The Spanish word 'río' meaning 'river' comes from Vulgar Latin 'rius', which itself developed from Classical Latin 'rivus' meaning 'stream' or 'brook'. The evolution from Latin 'rivus' to modern Spanish 'río' shows a common sound change pattern in Spanish where the 'v' sound was lost between vowels, and the final '-us' ending typical of Latin masculine nouns was simplified to just '-o' in Spanish.

Some related Spanish words include 'riachuelo' (small river, stream), 'ribera' (riverbank), and 'riada' (flood). All these words share the same Latin root 'rivus' and maintain connections to flowing water and rivers in their meanings.

English words related to 'río' include 'rivulet' (a small stream), 'rival' (which originally meant 'one using the same stream as another'), and 'derive' (which literally meant 'to draw from a stream'). These words all come from the same Latin root 'rivus'. While 'río' maintained its direct connection to water, the English derivatives have either stayed close to the water meaning (rivulet) or developed more metaphorical meanings (rival, derive).