“brook of Kanah.” This is a perennial stream, even if at the end of summer the water is only a trickle. The brook of Kanah had very steep sides on both sides and the canyon was hundreds of feet deep in places. The brook runs in a deep canyon and is a clearly definable geographic feature, which is why it makes such a good border, there was no mistaking it. And the canyon runs all the way out to the Mediterranean Sea.
“belonged to Ephraim among the cities of Manasseh.” It seems that people from Manasseh leaked over into what was technically Ephraim.