Continuous water fl ow is a unique feature of streams and

distinguishes them from all other ecosystems. The main fl ow

is always downstream but it varies in time and space and can

be diffi cult to measure and describe. The interest of hydrologists,

geologists, biologists and farmers in water fl ow, and

its physical impact, depends on whether the main focus is on

the entire stream system, the adjacent fi elds, the individual

reaches or the habitats of different species. It is important to

learn how to manage fl ow at all scales, in order to understand

the ecology of streams and the biology of their inhabitants.