Hercules, that great mythical giant and arguably the globe’s first environmental engineer, discovered that the solution to stream pollution is dilution-that is, dilution is the solution. In reality, today’s humans depend on various human-made water treatment processes to restore water to potable and palatable condition. However, it should be pointed out that Nature, as Hercules observed, is not defenseless in its fight against water pollution. For example, when a river or stream is contaminated, natural processes (including dilution) immediately kick in to restore the water body and its contents back to its natural state. If the level of contamination is not excessive, the stream or river can restore itself to normal conditions in a relatively short period of time. In this section, Nature’s ability to purify and restore typical river systems to normal conditions is discussed.