local topographic minimum), it forms a lake. So, the striking thing is that there are relatively few such topographic minima, lakes or marshes, in our continents.
3. If the river ends in the ocean, the fact that it is long is just imposed by the size of the continent and its sinuosity. If it ends in an endorheic (closed, internally-drained, intracontinental) basin, then by definition the river will form a lake at the lowest spot of the basin.
4. Because continents are above the level of the surrounding ocean, it is clear that between any location at the continent and the coast, every path will be, on average, downslope. Continental areas below sea level are by definition endorheic (see 3) and therefore they drain to internally-drained lakes.
5. Provided some slope, erosion makes the river network gradually entrench the terrain at typical rates of 0.01 to 1 mm/yr. Rivers move laterally more easily at very flat regions, where they loose energy and deposit their sediment load, forming meanders and marshes. Over geological time, the river network, together with other *surface transport* processes, make the landscape decay towards a flat (peneplain) surface at the level of the ocean.

So what would be the opposite to a landscape dominated by long rivers? Answer: Having many locations with no downhill slope to follow, topographic minima (basins) where **lakes** form.

Therefore your question reduces to this one:

**"Why aren't there more lakes?"**

The 117 million lakes on Earth occupy only 4% of the land surface[1]. The rest of the continental surface has a downhill slope for water to flow. Even endorheic drainage basins are drained mostly by rivers, although these drain to close lakes instead of the ocean.

Importantly, large lakes are relatively rare because they are ephemeral: once they form by tectonic and geodynamic changes of the topography they tend to disappear due to two processes:

A) Lake basins tend to fill up with the sediment carried by tributary rivers[2].

B) Lake basins tend to be lowered in level due to the erosion along their outlet river[3], see my point #5 above.

Consider a landscape formed with a purely random topography. It will contain many lakes because their steepest descent path will not lead to the sea. Processes A and B would tend to fill the lakes and erode their outlets, making this topographic minima disappear and establishing a *normal* fluvial network with progressively fewer lakes.

On Earth, erosion is a process trying to remove lake basins in timescales of hundreds of thousands to tens of millions of years, depending on the size or elevation of the lake and its drainage basin. If we still have this 4% of the planet surface occupied by lakes it is because river erosion competes with relief-generating mechanisms such as plate tectonics or glaciers. But that's a different story.

[1]: National Geographic: [117 Milion lakes in the planet](http://voices.nationalgeographic.com/2014/09/15/117-million-lakes-found-in-latest-world-count/).

[2]: Carroll, A.R., and Bohacs, K.M., 1999, Stratigraphic classification of ancient lakes: Balancing tectonic and climatic controls: Geology, v. 27, p. 99–102, doi:10.1130/0091-7613(1999)027<0099:SCOALB>2.3.CO;2.

[3]: Garcia-Castellanos, D., 2006. Long-term evolution of tectonic lakes: Climatic controls on the development of internally drained basins. In: Tectonics, Climate, and Landscape evolution. Eds.: S.D. Willett, N. Hovius, M.T. Brandon & D.M. Fisher. GSA Special Paper 398. 283-294. doi:10.1130/2006.2398(17)

> **Comments:**
> **username_6**: *"Rivers move laterally only when they reach very flat regions"* - that's not really true. Rivers are constantly carving chunks out of hill sides. They definitely move laterally slower and less extremely in mountainous regions, but they still move.  
> **username_4**: you're right, i will change that  
> **username_5**: A small lake formed by the 1903 [Frank slide](https://en.wikipedia.org/wiki/Frank_Slide#Aftermath) is almost filled with sediment already.  

---

