An Etymological Dictionary of the German Language/Annotated/Quelle
Quelle, feminine, ‘spring, source,’ first occurs in early Modern High German (naturalised by Luther); a late derivative of the Modern High German strong verb quellen, Middle High German quëllen, Old High German quëllan. From the variant kal (by gradation qel), derived from Old Icelandic kelda, ‘spring’ (whence Finnish kaltio), and Gothic *kaldiggs, which is implied by Old Slovenian kladęzĭ, ‘spring’; likewise Anglo-Saxon collen, ‘swollen.’ The prehistoric root gel (gol) is related to Sanscrit jala, ‘water,’ gal, ‘to curl.’ — quellen, ‘to soak, cause to swell,’ is a factitive of Old High German quëllan, ‘to swell.’