Seasonal rainfall patterns in large parts of the tropics and temperate regions generate flows that are hugely variable in narrow and steep upland streams. Globally, these streams account for the greatest stream length of any flowing ecosystem. During the dry season, flows in some streams are reduced to very levels, while in others flow ceases altogether and water persists only in isolated stagnant pools. Algae and leaf fall support moderate productivity, with seasonal floods sending organic matter downstream. The diversity of organisms fluctuates seasonally, with many localised (endemic) species, and specialised adaptations that enable animals to survive both flooding and dry conditions.