Old European Hydronomy.
The hydronyms of Europe and northwesternmost Asia are the oldest echoes of European substrata. Since their discovery by Hans Krahe in 1964, the catch-all term "Old European Hydronymy" has been used to describe the similar but un-analyzable names of ancient waterways of Europe. He pointed to the prevalence of a-vocalism in the words, as well as non-Germanic alien morphemes. The high frequency of a-vocalism is an outdated mystery, as it can be explained as laryngeal weaking of PIE *h2 > *a, among other mechanisms (Udolph 2002). An emphasis on non-Germanic morphology, was a limited perspective; support from other language families was needed and Krahe's list is not an accurate depiction of modern putative hydronyms (e.g., see discussion of Krahe's *el-/*ol- under *alant-). Here is brief depiction of Krahe's morphological data printed in Vennemann (2003), but edited and reconstituted here. Each of the following is the name of a river in Europe and can be broken into its lexical components:
There are multiple ways to interpret the data. The first and oldest is that this is the product of coincidence (Kitson 1996). The second that these are representative of the earliest substrata in Europe, vestiges from a time when the first explorers left the south for the north in search of game and crops, shortly after the retreat of the glaciers. The third is that these are Indo-European in nature, which was Krahe's own argument. The fourth is that these are extremely ancient substratal lexemes but dating them is fruitless. The first interpretation is somewhat a minority opinion, as in some instances when an unattested but Indo-European branch that then disappeared across Europe prior to writing must be hypothesized; the fourth is nearest to what can be described as the community's opinion.
The rivers, lakes, and streams of Europe certainly have similar names that ante-date the arrival of the Indo-Europeans. Many of these names betray an Iberian or Vasconic phonology which has led people to fringe conclusions such as Vennemann's Europa Vasconica theory or Blanca-Ferrer's Proto-Vascuence theory. However, most of these hydronyms that resemble Basque or Iberian are, not surprisingly, endemic to Iberia anyway. Moving outside of Iberia, we find a hydronyms of alternative phonotactics. A safer diagnosis is to suppose that many of the hydronyms were Iberian or Basque but their original meanings have been lost.
Aside from the names for hydronyms, several non-hydrolic Wanderwörter can be included. These pervasive Western European lexemes have robust attestation in Western European languages and are tantalizing remnants of the first peoples of Europe. To limit ourselves to just the waterways of Europe is to ignore several important lexical items that wend their way through Europe much like the rivers. These extra-hydronymic examples are *-asyo- name-forming suffix; *kar(r)- "stone;" .
*al- "to flow" Old European Hydronomy. Villar & Prosper 2005; Ballester 2007.
*alant-, hydronym. Old European Hydronomy. See *alant-.
*alant-, hydronym. Old European Hydronomy. Villar & Prosper note that this etymon's range matches the historical range of genetic Haplogroups V and H3 [I am skeptical of its significance]. Alternate reconstruction of Baltic / North Germania reflexes found in *el/*ol by Krahe (1964) but has come under serious fire (see Undolph 2010 for discussion). [In addition to said criticisms, Celtic root *al-o- "feeds" is a root in tributaries, in the sense of feeding water to a larger lake (cf. Gaulish alaunos, alauna); see Matasovic 2014]. Villar & Prosper 2005; Ballester 2007; Cuchin 2008.
Aland, a river in Lower Saxony.
Alande, a river [in Latvia?].
Alando, [Corsican river/town?].
Alanta, a tributary river in Lithuania.
Alante [I can't find this in primary sources! It's listed in Ballester 2007. Please contact me if you can help].
Alanzabas, [listed in Villar & Prosper 2005 as Cordoba waterway but I can't find it; alanzabes is a common Spanish surname, however this may just trace to Spanish alanzar].
Alonta [may represent Celtic *al-o- "feeds" from an unattested Continental Celtic language].
*-asyo- Name-forming suffix. Unknown; [Old European Hydronomy?]. Curchin 2008.
Turasia, a lake. See also *tur-.
Makasia, personal name. On Botorrita Bronze I.
Kamasio, personal name. Villar & Prosper 2005.
*(a)ur-, "water." Old European Hydronomy. [From Latin aur-um "gold"? See *(h)arus.] Villar 2000.
*urc-, a hydronym. See corresponding entry here in the Dictionary.
*(h)arus, original name of a river. Old European Hydronomy? Alternative explanation is that this is a corruption of Latin aurum "gold," as the Greek name for guadiaru was "the gold river." [I would have ignored this squib if it were not for the interesting similarity to *(a)ur-]. Correa 2013.
Guadiaru, a river. From Arabic wadi yaru "the river Yaru." Likely Yaru is from Gothic *aro, a Germanicization of Latin *ar-us [vulgar corruption of aurum? Neuter Latin suffix -um > -o anyway, so why ask for masculine gender?].
*iber- Unknown meaning. Iberian(?). Curchin 2008.
[Ebro, a valley.]
(H)iberus, a river. The h-initial is an Iberian Romanism and can be safely disregarded if need be.
Iberia, a demonym.
*kar(r)- "stone," Western European Wanderwort. A generally accepted paleo-IE root. Cuchin 2008; Trask 2008; Kroonen 2014.
Cara(e), a town.
Caraca, a town. Modern Carpentani.
Careni, a demonym.
Carietes, a demonym.
Caristi, a demonym.
Caronium, a town. Modern Galicia.
(h)arri, "stone." Basque.
*kark-ú- "pile of stones forming a sacrificial mound." Pre-Proto-Germanic. See entry in Pre-Proto-Germanic.
*kar-w- "stone." See entry *kar-w- in Iberian Peninsula Languages.
*sal- Unknown meaning. Curchin 2008.
Sala, a river.
Salia, a river.
Salo, a river.
*tam- Unknown meaning, mostly in towns, nearly entirely in Iberia. Ballester 2007.
Tamarici, a location in Iberia listed by Pliny.
Tamaris, towns in northern Italy and Galicia. Modern Galician Tambre.
Támega, a town in Galicia
Támesis, a town listed by Caesar as Londonese Latin. [Probably fair to discount this one as coincidence].
Támoga, a town in Galicia.
Tamuxe, a town in Galicia.
Thames, a river in London. Highly controversial. Kitson (1996) vehemently disagrees and invents a Pre-Celtic, Indo-European language to explain its etymology. [I find Kitson's explanation equally implausible. I say ignore Thames and Themsche as the aspiratism is problematic].
Temsica, a town in Flanders.
Themsche, a town in Flanders.
*tur- / *ter- Unknown meaning. Unknown; [Old European Hydronomy?]. Traditionally regarded as Iberian but questionable. Curchin 2008.
Nemanturista, a town. Probably Indo-European and not a substratum.
Turasia, a lake. With suffix *-asyo-.
Turia, rivers in the south and north of Spain.
*urc- Unknown meaning. Derived from *(a)ur-. Villar 2000.
Orcelis, a river.
Orcia, a deity.
Orcia, a town. Most believe this to be Celtic, however. Curchin 2008.
Orcia, rivers in Greece and Dacia.
Orcius, personal name
Ballester, Xaverio. "Hidronimia Paleoeuropea: una Aproximacion Paleolitica". Quaderna di Semantica. Vol. 28, Is. 1. 2007.
Correa, Jose A. "El hidronimo Guadiaro: Nota a Avieno". Palaeohispanica. Vol. 13. 2013.
Cuchin, Leonard A. "Place-Names of the Ebro Valley: Their Linguistic Origins". Palaeohispanica. Vol. 8. 2008.
Kitson, P. R. "British and European River-names". Transactions of the Philological Society. Vol. 94, Is. 2. 1996.
Kroonen, Guus. Etymological Dictionary of Proto-Germanic. Ed. by Alexander Lubotsky. Brill, 2014. 'Brill Online'. 10 February 2014.
Matasovic, Ranko. Etymological Dictionary of Proto-Celtic. Ed. by Alexander Lubotsky. Brill, 2014. Brill Online. February 24, 2014.
Udolph, Jurgen. "Nordic, Germanic, Indo-European and the structure of the Germanic language family" in The Nordic Languages, Volume 1. Ed. by Walter de Gruyter. 2002.
"Alteuropa, Iller, Alster, Elster und Aléti". Acta Linguistica Lithuanica. Discussion Paper. 2010.
Trask, R. L. Etymological Dictionary of Basque. University of Sussex. 2008.
Vennemann, Theo. Europa Vasconica - Europa Semitica. Walter de Gruyter. 2003.
Villar, Francisco. Indoeuropeos y no indoeuropeos en la Hispania prerromana. Salamanca. 2000.
Villar, Francisco & Blanca M. Prosper. Vascos, celtas e indoeuropeos. Genes y lenguas. Salamanca 2005.