Science of Water : Concepts and Applications

Water Ecology 175
In rain events where the infi ltration capacity of the soil is not exceeded, rain penetrates the soil
and eventually reaches the groundwater, from which it discharges to the stream slowly and over a long
period. This phenomenon helps explain why stream fl ow through a dry weather region remains constant;
the fl ow is continuously augmented by groundwater. This type of stream is known as a perennial
stream, as opposed to an intermittent one, because the fl ow continues during periods of no rainfall.
When a stream courses through a humid region, it is fed water via the water table, which slopes
toward the stream channel. Discharge from the water table into the stream accounts for fl ow during
periods without precipitation and explains why this fl ow increases, even without tributary input, as
one proceeds downstream. Such streams are called gaining or effl uent, as opposed to losing or infl uent
streams that lose water into the ground (see Figure 6.14). The same stream can shift between gaining
and losing conditions along its course because of changes in underlying strata and local climate.
STREAM WATER DISCHARGE
The current velocity (speed) of water (driven by gravitational energy) in a channel varies considerably
within a stream’s cross-section owing to friction with the bottom and sides, with sediment,
and the atmosphere, and to sinuosity (bending or curving) and obstructions. Highest velocities,
obviously, are found where friction is least, generally at or near the surface and near the center of
the channel. In deeper streams, current velocity is greatest just below the surface due to the friction
with the atmosphere; in shallower streams, current velocity is greatest at the surface due to friction
with the bed. Velocity decreases as a function of depth, approaching zero at the substrate surface.
TRANSPORT OF MATERIAL
Water table
Water table
Baseflow
stage
FIGURE 6.14 (a) Cross-section of a gaining stream. (b) Cross-section of a losing stream.
(a)
Stream
(b)
Water fl owing in a channel may exhibit laminar fl ow (parallel layers of water shear over one another
vertically) or turbulent fl ow (complex mixing). In streams, laminar fl ow is uncommon, except at
boundaries where fl ow is very low and in groundwater. Thus, the fl ow in streams generally is