One by one, the Classic cities in the southern lowlands were abandoned, and by A.D. 900, Maya civilization in that region had collapsed. The reason for this mysterious decline is unknown, though scholars have developed several competing theories. which is Maya had exhausted the environment around them to the point that it could no longer sustain a very large population. <span>Other Maya scholars argue that constant warfare among competing city-states led the complicated military, family (by marriage) and trade alliances between them to break down, along with the traditional system of dynastic power. As the stature of the holy lords diminished, their complex traditions of rituals and ceremonies dissolved into chaos. Finally, some catastrophic environmental change–like an extremely long, intense period of drought–may have wiped out the Classic Maya civilization. Drought would have hit cities like Tikal–where rainwater was necessary for drinking as well as for crop irrigation–especially hard</span>
The two consequences of the California Gold Rush is it had an severe environmental impact. Another consequence was the rivers were clogged with sediment.
