One of the truisms in geology or environmental history or physical geography is that the river is going to win. Water will win. Erosion wears away mountains faster than they can rise (with a few notable exceptions at least in the current moment). But rivers can and do disappear from time to time, sometimes with a little help from humans, sometimes all on their own. Why is often intriguing.
Climate change and tectonic change are two of the major reasons rivers disappear, never to return. Geographers and geologists have found evidence of numerous rivers in what is now the Sahara Desert in Africa, streams that existed in a far wetter, cooler world with less evaporation and different storm tracks.
As flow weakened and the dunes became more active, both caused by increasing aridity, dunes buried some rivers, leaving traces on the surface and groundwater in a few hidden locations.
Far more important to living cultures is the Sarasvati, or Saraswati River, mentioned in the Rigveda and other very, very early South Asian writings. Hindu tradition maintains that the river continues to flow in spirit and is a powerful source of blessing and a place of pilgrimage where it joins the still-flowing Ganges. For a long time the Sarasvati and another river were thought to be poetic, like the four rivers that flowed out of the Garden of Eden were considered to be. But the use of remote sensing, some ground checks, and the discovery of a number of Harappan villages where no river flows today revealed the location of the original, water-filled Sarasvati.
So what happened? At the end of Vedic time, around AD 500 CE or so, the Himalaya and associated ranges rose enough to shift the flow of the waters. Also, climatic changes probably didn’t help as storm tracks shifted north for a time. The villages that had been along the river faded away, as did memories of the stream’s physical location. It was not until the British began poking around in the region in the 1880s and 1890s reported a river too small for its current bed, and proposed that a far larger stream had once flowed there. Modern archaeology and remote sensing, along with oil and gas exploration, has since helped confirm the location of the former glacier-fed stream.
The finding of the route of the original river is rather controversial, although not for scientific reasons (other than the usual – great claims demand great amounts of evidence). No, the arguments come from Pakistani sources concerned about Indian claims to where the river had once been and how it might affect international borders and the Kashmir controversy. Also from different groups within Hindu nationalist groups, some of whom contest the idea of the Indo-Aryan/ Indo-European migration and the historical validity of the Vedas. http://bharatkalyan97.blogspot.com/2015/05/ground-truth-of-vedic-river-sarasvati.html This page gives you a bit more technical imagery and some information about political goings on.
*Since WWII, the English-language convention is to refer to speakers of the languages that became Sanscrit, Bengali, Persian/Iranian, Latin, German, and a few others as Indo-European, and the ancestral tongue as Indo-European. In South Asia, the use of Indo-Aryan remains standard, as it had once been for English, and has absolutely no links to the insanity that developed when H. S. Chamberlin combined bad linguistics with worse biology and a large slug of nationalism to create the blond, blue-eyed Aryan super-race. Aryan in South Asia refers to the peoples who moved into the continent from the steppes around 1700 BC/BCE. They were not blue-eyed, blond, or the least bit “Nordic.”