The top of Chimborazo is completely covered by glaciers, with some north-eastern glacier arms flowing down to 4,600 m. Its glacier is the source of water for the population of the Bolivar and Chimborazo provinces of Ecuador. Chimborazo glacier's ice mass has decreased over the past decades, which is thought by some to be due to the combined influences of global warming, ash covers from recent volcanic activity of Tungurahua, and the El Niño phenomenon.