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