The Amazon basin formerly drained westwards into the Pacific Ocean, until the Andes rose and reversed the drainage. The Congo basin is completely surrounded by high land, except for its long narrow exit valley past Kinshasa, including waterfalls around Manyanga. That gives the impression that most of the Congo basin was formerly on a much higher land level and that it was rejuvenated by much of its lower course being removed. Before Gondwanaland broke up due to continental drift, the Congo would likely have flowed into the Amazon.
West Siberian Glacial Lake drainage
Main article: West Siberian Glacial Lake This river would have been about 10,000 km (6,200 mi) long, in the last Ice Age. Its longest headwater was the Selenga river of Mongolia: it drained through ice-dammed lakes and the Aral Sea and the Caspian Sea to the Black Sea.
Formerly Lake Tanganyika drained northwards into the White Nile, making the Nile somewhere around 1,400 kilometres (870 mi) longer, until in the Miocene the Virunga Volcanoes arose and blocked its course. Also, when the Mediterranean was dry during the Messinian Salinity Crisis, the Nile extended northwards over the dry seabed and thus may have gained 160 kilometres (99 mi) or more in length.
When the Mediterranean Sea was dry or much lower during the Messinian Salinity Crisis, the Po would have extended its course south-eastwards in what is today the seabed of the Adriatic Sea, more or less doubling its current length (652 km), likely varying seasonally according to how far it managed to flow across the hot dry seabed until it dried.