or as soon as we set eyes on it we understand how it got its name ....

or on seeing las piedras its name becomes clear

many options and some may fit better by rewriting the whole sentance

Utilizing Charles' re-punctuation of the ST - "Los procesos erosivos ocasionan constantes desprendimientos en la Caldera. Este barranco es un ejemplo de ello. Su nombre, "de las Piedras Redondas", lo comprobamos visualmente. Mirando hacia las paredes superiores, rápidamente nos haremos una idea de cómo se ha formado a lo largo del tiempo este espectacular cauce." - the translation falls right out. I would use "rounded rocks", which is a perfectly good term from sedimentology. And while all river rock is generally rounded rock, not all rounded rock is river rock - there is a phenomenon called spheroidal weathering, which is most common in granitic and related rock types, in which chemical weathering takes place along fracture planes in buried bedrock, resulting in very rounded, almost spherical, boulders, which can be exposed by erosion from the surface downwards. https://en.wikipedia.org/wiki/Spheroidal_weathering