well, the analogy does not work, because a river is not a system, but only a huge amount of water molecules following gravity, whereas a body is a system, and it's the system and its structure that survives, not the parts it's made of

@Thibaud70 No, even if the river dries up, then when the rains come again, then the water will follow the same channels, know as the river system, and including such man made structures as bridges, which it will flow under, quays that it will rise against, and dams which it will build behind.

@Fernapple A river is not a system. There might be man-made irrigation systems, but a normal river is nothing but an amount of water following gravity. Like a sand dune, which isn't a system either.
A system (ancient Greek sýstēma "whole composed of several individual parts" ) is something that consists of various components with different properties, which (can) be regarded as a common whole due to certain ordered relationships between them and can thus be distinguished from others.
Where are the parts and ordered relationships of a river ??

@Thibaud70 Parts. Two banks, a bed, several ecological zones, feeder streams, plus man made parts and parts made by other animals such as beavers. And in any case, the fact is that rivers are, wrongly or rightly, always referred to as, "systems," in English, whatever the ancient Greek origins of the word.