The secret river actually carries salty sludge from the evaporation pools of the Dead Sea Works run by Israel Chemicals Ltd

The water evaporates, leaving potash, a potassium-rich salt, and halite for the factories’ use, while the remaining effluent — which is very dense and salty — is channeled back to the lake.
Half a century ago, the Dead Sea was one body of water and it was easy to direct the effluent back from the evaporation pools.

But since 1976, the lake’s surface area has almost halved and its elevation has dropped more than 40 meters (130 feet) — from 390 meters (1,280 feet) below sea level to minus 434 meters (minus 1,425 feet).

The Dead Sea had lost so much water that it split into two and a tract of land that had previously been under water became exposed.