How does episode 2 work (can't find the rule snippet anywhere)?

[Affensohn]

In episode 2, we're being told that it is important to have connected coloured areas.
Since I am nowhere able to see this message again (or am I?), I wonder whether the river divides a connected coloured area or doesn't.
Might anyone help?

Would be nice to find those additional rules anywhere on the site. Thanks in advance!

[HaetteHaette]

I think the river is irrelevant for that rule.

But I haven't seen an explicit rule saying that.