Environmental Engineering Reference
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For example, small meanders are typically associated with high gradients and coarse substrates, while
large meanders are associated with low gradients and ine substrates, such as in the depositional zone.
The channel sinuosity has large impacts on the channel characteristics and has often been reduced in
regulated rivers. A frequent goal of restoration efforts is to restore natural sinuosity.
Streams may allow differences in the presence and frequency, or absence, of pool versus rifle areas.
Rifles are areas of relatively shallow swifter-lowing waters, where the surface is usually turbulent,
which usually alternate with, or are separated by, deeper more slowly moving pools. The grain size dis-
tribution also varies, with the rifle areas more armored with larger rocks or pebbles, while the pools are
more depositional, with smaller grain sizes. The formation and stability of pools and rifles are largely a
function of the streambed makeup (Interagency Stream Restoration Working Group, 1998):
Gravel and cobble streams typically have regularly spaced pools and rifles and are stable
in rapidly luctuating streamlows.
Sandbed systems typically do not have pools and rifles, since the grain size is the same
in the shallow and in the deeper areas, but they typically do have alternating deep pools.
High-gradient and high-velocity streams typically have pools but not rifles, with the water
moving from pool to pool in a stair-step fashion.
The location of the pools and rifles also varies with the sinuosity of the stream (Figure 2.11). For
example, in sinuous streams, rifles are found at the entrance and exit of meanders, and control the
streambed elevation, while pools are located at the outside bend of the meander.
Low-gradient streams typically form pool-rifle systems while high-gradient and high-velocity
streams typically have step -rifle systems, as plunge pools are formed as the stream cascades over a
step (Figure 2.12). Pools may also form downstream of boulders or logs, as the falling water scours
a pool, and aid in dissipating energy. Backwater impacts due to logs, root wads, and debris blocking
or partially blocking a channel may also result in pools. Pools may also be of biological origin, such
as due to beaver dams. The upward-sloping area of the pool from the bed to the head of a rifle is
known as a glide , while the transitional feature between a rifle and a pool is known as a run . Glides
and runs typically have a latter water surface than rifles.
Pools and rifles, along with other channel features such as runs, steps, and undercut banks,
provide a necessarily diverse instream habitat for different organisms. Rifles provide cover for
Straight
Rie
Pool
alweg
line
Sinuous
Pool
Rie
or crossover
FIGURE 2.11 Sequence of pools and rifles in straight and sinuous streams. (From FISRWG, Stream
Corridor Restoration: Principles, Processes, and Practices , Federal Interagency Stream Restoration Working
Group, 1998.)
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