A new study shows water levels in some of the world's most important rivers fell sharply between 1948 - 2004.
Records from 925 big rivers were examined.
They include the Yellow River in northern China, the Ganges in India, the Niger in West Africa, and the Colorado in the southwestern United States.
Annual freshwater discharge into the Pacific Ocean fell by about 6%, or 526 cubic kilometers - about the equivalent volume of water flowing out of the Mississippi River each year.
Annual river flow into the Indian Ocean dropped by about 3% during the period period, or 140 cubic kilometers.
The Columbia River in the Northwest United States lost about 14% of its volume from 1948 - 2004.
Researchers say climate change and human activities such as dam building are to blame.
The research was caried out by the National Center for Atmospheric Research in Boulder, Colorado. Results have been published in the
American Meteorological Society's Journal of Climate.