How (un)likely is a split of one major river into two others?

I inherited a worldbuilding project, and the former authors liked some things that seem unrealistic. One of them is that one of the world's biggests rivers splits into two other rivers some 300-400 kms before reaching the sea, definitely before area where the delta should be expected.

Map legend: Galvina river (flowing from the north) splits into Galvina and Molana (the red circles), both flow into the sea. One square is 100km times 100km.

How likely is this to happen naturally? Is this even possible to occur and stay for more than 1000 years?

The terrain around the rivers is covered by jungle, and it wasn't specified how flat or hilly it is, so I guess that there should be hills around but valleys where the rivers should flow. However, as I understand geology, one of the river should prevail and make its channel so deep that the other river would get dry. However, I don't know how long would this take and what could produce this phenomenon in the first place. The world's recorded history spans for 1000 years and the rivers should stay as in the map at least for few more centuries.

posted over 9 years ago

Pavel V.‭

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