thalweg
Concise Oxford English Dictionary © 2008 Oxford University Press:
thalweg /ˈtɑːlvɛg, ˈθɑːlwɛg/
▶noun Geology a line connecting the lowest points of successive cross sections of a valley.
– origin C19: from Ger., from obs. Thal ‘valley, dale’ + Weg ‘way’.
'thalweg' also found in these Oxford entries:

