into them. As shown in Figure 2-1, headwater streams are classified as first order, with order number increasing in a downstream direction. Headwater networks of very small streams accumulate rainfall, overland flow, snowmelt, or aquifer discharge, sending variable amounts of water downstream to increasingly larger channels.

The water budget of all streams and rivers is determined by climate and by other watershed attributes such as topography, soil type, bedrock substrata, groundwater discharge, and vegetation. Natural flow patterns—unregulated by dams and water diversion—will vary with the dynamics of water delivery and cycling, unless the source is a spring fed by a deep (phreatic) aquifer that has very little surface connection (Gibert et al., 1994; Vervier, 1990). According to Poff et al. (1997), the flow regime of a river can be distinguished by several major components, including magnitude, frequency, duration, timing, and rate of change, as described in Box 2-1. River flows are often described using one or more of these components. Thus, for example the bank-full flow, which defines the bank-

FIGURE 2-1 Stream orders for a watershed that includes first- to fourth-order streams. Ephemeral streams are not shown on this diagram. SOURCE: Reprinted, with permission, Strahler (1952). © 1952 by The Geological Society of America.