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I am designing the geography of a lake in which two rivers flow. Should it have a river flowing from it to the sea or vice versa?

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    $\begingroup$ I would also look at researching tidal creek as part of your project. $\endgroup$ – sport_Billy Feb 2 '17 at 8:02
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    $\begingroup$ Have you looked at planet earth? You might find some clues there $\endgroup$ – Mawg Feb 2 '17 at 8:44
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    $\begingroup$ Real World incident where the ocean flew into a Lousiana lake (for a short time): en.wikipedia.org/wiki/Lake_Peigneur $\endgroup$ – Mindwin Feb 2 '17 at 12:38
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    $\begingroup$ @Mindwin Odd. I don't remember ever hearing about a flying ocean... $\endgroup$ – Frostfyre Feb 2 '17 at 13:07
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    $\begingroup$ There are plenty of real-life examples of rivers flowing from the sea and vice versa, some rivers flow into lakes, and others create new seas with circulation issues. Could you explain what features you having a problem with. Do both rivers flow into the lake, or does one flow in and another out? Do you have a specific latitude and climate in mind? What story details are set in your mind and what can be altered to incorporate details from answers? $\endgroup$ – EveryBitHelps Feb 2 '17 at 17:55

12 Answers 12

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Normally, a lake will have a fresh water river flowing into it from higher ground, and a river flowing out of it to the sea.

Fresh water source
                  \
                   \
                    --- Lake ---
                                \
                                 \
                                  Ocean

If you do it the other way around, water can't flow out of the lake. The lake would have to be below sea level. It would fill and overflow until the level of the lake has reached sea level and merged with the sea. Then you'd get something like the Mediterranean Sea fed by the Strait of Gibraltar or further in the Black Sea fed by the Dardanelles. Both the Mediterranean and the Black Sea are at the global sea level. Both straits have both inflow and outflow.

There are lakes below sea level, the Dead Sea for example, but it's cut off from the sea and fed by the Jordan River. Its "outflow" is evaporation.

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    $\begingroup$ Actually the Black Sea is fed by the Danube, the Dniester, the Dnieper and the Don and flows into the Mediterranean. Since the opening of the Suez Canal, the Red Sea also flows into the Med. It's not a large gradient, but it is there. See Lessepsian Migration. $\endgroup$ – AlexP Feb 2 '17 at 6:02
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    $\begingroup$ Obligatory xkcd what if. Briefly: yes, but it would not end pleasantly. $\endgroup$ – Spratty Feb 2 '17 at 10:03
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    $\begingroup$ @RichardTingle The current size of the Dead Sea is such that the water lost through evaporation is roughly the same amount as flows in through the Jordan River. If you make a river from the Mediterranean Sea to the Dead Sea, that balance will be disturbed and the Dead Sea will rise in water level until it's equal with the Mediterranean Sea. This would flood the entire Jordan Valley from the demilitarized zone with Syria all the way to Ein Yahav. I can't find proper math on this, but I think that's about the same size as the Bay Area. $\endgroup$ – Nzall Feb 2 '17 at 14:02
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    $\begingroup$ @Schwern: The average surface of the seas (the "local sea level") is not necessarily at the "mean sea level". In particular, the eastern Mediterranean is a about 1.2 meters lower than the Red Sea because the evaporation in the Med is intense while the flow of water from the Atlantic and the Black Sea is constricted by narrow straits. $\endgroup$ – AlexP Feb 2 '17 at 22:17
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    $\begingroup$ @Schwern: It is true than water flows both ways both through the Gibraltar and the Bosphorus. But more water flows into the Med from the Atlantic and and Black Sea than from the Med into the them. $\endgroup$ – AlexP Feb 3 '17 at 0:14
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If a river flows from ocean into lake, you better have another river that flows out of the lake into someplace else, otherwise that ocean is quickly going to fill the lake, the surrounding area, and continue until everything below sea level is filled up, at which point it will stop flowing. If you have a second river flowing to somewhere else to drain the lake, great, but it has to be SOMEWHERE ELSE, as in, not a normal place, because if you just connect it across to another ocean, again, it'll just fill up from both sides and then cease to flow. So if you have an infinitely deep pit to dispose of the water, or a superheat source that continually evaporates the lake, or a dimensional portal, then, yes, you can have a river from an ocean to a lake, but otherwise, you cannot.

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    $\begingroup$ If you have a river flowing from the sea into a lake, you have what happened to the Black Sea Basin sometime after the last ice age. $\endgroup$ – jamesqf Feb 2 '17 at 2:50
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    $\begingroup$ And just like then it will not stay a river for long tides, waves, and high flow erosion rate carve a river into a sea very quickly. $\endgroup$ – John Feb 2 '17 at 2:54
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    $\begingroup$ And even if it does not, the precipitating salt will fill up the lake until it reaches sea level. $\endgroup$ – Karl Feb 2 '17 at 5:37
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    $\begingroup$ @jamesqf ISTR it's been dated to around 8000BC. There would have been people living in the basin below sea level at the time, and I find the distribution of legends similar to the biblical flood rather suggestive. $\endgroup$ – nigel222 Feb 2 '17 at 11:22
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You could, of course, do some exotic things with neutronium to warp gravity the way you want it. But it's not necessary; lakes like this already exist in nature (sans neutronium).

The best example is probably Lake Assal in Djibouti.

This is near where the East African Rift comes out into the Red Sea. Instead of ocean water flowing into the lake via a river, however, it comes in by underground cracks from Lake Goubet, which is connected to the ocean. In a few million years time, this will be part of a new sea dividing East and West Africa.

The salient (pardon the pun) conditions are similar to what Scott Whitlock described:

  • The lake is in a desert (the Danakil), so that evaporation balances or exceeds inflow. In this case, salt from the ocean gets concentrated in this lake to make it one of the saltiest in the world.
  • The lake is below sea level so there is a potential energy difference to make the water flow out of the ocean rather than into it.
  • The lake is close to the ocean, so that the connection can form naturally.
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    $\begingroup$ Damn, I saw the title, did some research and uncovered Lake Assal, then came here and saw your answer. $\endgroup$ – Malvolio Feb 2 '17 at 15:46
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Yes, but you need to have some water sink at the lake end. One simple solution is evaporation. Water enters from the ocean into the lake (make it a small trickle of water) and because it's in a hot valley, it evaporates and more flows in. Since it's salt water, it would have a lot of dissolved solids, and these would be left over after evaporation. This would make the lake "brackish" (?) and unsuitable for most living things. Also, the sediment would slowly accumulate so I don't think it could last forever, unless you had some way of lowering the lake bed - perhaps a geological feature?

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    $\begingroup$ See the Salton Sea as an example of the described problems. The resulting salinity kills off fish, creates clouds of dust, interferes with bird migrations, etc. $\endgroup$ – GrinningX Feb 2 '17 at 3:23
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    $\begingroup$ @GrinningX Note that inflow to the Salton Sea is from rivers, not the ocean. The Salton Sea is particularly interesting because while historically it's been a sea, its current state is mostly the result of man-made canals and their failures. Veritasium did an interesting piece on it, Should This Lake Exist? $\endgroup$ – Schwern Feb 2 '17 at 4:34
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    $\begingroup$ With evaporation all you get in the end is salt level equal sea level. Salt from ocean water won't evaporate $\endgroup$ – Mołot Feb 2 '17 at 6:33
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    $\begingroup$ @Molot - yes, that's why I ended the answer with, "Also, the sediment would slowly accumulate so I don't think it could last forever, unless you had some way of lowering the lake bed - perhaps a geological feature?" $\endgroup$ – Scott Whitlock Feb 3 '17 at 17:20
  • $\begingroup$ Sorry, didn't get it is about salt, too. No excuse. $\endgroup$ – Mołot Feb 3 '17 at 17:26
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What the other answers seem to have overlooked is the possibility of the tidal inland lake.

Given a coastal barrier of harder rock but softer ground further inland it's possible to have a tidal river running from an inland lake to the sea. As the tide comes in the river flows from the sea to the lake, as the tide goes out it flows from lake to sea.

Due to the restricted flow of the water up the river, it's unlikely that the lake will ever reach the full high or low of the tide. While there will be 4 brief points of slack water in the river during a 24 hour tide cycle, you will have flow in the river most of the time.

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  • $\begingroup$ This is also a great idea; interestingly, it makes "in or out" a false choice. $\endgroup$ – Spencer Feb 2 '17 at 17:33
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    $\begingroup$ You might want to research the Norfolk Broads in the UK which are a network of rivers and (mostly small} "lakes" - except for the big one nearest to the coast near Great Yarmouth. In fact the tidal flows can get quite complicated in a system like this, since there may be several hours time difference in "high water" at different points on a long river, and multiple tidal rivers connecting the same lakes can be of different lengths. A google map of the whole system (the bright green area) is at google.co.uk/maps/@52.6458771,1.5142793,10.86z $\endgroup$ – alephzero Feb 3 '17 at 10:56
  • $\begingroup$ @alephzero, also the Walton backwaters, but I was keeping the system simple :) $\endgroup$ – Separatrix Feb 3 '17 at 11:02
  • $\begingroup$ This answer also nicely addresses the issues that some other answers have (e.g. evaporation limiting the lake level, and leaving salt deposits). By backflowing, the water carries its salt back with it into the ocean, so while you will still have some evaporation (and thus salt buildup) it will be much less than that of a lake that is continuously supplied/drained by a tidal river. $\endgroup$ – Doktor J Feb 3 '17 at 19:12
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TL;DR: Yes, but only if meeting very specific conditions. I would however actually encourage this for the unique thematic effect.

If the water flowing from the ocean into the lake either:

a) Evaporates at the same or higher rate than the inflow.

OR

b) Continues elsewhere underground (at such rate) until it spews from some warm water geiser.

Additionally the inflow from the ocean will increase over time as more ground is dragged along from the riverbed and ocean. This inflow of ground would be able to fill the underground geiser tunnels if it keeps increasing.

If you really want the salt water river for thematic effects, then do so it adds to the uniqueness of your story, but if I were the one writing I'd let the water come trough a rock based canyon(caused by earthquake back in time) into a lower valley. This way the input from the ocean will remain the same and if the initial rock debris didn't close the underground geiser caverns then the much lower amount that will come in over time is unlikely to do it any time soon.

Intelligent beings are also likely to:

a) Close any leakage of the canyon except at it's final destination where this is impossible, as it would affect their farmland badly trough salting the earth if any small streams diverged from the canyon at other locations.

b) Create a successful salt production settlement, as the area is far from the ocean and possibly low in salt mines. Creating supply in a low supply area, easily becoming an important trading hub.

c) Goods could be transported downstream easily, again making this inland settlement a potentially vibrant trading hub where other people come to buy what they can't elsewhere.

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There is such a situation on Earth, alltho' not from the Ocean but from Caspian Sea: There is a river of salt water, even having a cataract of salt water, into another salt lake called Kara Bogaz Gol.

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Easily, but not for long.

The water in a desert lake fed from the sea would be supersaturated with salt, and the inflowing salt would precipitate all the time, slowly filling up the depression.

Unless the channel from the sea widens too quickly, in which case you get something like the mediterranean sea.

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  1. At high tide water flows into the lake due to the gravtational pull of the moon(s). At low tide it flows back into the ocean.
  2. The desalination settlement (DeveloperDoge suggested) but with the twist that it flows into an active lava flow and evaporates.Perhaps even flowing into a sunken volcano or caldera.Steam technology?
  3. Engineer a dam or a system of locks that connect to the lake, controlling water flow.
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short answer: all rivers between a lake and a sea will flow to the sea

basically, if the sea can flow into an area (e.g. a lake), it will quickly fill that area and stop flowing. rivers constantly flow into the sea because the sea is constantly evaporating away due to it's huge surface area. there are exceptions, but they are far less common than the standard rain to river to lake to sea paradigm. just for fun, i'll list some exceptions:

  1. some rivers end in a salt lake, salt flats or salt marsh. the water flows into the salty area, then evaporates away leaving behind the salt and other sediment. this area might not be called a "sea" if it isn't a large enough body of standing water.
  2. some rivers will flow "backwards" at high tide. nearly all rivers do this to some extent. they generally don't flow backwards long or hard enough to actually reach an area called a "lake".
  3. after earthquakes, it may take a short time for the sea to flow into a newly created depression. this situation generally resolves within a few days.

if you just want a semantic solution, you could have a big lake draining into a small lake, which later drains into the ocean. the locals might call the big lake a "sea" and the small lake a "lake", but a "sea" is by definition salty, which means the lake would be salty too. the connection between two would be a salty river. salty rivers on earth tend to be extremely rare, short and short lived (in geological time). but hey, they happen.

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If you just want a lake with a river flowing into it, that's entirely possible (and actually exists on Earth), so long as there is enough evaporation (or maybe some industry using the water for something else) to compensate for the influx of water.

A problem arises when seawater flows into your lake, as seawater contains salt it will turn your lake into brine quite fast.

But if the rivers flowing into the lake are clean and fresh water, your lake should stay relatively clean. Especially if the lake is very deep and pollutants just sink to the bottom.

There are numerous lakes like this around the Gobi desert, being fed from runoff from glaciers in the Himalayas, and losing their water to evaporation. Example: Bosten lake, wiki, google maps.

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Take a look at the Zanclean flood. This is when the Mediterranean was flooded by the Atlantic. Estimates of the time taken vary from a few years to 10,000 years, so you could take your pick. Do bear in mind that water will erode whatever channel it is running through, so if you want the process to take a long time then there is going to have to be some hard rock creating a bottleneck. There is also a lot of scope for economic, social and military conflict driven by the rising water levels, madcap schemes for damming the flow that might just work, and nefarious schemes for destroying the bottleneck to make the flow dramatically increase.

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