Physiography of Flowing Water - Freshwater Ecology: Concepts and Environmental Applications - page 78

occurred just last year. The severity of floods is related to watershed char-
acteristics as well as to the intensity of precipitation events. Where runoff
is more rapid and infiltration is less, floods are often more severe. Chan-
nelization and increases in impermeable surfaces (pavement and buildings)
associated with urbanization cause increased flooding (Figs. 5.8 and 5.9).
Water velocity also varies within stream channels. As we move down
a small stream, we find some shallow areas where the influence of the bot-
tom can be seen at the surface of the flowing water. These turbulent, shal-
low areas are called riffles . Deep areas with relatively low water velocity
are called pools, and areas with rapidly moving water but a smooth sur-
face are called runs (Figs. 5.10 and 5.11). Pools tend to accumulate fine
sediments, and runs have coarser substrata. A section of river with several
runs, pools, and riffles is called a reach .
The distance downstream between successive riffle and pool areas is
approximately five to seven river widths in some streams (Leopold, 1994).
The alternating riffle and pool pattern is absent when the bottom material
consists of fine sand or smaller particles, is present with gravel and small
rocks, and occurs sometimes when large boulders make up the stream bot-
tom. In steep mountainous regions with large rocks, pools alternate with
small falls. These cascades depend on geographical structure and do not
FIGURE 5.10 Conceptual diagrams of stream geomorphology. (Top) The side view is a cross-
sectional lengthwise view showing pool and riffle sequence. (Middle) The top view shows a
meandering stream, the thalweg (line of maximum velocity), and zones of erosion and depo-
sition (point bars). (Bottom) The water velocity contours (cross-sectional across the channel)
show how the maximum velocity is outside of the bend and the lateral current direction. When
the thalweg crosses the channel, the maximum velocity is in the center of the channel.