The hydrologic cycle encompasses all movements of water molecules through the atmosphere, hydrosphere, and groundwater zones. One key part is the drainage of surface waters and groundwater back to the ocean. Natural drainage systems are well-adjusted to their climate, vegetation, and geology. Hydrologists and fluvial geomorphologists have studied these systems in great detail; human disturbances are readily apparent. Three impacts are considered here: wetland drainage and irrigation; dam construction and mining alterations; and urbanization.

Robert E. Horton identified many drainage characteristics during the 1930s and 1940s. Later researchers added other useful quantitative parameters. All of these show the predictable patterns found in natural drainage systems. Best known are stream order and drainage density. Although influenced by the detail of the maps used (commonly 1:24,000 or 1:62,500 topographic maps), stream order provides a simple system for ranking tributaries. The smallest ones are labeled first order, second-order streams have at least two...