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.

• Does it have to be completely natural, or can there be human influence? I can see bifurcation happening and lasting if each branch of the river has a decent drainage basin downstream of the split. However, as pointed out in a lot of answers a bifurcated river will tend to favour one route over the other, leaving the other very small and small flowing (between the split and the drainage basin). If there was a significant advantage to keeping both rivers flowing strongly - most likely for navigation - then human interference with the flow could keep each branch roughly equal. – Matt Bowyer Feb 3 '18 at 23:25
• @MattBowyer: I had "completely natural" in mind. Human influence can make it more durable, but I am interested in how long (centuries? thousands of years? even more?) can a natural bifurcation last. – Pavel V. Feb 5 '18 at 9:04

What you are talking about is called bifurcation and yes it happens. However usually over a long period of time (like 1000 years) one branch or the other will get silted in and turn become the minor branch. Having 2 major branches is not something that generally lasts.

You are going to need some very specific feature for something like this to work long term. First in order for a split to last you need the river to flow through an area of hard rock that can resist weathering and erosion by fast moving water in the river. The split would have happened because the natural erosion of the river came in contact with this harder stone that resisted erosion and thus caused some of the current to be redirected to the other side. You can see this happen in rivers where there are islands and short term splits like in the Grand Canyon.

The next feature that will be required is that both channels will need to be moving quickly enough that they resist a build of of silt that would normally close off one of the channels over time. So the chances are the split happened at a place that is elevated sigifigantly from just a few miles down stream. It could be that both river forks flow into different valleys.

One thing I would note is that places like this tend to attact settlements. These are places where a large amounts of goods will likely pass on their way down river and since there is likely to be rapids that will have to be negotiated or bypassed it going to naturally be attractive to tradesmen who will look to make a buck(or pence or whatever) off of the people who will be making camp or even just taking a break before the challenging task ahead. Not to mention a place where goods may get stranded or abandoned as over agressive traders are forced to lighten their load before attempting to transverse the rapids, and goods get salvaged after some fail that transversal. This area is going to become a natural area for civilization to pop up in support.

• Hard rock under thick soil was an idea that occured to me just after reading Cal West's answer. The situation you describe (rapids, hills around) could support the bifurcation for more than 1000 years, especially if the smaller river is in a steeper, but narrow valley, so the rest of the water formed a lake, before it opened the other, wider valley. Btw. I just asked a question on Earthscience.SE on the age and reasons for survival of the Earth's biggest bifurcation; I guess it will just approve what your answer. – Pavel V. Sep 17 '14 at 19:02
• Another way of stabilizing the bifurcation (without the benefit of hard rock) is human intervention by building dykes and regulating the waterflow with dams and locks. Take a look at The Netherlands and what the Dutch have done with the Rine/Meuse delta. en.wikipedia.org/wiki/Rhine#/media/… – Tonny Feb 2 '17 at 13:51
• @Tonny Agreed. The locals probably have it in their best interests to both (a) keep both sides flowing and (b) keep both navigable. – Demi Feb 5 '17 at 1:22
• @demi Let's not forget reason nr C: To keep our feet dry. Not only is about 40% of our country below sea-level, at least 25% of the rest is in the floodplain of one of those rivers. If we don't regulate the rivers then every spring in the Alps will result in major flooding 2-3 weeks later when the melt-water hits us. – Tonny Feb 5 '17 at 17:35

That depends on how close to the time of the split you are asking from. The closer to the time, the more likely it is to be visible.

Once a river finds a new route it will flow in that direction. Over time the majority of water will continue on this new route eroding more material, making it even easier for more water to choose the new route.

Eventually the new route of the river would be made deep enough to accommodate the full watershed making the original route obsolete. It would become a dry-river bed. Think about gorges or creeks in this regard.

It is very unlikely that both possible routes for the river to reach the sea will erode material at similar enough speeds to keep them completely equal.

EDIT:

To answer more fully, the speed of the shift from one route to another depends on the comparison of the erodibility of the material that the two routes flow through/over.

Old route stone vs. new route stone the process will be slower. Old route stone vs. new route clay the process will be much quicker.

Not only is this possible, but there are already living, long-lasting examples of bifurcations. Check https://en.wikipedia.org/wiki/Casiquiare_canal for a branch of Orinoco river that flows away into Amazonas river. These bifurcations are called distributaries (https://en.wikipedia.org/wiki/Distributary) and are not uncommon, albeit most of them are in deltas.