The happiness in a riverbed

The happiness in a riverbed in infinite

The animals are happy, feeding and breeding.

The water is warm, as the sun shines brightly overhead

Plants grow wildly as nature deems it so, and feeds her young

However, a gloom comes over head, with a new character entering the riverbed. 

Mankind has found it, and want all of it. 

The fish is takes out to be used as food.

The plants torn for either decoration or room for Men's own plants. 

And destroys the freshwater with their muck. 

Why does one do this, to the poor riverbed?