Two men ride horses on the dried-up riverbed of the Yongding River located on the outskirts of Beijing March 21, 2010. No water has flowed down the river for many years as a result of the ever increasing demands for water from China's capital city. The Earth is literally covered in water, but more than a billion people lack access to clean water for drinking or sanitation as most water is salty or dirty. March 22 is World Water Day. REUTERS/David Gray (CHINA - Tags: ENVIRONMENT SOCIETY)