[CarrotAndStick]

The supposed sudden disappearance of the Saraswati river in ancient India, and not the Aryan influx, is thought to have led to the demise of the Indus Valley Civilisation.

[SunkenCiv]

from a (probably) dead link:

Evolution in Your Face
by Patrick Huyghe
Omni
Wayback Machine versions

Lake Victoria, Africa's largest lake, is home to more than 300 species of cichlids. These fish, which are popular in aquariums, are deep-bodied and have one nostril, rather than the usual two, on each side of the head. Seismic profiles and cores of the lake taken by a team headed by Thomas C. Johnson of the University of Minnesota, reveal that the lake dried up completely about 12,400 years ago. This means that the rate of speciation of cichlid fishes has been extremely rapid: something on average of one new species every 40 years!