Many streams have been dammed and many of
these dams are water diversionary structures. Below
such structures, streams may be locked into permanent
drought. Thus, a critical step in environmental
flow setting is to ensure that the restored flow regime
does not contain extended periods of extreme low flow
or no flow. Other streams, such as many in the
southern part of the Murray–Darling basin, Australia,
are used to supply irrigation flow and this means that
they have high flows in summer and low flows in
winter. Droughts or extended periods of low flow in
summer are natural phenomena and as for other
disturbances, serve as a major force in maintaining
biodiversity (Everard, 1996). For fish, low flow periods
may be a time at which successful recruitment occurs
for some species (Humphries et al., 1999) and droughts
can also selectively deplete populations of invaders
(e.g. Closs & Lake, 1995). Thus, for the restoration of
streams used for irrigation water delivery, it would be
desirable, if not necessary, for the maintenance of
biodiversity to implement low flows at the natural
time of the year