(I) Many different conditions exist along the length of a river or stream. (II) The nature of a flowing-water ecosystem changes greatly from its source, where it begins, to its mouth, where it empties into another body of water. (III) For example, headwater streams are usually shallow, cold, swiftly flowing. (IV) In contrast, rivers downstream from the headwaters are wider and deeper, less cold and slower-flowing. (V) Unless strong conservation measures are initiated soon, human population growth and industrialization in tropical countries will spell the end of tropical rain forests by the middle of the century.