Answer: Dried Up Lake Bed

When we think about sources of pollution, what usually comes to mind are the results of human endeavors: coal-fired plants belching clouds of byproduct into the air, strip mines leaving land exposed and prone to erosion and heavy-metal-laden runoff, and the like.

When it comes to dust pollution in the United States, however, the biggest single source of pollution is a dried up lake bed in California. Let’s not be too hasty in letting humanity off the hook though—the lake was full of water and not the least bit dusty until we came along. A century ago, engineers tapped into the Owens River, which fed into the now non-existent Owens Lake, which, in turn, diverted the flow of the river down an aqueduct to bring water to the thirsty city of Los Angeles 200 miles away.

The resulting dried up lake bed is now a salt flat roughly the size of San Francisco, and when the winds kick up, as they often do in the mountains where the lake bed is located, dangerously high levels of dust blow off the lake bed and into the surrounding areas of California.

As you can imagine, big environmental problems require big control measures, and the efforts to control the dust—funded by the city of Los Angeles, the recipient of all that water and the cause of the dust in the first place—has run into the billions of dollars over the years. To date, the most successful control measures have been, rather ironically, pumping water from nearby city-controlled water sources to coat the massive and flat lake bed with a very shallow layer of water to keep the dust down. Every day, the city pumps a stadium-sized volume of water into the lake bed to help control the dust.