structural:scholar_edition, quality:correction
Non-perennial rivers

About half of all waterways on Earth are intermittent rivers, which do not always have a continuous flow of water throughout the year. This may be because an arid climate is too dry depending on the season to support a stream, or because a river is seasonally frozen in the winter (such as in an area with substantial permafrost), or in the headwaters of rivers in mountains, where snowmelt is required to fuel the river. These rivers can appear in a variety of climates, and still provide a habitat for aquatic life and perform other ecological functions. Non-perennial rivers are believed to have once flowed on Mars.