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In my book series, there is a very small planet (slightly smaller than Earth's moon). This is called Planet Squid, and it has a climate and group of cultures very similar to those of sub-saharan Africa. Most of the planet is dense jungle, with some desert in the west and a savannah area in the southwest. There is only one continent, almost all of which is in the drainage basin of the Enkulu River.

The Enkulu river is fed by a lake in a caldera at the top of Shining Mountain. However, due to a magic ritual that I don't have time to explain and isn't really important to the question, the lake's water level has dropped by around 90%. What was once a 500-foot-feep lake now barely cracks 50 at its deepest part. The civilization uses the Enkulu River Valley for pretty much all of its crop farming, but most of their ranching/pastoralism is done in the Savannah and the area near Zulundi.

Anyway, the question is: How would this sudden reduction in water level affect the viability of agriculture in the Enkulu River Valley? Wil the people who depend on this river for irrigation have to move to a better water source or will the river still trickle out enough to grow their crops?

I'll include a grossly oversimplified map of Planet Squid (at least the areas seen in my book series) so you at least kind of know the lay of the land (ignore the victory markings on the map, it was originally used for something else): Map

Here is a link to my website's page on Planet Squid in case you need even more detail: Link

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closed as unclear what you're asking by Mołot, Ash, Ryan_L, Trish, jdunlop Sep 24 '18 at 17:33

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ You ask about "the area" in the title, "civilization" in the body, and have tagged this "medical". Which will it be? Please Edit to clarify. $\endgroup$ – a CVn Sep 24 '18 at 15:06
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    $\begingroup$ Without knowing what percentage of the river's water comes from the lake, as opposed to drainage from the rest of the basin, we can't really answer this question with anything useful. $\endgroup$ – Ash Sep 24 '18 at 15:06
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    $\begingroup$ How can a river be fed by a lake in the caldera of a volcano? How is the lake replenished? Is there precipitation on your world? Would rain falling on slopes of the volcano not be a larger catchment area for the river? $\endgroup$ – Binary Worrier Sep 24 '18 at 15:09
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    $\begingroup$ Calderas tend to accumulate precipitation only, and only for a limited time - volcanic activity tends to fracture nonporous strata, giving water an easier path to springs lower down the mountain. The same activity will eventually fracture the caldera sides and drain the water from the crater. $\endgroup$ – user535733 Sep 24 '18 at 15:24
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    $\begingroup$ If the civilization draws it's water from the lake, they will need to migrate to a new reliable water supply. If they (foolishly) dropped their wastes into the same lake (yes, people do that sometimes), then the civilization will see a big decrease in water-borne diseases after they migrate. More children will survive. $\endgroup$ – user535733 Sep 24 '18 at 15:28
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As Binary Worrier said, the lake on top of a mountain will can only contain a small quantity of water, and draining that lake will have very little effect on the river.

In your map, your river flows by the jungle. Jungles require rain. All that rain will flow into the river, and keep it at the same level.

If you want river level to drop, you need a setup like Nile and lake Victoria: large flatland lake fed by smaller rivers, and a major river flowing out of the lake, and through a desert (so no additional water).

Putting an alternative drain out of that lake will cause drop in river level, so crops die, animals go mad with thirst, etc.

If you want a volcano in this scenario, have it erupt under the lake, so it evaporates the water currently in there, and redirects the water from feeder rivers.

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I have farmed in drought years where the water level in the ponds dropped to almost 0% and the lakes fell by around 40 to 45%. This can cause year round creeks to go dry, which has some effect on the whole riparian zone. Most of those areas were fed by aquifers that had a limit on irrigation on drought years. That also depends on if this culture your writing about has the knowledge of utilizing aquifers to irrigate and the capacity of those aquifers if there are any. A 90% drop in the feeding source of a river would certainly effect the farmers 100% if that is their primary irrigation source and if there isn't rainfall as well. It also depends on how those jungles and that desert move the moisture and how much the lake effects the movement of that moisture. If there's full or mostly full aquifers and average rainfall, then they'd be able to make it. It also depends on where that mountain moisture went. If it evaporated, then there may be to much rain down in the valley and there would be great floods and the farm bottom land would be unable to farm and most vegetation would rot, especially root crops and the soil would spoil. That happened to me one year in Oklahoma. It rained from early April until almost June, on average of 1-3 inches every day almost and some days it dumped 6 inches. I planted crops three times in a couple 3 or 4 day breaks about every ten days. Nothing survived. potatoes rotted in the ground, Kale died from root rot and the soil smelled like rot. I didn't get plants in the ground that survived until the first weeks of June. Then it got hot and humid to the point it almost killed those plants. So these are all things that have to be factored in.

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  • $\begingroup$ Welcome to Worldbuilding, Roberta. This is actually a good answer, especially for someone new to the site. That said, for the sake of readability, may I suggest you edit the answer to break it up into paragraphs? If more people find it easier to read all the way through, you'll automatically attract more upvotes by virtue of having something interesting to contribute. $\endgroup$ – Tim B II Oct 4 '18 at 3:23

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