How do long rivers exist?

Question

I was recently on a long-haul flight over Siberia and it struck me as rather remarkable that something like the 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, 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?

Answer 1

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 due to the geology and topography.

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, 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.

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).

Answer 2

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.

[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)