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Landscape Characterization / Impervious Surfaces / What are watersheds?
What are watersheds?
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Stream Order
The streams occupying a drainage basin form a hierarchical network
of channels that, in humid areas, hold increasingly larger volumes of water
as you move toward the mouth of the basin. A stream's order is its rank, or
relative position, within the network. A first-order stream is a channel with
no tributaries--that is, a channel at its upper reaches and near its source.
A second-order stream is a channel fed by at least two first-order tributaries.
The joining of two-second order streams forms a third-order stream. Stream ranking
continues in this manner until the highest-ordered channel is reached. First
and second-order streams are located in the headwater areas of watersheds and
typically convey small volumes of water. These lower-order streams are vulnerable
to pollution because they are unable to dilute contaminants, assimilate much
organic waste, or remove sediments deposited on channel bottoms.
 Stream order classification based on system developed
by Strahler. Source: Marsh 1998, 169.
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