Each stream has a
typical pattern of flows throughout the year.
Streams with significantly different watershed
characteristics (such as a stream with a high
groundwater content compared to a stream fed almost
entirely by runoff) will exhibit different
hydrographs. Streams in different regions, with
different climates, will exhibit vastly different
patterns of intra-annual variability.
Left: Stream at low flow.
Right: The same stream at high flow.
Seasonal variability is natural and
necessary for the functioning of the river
ecosystem. It is important to remember that river
organisms have evolved under a regime of high flows
and low flows over millennia. Bank-high flows form
and maintain the shape of the river channel; flush
sediment from feeding and spawning areas; signal
migration for certain species; and allow passage
upstream. For example, walleye, northern pike, and
sturgeon need a seasonal pulse of high water that
submerges rapids to be able to reach spawning