Two premises
1) The river names from *wip-/*wib- etc have -p- in the present
Germanic area (fact).
2) Those areas were Germanic speaking at the time of the Grimm-shift
(traditional assumption)
=>
conclusion:
Those river names were names in *wib- before the Grimm-shift.
That's also how Udolph explains it. But where are then the *wip-
forms? Why aren't there any river names in *wif-?
Check the wib-/wip-map
http://tinyurl.com/2tfnfj
Note that the rivers marked as *wib- have been marked so by Udolph
himself according to his Germanic-continuity-preserving theory that
PPPGmc. *p > PPGerm. *b > PGmc *p. In actuality, all those rivers in
Germany have -p-. Now with names in -p- all over the continent, where
did the Grimm-shift happen?
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*vab- is attested in Celtic Gaulish
as "small river"
Arnaud
====================