You're looking for a feature of "El río viviente" (The Living River) by Isaac Asimov!
You can find "El río viviente" (The Living River) in various Isaac Asimov collections. Unfortunately, I couldn't find a direct link to a PDF version of the story. However, I can suggest some sources where you might be able to find it: el rio viviente isaac asimov pdf
Here's one:
If you landed here looking for a free PDF of Isaac Asimov’s El Rio Viviente (The Living River), you’re not alone. This short but brilliant essay is a staple for Spanish-language science enthusiasts and students alike. You're looking for a feature of "El río
8vo, tan buckram cloth, white titles, 232pp. First Edition. Price-clipped dust jacket chipped and creased at upper edge, Brentano? tan buckram cloth