Water supply and wastewater

Das Wehr is the English weir, and not by coincidence: both go back to a Germanic word for something that keeps a thing out or holds it back. English kept the river sense and let the rest go. German kept the whole family together, and you can still hear it in the verb wehren, to fend off, and in sich wehren, to defend yourself. A weir holds back a river the way a defender holds back an attack. The same root gives a second noun that differs only in its gender: die Wehr is defence, and it lives on in die Feuerwehr, the fire brigade, and in die Bundeswehr, the German armed forces. So the article is doing real work here. Das Wehr stands in the river; die Feuerwehr is who turns up when the river comes over the top.