The second most populous city in the United States, Los Angeles has perhaps the most fascinating environmental history of any urban area in the country. The Los Angeles Basin, into which more than 80 communities of Los Angeles County are crowded, is a trough-shaped region bounded on three sides by the Santa Monica, Santa Susana, San Gabriel, San Bernadino, and Santa Ana Mountains. On its fourth side, the county looks out over the Pacific Ocean.

The earliest settlers arrived in the Basin in 1769 when Spaniard Gaspar de Portolá and his expedition set up camp along what is now known as the Los Angeles River. The site was eventually given the name El Pueblo de la Reyna de Los Angeles (the Town of the Queen of the Angels).

For the first century of its history, Los Angeles grew very slowly. Its population in 1835 was only 1,250. By...