Rivers and Streams, Characteristics - Hydro-Environmental Analysis: Freshwater Environments - page 24

Plan view

Cross-sectional views

1

Pool

2

Step

3

Run

4

Pool

e location of cross-section 3
is recommended for bank-full
measurements.

1

2

3

4

Profile view

FIGURE 2.12 Location of features in a step-pool system. (From Rosgen, D.L., Applied River Morphology ,
Wildland Hydrology Books, Pagosa Springs, CO, 1996. With permission.)

macroinvertebrates and deliver food for ish in microhabitats within the rifle or downstream pool.
For example, certain caddislies occur typically only in rifles. The coho salmon generally rear in
pools formed by large woody debris. The turbulence associated with rifles also aids in maintain-
ing high concentrations of dissolved oxygen. Often, as sedimentation increases due to the cumula-
tive impacts of human or natural inluences on watershed changes, the number of glides and runs
increases in relation to pools and rifles. The re-establishment of pools and rifles is a common goal
of stream restoration.

2.2.1.4 Channel Processes and Stream Morphology
2.2.1.4.1 Gradient
The slope or the rate of change in a bottom elevation with respect to distance (e.g., expressed as
meter per kilometer [m km -1 ], feet per mile [ft. mile -1 ], or percent) is one longitudinal morphological
characteristic of streams impacting other characteristics. The stream slope typically decreases as
streams move from the headwater to the depositional zones (Figure 2.5).
An increasing slope, or gradient, is generally associated with higher stream velocities. In hydrau-
lics, stream velocities are separated into two types of low based on a comparison of the stream
velocity to the celerity of a gravity wave ( c = , where c is the celerity, g is the gravitational
acceleration [32.17 ft. s -2 or 9.81 m s -2 ], and Y is the stream depth [feet or meters]). If the stream
velocity is equal to the celerity of a gravity wave ( c ), then the low is referred to as a critical low. If
the velocity is greater than c , then the low is referred to as supercritical , and if smaller, subcritical .