A hydrograph is a graph of the flow in a stream over a period of time. Below
is a picture of a hydrograph, with stream flow (discharge) in cubic feet per
second on the y-axis and time in months on the x-axis. Peaks in the hydrograph
are usually a result of precipitation events, while troughs represent drier
times.
To read a hydrograph, pick an approximate date and trace a vertical
line up to where it intersects the graph. Reading horizontally to the left,
you can determine the discharge of the stream for that date.
To say that a hydrograph is flashy means that the graph depicts sharp vertical
jumps and equally steep vertical declines. What this means for the actual
stream represented by the graph is that a flashy stream is one that exhibits
significantly increased flows immediately following the onset of a precipitation
event and a rapid return to pre-rain conditions shortly after the end of the
precipitation. That is to say, water that precipitates within a flashy stream's
watershed will make its way quickly from the land into the stream and be flushed
through the system rapidly. On the other hand, in watersheds supplying a stream
that is not flashy, the transport of water will be slowed through absorption
into and seepage through soils, containment on the surface in lakes, and retention
in the soil as moisture.
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