For example, when a river bifurcates into two separate streams of water, and then these two streams rejoin further along to become a single river once more.
Bifurcate itself is not generalized enough for the title purpose because (a) it implies a split into 2 parts, rather than n; and (b) it doesn't suggest anything about rejoining.
Example sentence can be as simple as "the river [term] as it travels along the flat valley", with the meaning as above, though the specific form might vary.
Perhaps branching could provide the n-parts concept, but I'm not sure how to incorporate rejoining in an elegant way. Perhaps also there is some computer science or mathematical term for this idea, since it seems it might be useful in those fields.