In weakness there is often hidden strength. Water droplets destroy mountains and wash them into the sea by breaking down the stone one tiny piece at a time and carrying the debris off, first in brooks, then in streams, then larger and larger rivers, until finally it reaches the huge natural waterways that drain continents. These great rivers never die because they are self-cleansing, the water that runs through them constantly recycles itself, and they are linked with the land in a long, slow, ever-changing cycle that ensures there will always be a great river present.
♦ Rivers are survivors
The self-cleaning capability of all rivers is not something we are overconfident of today, but I have witnessed it. Some of our neighborhood streams back in the 1960s were strange shades of blue-gray and livid green and had been this way even before the Monsanto company built a chemical plant a few miles down the road from our home. They stank, and nothing grew in them. However, the environmental movement was gaining momentum back then, and soon strict state and federal laws were passed, protecting these and other streams and rivers. Today people can fish in them. Our massive environmental efforts really are aimed at giving rivers a break so that they can clean themselves in various ways, by filtering water through river sediments and nearby wetlands, replenishing the oxygen content with aquatic plants, and so forth.
♦ The hydrologic cycle
Water seldom disappears completely from Earth. It just changes form, circulating through our environment in a closed system that could be summed up as "what goes up must come down, and then go up again." In this hydrologic cycle, sunlight evaporates water from ocean surfaces. As the water rises and cools, it condenses into clouds and eventually falls back down as rain or snow, where some of it percolates into the ground to reappear, perhaps thousands of years from now, from a cave mouth, a volcanic vent, or just as quiet seepage from the water table into a brook or stream. Most of it, of course, stays on the surface and travels relatively quickly over the land to the nearest channel, eventually returning to the sea via one of the world’s great rivers.
♦ Rivers ride the changing continents
Great rivers are those large enough to drain part of a continent: the Mississippi and Saint Lawrence rivers in North America, for example; the Amazon and Orinoco rivers in South America; the Nile and Zambezi rivers in Africa; the Yangtze and Mekong rivers in Asia; the Volga and Danube rivers in Europe; or the Murray-Darling and Murrumbidgee rivers in Australia. Plate tectonics theory shows how Earth’s huge landmasses change over time, and now we are also learning that great rivers have adapted to these changes. In 2010, scientists reported finding evidence from the Jurassic era that a great river may well have drained what are now the Appalachians and flowed westward at least as far as what is now the Colorado Plateau. As geologic processes reshaped the continent, the water flow of all North America's great rivers changed with the land, and now most of them flow generally south (with the Saint Lawrence’s flow toward the east being a prominent exception).
Humanity is really a bystander to this process. While it seems that we have the ability to totally destroy a river, they do in fact come back, even when we abuse them to the point where they burn, as the small Cuyahoga River did many times between the 1860s and 1969—now over 40 species of fish thrive in some parts of the Cuyahoga. Great rivers can be leashed, but they cannot be killed: The Colorado River often runs dry at its mouth now, but water still evaporates daily off the seas and falls into the river’s watershed; and when the dams disappear, as all works of man ultimately must, the huge river will once again pour into the Gulf of California. Even the shifting changes of continental movement cannot destroy great rivers. The great rivers of the world will outlive us all.
Sid Perkins, “Before the Mississippi, minerals show ancient rivers flowed west.” Retrieved on February 2, 2011, from http://www.sciencenews.org/view/generic/id/59998/title/Before_the_Mississippi,_minerals_show_ancient_rivers_flowed_west_