As far as geological mysteries go, it's an intriguing one: For decades, China has reported being home to 50,000 rivers of at least about 40 square miles. But the country's three-year census of its water—a first-of-its-kind effort involving 800,000 surveyors and released last week—revealed a drastically different count. As of 2011, just 22,909, reports the Verge, which asks the million-dollar question: Why did 27,000 rivers vanish? The answer is an easy one, a census director tells the South China Morning Post: That 50,000 figure was arrived at using topographical maps as many as 60 years old, which were incomplete.
The Post cited climate change and water and soil loss as well, but the Verge reports that China itself may have had a heavier hand in the decrease. It spoke with Ma Jun, the director of the Institute of Public and Environmental Affairs in Beijing, who agreed that the answers offered by the government likely played a role; but he cited pollution and overpopulation as key factors. As the Verge explains, the destruction caused by flooding in the 1960s led the country to erect dams and reservoirs that reined in flooding but messed with the ecological system, causing rivers to go dry. Add in a more than fivefold surge in water use by a booming population in about the same period, along with pollution that only compounds the problem: It's "destroying the limited clean resources we have," Ma said.