Comment by shkkmo

> If it were not for beavers evolving beside us, the Eastern US and much of the world would very nearly resemble the driest American Southwest of today, with rain being gathered after a brief overland wash into deep river gorges, with little water left behind close to surface.

The rainfall patterns are very different in those two areas. I don't doubt that beavers have important erosion and sediment retention impacts that over time do have a massive impact on the ecosystem and landscape. However, sediment rention is far from only reason why the american SW looks so different from other parts of the country.