This can happen for several reasons, including: The additional water flowing down the capturing stream may accelerate erosion and encourage the development of a canyon (gorge).
The Slims River was previously fed by meltwater from the Kaskawulsh Glacier in the Saint Elias Mountains in the Yukon and its waters flowed into Kluane Lake and on to the Bering Sea.
[8][9] Geological uplift in the southern South Island led to the divergence of freshwater galaxiid populations isolated by river capture.
In the last two million years erosion has reduced the Great Dividing Range to a critical point where west-to-east river capture events have been possible.
Examples include: Olive perchlet (Ambassis agassizii), western carp gudgeon (Hypseleotris klungzingeri), pygmy perch (Nannoperca australis) and Australian smelt (Retropinna semoni) also appear to have made crossings into coastal systems, the last two species seemingly many times as they are found in most or all coastal streams in south eastern Australia as well as the Murray-Darling system.