10. Mississippi River, U.S.A The term Mississippi refers to “big river” or “great river”. It is the main river of the largest drainage system in the entire North America and also acts as the greatest natural and economic resource for the US. This river flows all the way down in the US and rises in the northern Minnesota, meandering slowly. Mississippi river valley is one of the most fertile agricultural region. But since the modern development took its girth in the basin, this cardinal river has started getting polluted with all agricultural sewage making the surrounding areas of the river mouth as a “dead zone”.