In the case of distributaries (the lower Ganges, in this case), the water is still following the path of steepest descent. The main channel starts to fill with silt until it is no longer able to carry the entire flow, which forces a side channel that can carry the rest of the flow. This side channel appears because the amount of water + the reduced channel size makes the total water column height greater than the surrounding landscape. This sort of thing is common in deltas all over the world, but the length of the distributaries is always a tiny fraction of the overall river length.
Many of the river splits we see here at the guild take place far up along the river, which is very unstable, geologically speaking, and will not last any significant length of time.