Vltava, Sázava, Mumlava.

It’s always nice to discover a blogging linguist I hadn’t known about [actually I had; see below]; Danny L. Bate (“Linguist, broadcaster, writer, cat fanatic”) has been doing it since June 9, 2020 (I like the way his introductory post lays out “the standard practice among linguists,” so that laypeople can follow along: single quotes for translations, italics for words in a given language, etc.), and his latest post, Vltava, Sázava, Mumlava: A Mumble of Voices Almost Lost, does a nice job of linking local river names to the great sea of Indo-European:

The chief of the rivers that flow through Krkonoše is the Elbe; I’ve previously written about that prince of waterways in great detail. My most recent visit instead inspired linguistic reflections about a considerably less famous river: the Mumlava.

Etymologically, it’s the mumbler (in older German: mummeln; in Czech: mumlat). It by no means ranks among the great rivers of Europe; the Mumlava rises just to the south of the source of the Elbe, mumbles its way for twelve kilometres, then spills out into the Jizera. It wouldn’t be known at all beyond the wardens and fans of Krkonoše, were it not for the Mumlava Waterfall, the largest in the country. […] Given the region’s historical inhabitants, it seems to have been on the basis of the German verb mummeln that the Mumlava was first christened the Mummel. Czech speakers then modelled their own name for it on that German original. This they achieved by adding the ending -ava.

This Czechification of the name brought the Mumlava into line and rhyme with other rivers in the country; there’s the Sázava, the Jihlava, the Úhlava, the Otava, the Oskava, the Opava, the Morava and the Vltava. […] Their common -ava ending was bestowed on the Mumlava, a sort of hydronymic suffix to make it sound like a proper Czech river. The thing is, this ending is not part of Czech’s core of Slavic vocabulary – it’s not something the language has inherited from its prehistoric Slavic origins. Instead, naming rivers with -ava is a later practice that the Czechs-to-be extracted from names already in use when they first arrived in Bohemia and Moravia.

He goes on to describe how Germanic *ahwō ‘river’ “is behind the -ava part of the river’s name,” and how that probably derives from a Proto-Indo-European *akʷā. But “the door is also open to an instance of borrowing”:

This alternative explanation would envision a word, in a language local to central and southern Europe, that was adopted into the Indo-European family from outside. Such an external origin was considered by the scholar Robert S. P. Beekes, for one. In Beekes’ view, *akʷā belonged to the prehistoric language behind so many European river names. It acted as a ‘substratum’ that donated words to the ascendant Indo-European languages. Those words in time became all that remained of it.

I think he explains these things very well (while providing some lovely photos); thanks, Scopulus!