The greatest rivers in the world usually begin on the cold, frozen slopes of high mountains as the sun begins to melt snow and ice, and from the trickle of crystal-clear water a steam begins to flow and eventually a cascade of water plummets down the mountain.  In the valley far below, the stream becomes a river, then a great river which we call the Nile, the Amazon, the Ganges, the Rhine, or whatever.  By the time these massive bodies of water reach the ocean, they may be kilometers wide, but they are filthy, muddy, and grossly polluted.