Science of Water : Concepts and Applications

Water Ecology 177
long profi le, cross-section, depth and slope profi le, and because these mutually adjust over a time
scale as short as years and as long as centuries or more, cause-and-effect relationships are diffi cult
to establish. Other variables that are presumed to interact as the stream achieves its graded state
include width and depth, velocity, size of sediment load, bed roughness, and the degree of braiding
(sinuosity).
STREAM PROFILES
Mainly because of gravity, most streams exhibit a downstream decrease in gradient along their
length. Beginning at the headwaters, the steep gradient lessens as one proceeds downstream, resulting
in a concave longitudinal profi le. Though diverse geography provides almost unlimited variation,
a lengthy stream that originates in a mountainous area typically comes into existence as a
series of springs and rivulets; these coalesce into a fast-fl owing, turbulent mountain stream, and the
addition of tributaries results in a large and smoothly fl owing river that winds through the lowlands
to the sea.
When studying a stream system of any length, it becomes readily apparent (almost from the
start) that what we are studying is a body of fl owing water that varies considerably from place to
place along its length. For example, a common variable—the results of which can be readily seen—
is whenever discharge increases, causing corresponding changes in the stream’s width, depth, and
velocity. In addition to physical changes that occur from location to location along a stream’s course,
there are a legion of biological variables that correlate with the stream size and distance downstream.
The most apparent and striking changes are in steepness of slope and in the transition from
a shallow stream with large boulders and a stony substrate to a deep stream with a sandy substrate.
The particle size of bed material at various locations is also variable along the stream’s course.
The particle size usually shifts from an abundance of coarser material upstream to mainly fi ner
material in downstream areas.
SINUOSITY
Unless forced by man in the form of heavily regulated and channelized streams, straight channels are
uncommon. Stream fl ow creates distinctive landforms composed of straight (usually in appearance
only), meandering, and braided channels, channel networks, and fl ood plains. Simply put: fl owing
water will follow a sinuous course. The most commonly used measure is the sinuosity index (SI).
Sinuosity equals 1 in straight channels and more than 1 in sinuous channels.
Meandering is the natural tendency for alluvial channels and is usually defi ned as an arbitrarily
extreme level of sinuosity, typically a SI greater than 1.5. Many variables affect the degree of
sinuosity.
Even in many natural channel sections of a stream course that appear straight, meandering
occurs in the line of maximum water or channel depth (known as the thalweg). Keep in mind that
a stream has to meander, that is how they renew themselves. By meandering, they wash plants
and soil from the land into their waters, and these serve as nutrients for the plants in the rivers. If
rivers are not allowed to meander, if they are channelized, the amount of life they can support will
gradually decrease. That means less fi sh, ultimately—and less bald eagles, herons, and other fi shing
birds (Cave, 2000).
Meander fl ow follows predictable pattern and causes regular regions of erosion and deposition
(see Figure 6.15). The streamlines of maximum velocity and the deepest part of the channel lie
close to the outer side of each bend and cross over near the point of infl ection between the banks
(see Figure 6.15). A huge elevation of water at the outside of a bend causes a helical fl ow of water
towards the opposite bank. In addition, a separation of surface fl ow causes a back eddy. The result is
zones of erosion and deposition, and this explains why point bars develop in a downstream direction
in depositional zones.