Okavango Delta is one of most probably suggestive examples.
I was wondering what would happen if a earthquake or a large landslide happened and blocked of the Niger from it's OTL path. Where would it go? Maybe if there is a way to get it across the Sahara and into the Mediterranean what would be the consequences?
The Guinean forest-savanna mosaic covers an area of 673,600 square kilometers (260,100 sq mi), extending from western Senegal to eastern Nigeria, and including portions of Gambia, Guinea Bissau, Guinea, Sierra Leone, Côte d'Ivoire, Ghana, Togo, and Benin. The Cameroon Highlands of eastern Nigeria and Cameroon separate the Guinean forest-savanna mosaic from the Northern Congolian forest-savanna mosaic, which lies to the east. The Dahomey Gap is a region of Togo and Benin where the forest-savanna mosaic extends to the coast, and separates the Upper Guinean forests of Guinea, Sierra Leone, Liberia, Côte d'Ivoire, and Ghana from the Lower Guinean forests of Nigeria and Cameroon.
https://en.wikipedia.org/wiki/Guinean_forest-savanna_mosaic
Without a series of earthquakes of a magnitude not known to have occurred in that part of the world during the several hundred thousand years or the sudden eruption of long-dead volcanoes or several multi-megaton blasts, what you're wondering about is not going to happen. There's a reason why the Niger has a boomerang shape; it's because the Sahel, to its north is at a higher elevation, and water doesn't naturally go up hill. All that would realistically happen if the Niger were blocked, would be massive flood in this area:
What about it going into lake Chad?