Basic Concepts and Management Issues of Rivers - River Dynamics and Integrated River Management - page 21

Sinuosity —Sinuosity is not a profile feature, but it does affect stream slope. Sinuosity is the stream length
between two points on a stream divided by the valley length between the two points. For example, if a
stream is 2,700 m long from point A to point B, and if the valley length distance between those two points
is 1,000 m, that stream has a sinuosity of 2.7. A stream can increase its length by increasing its sinuosity,
resulting in a decrease in slope.
Channel cross sections —Figure 1.19 presents the type of information that should be recorded when
collecting stream cross-sectional data. In stable alluvial streams, the high points on each bank represent
the top of the bankfull channel. Channel cross sectional data need to include enough points to define the
channel related with a portion of the floodplain on each side.
Fig. 1.19 Typical channel cross section
Resistance and velocity —Channel slope and roughness are important factors in determining stream
flow velocity. Flow velocity is used to help predict what discharge a cross section can convey. As discharge
increases, either flow velocity, flow area, or both must increase. Roughness plays an important role in
streams. It helps determine the depth or stage of flow in a stream reach. As flow velocity slows in a stream
reach due to roughness, the depth of flow has to increase to maintain the volume of flow that entered the
upstream end of the reach. This is the concept known as flow continuity. Typical roughness along the
boundaries of the stream includes: sediment particles of different sizes; bed forms; bank irregularities; the
type, amount, and distribution of living and dead vegetation; and other obstructions. Roughness generally
increases with increasing particle size. The shape and size of instream sediment deposits, or bed forms,
also contribute to roughness.
1.1.7 Chemical Features
Scientists have been able to define several interdependent cycles for many of the common dissolved
constituents in water. Central among these cycles is the behavior of oxygen, carbon, and nutrients, such
as nitrogen (N), phosphorus (P), sulfur (S), and smaller amounts of common trace elements.
The total concentration of all dissolved ions in water, also known as salinity , varies widely. Precipitation
typically contains only a few parts per thousand (ppt) of dissolved solids, while the salinity of seawater
averages about 35 ppt.
pH, alkalinity, and acidity— The acidic or basic (alkaline) nature of water is commonly quantified by
the negative logarithm of the hydrogen ion concentration, or pH. A pH value of 7 represents a neutral
condition; a pH value less than 5 indicates moderately acidic conditions; a pH value greater than 9 indicates
moderately alkaline conditions. Many biological processes, such as reproduction, cannot function in acidic
or alkaline waters.
Dissolved oxygen —Dissolved oxygen (DO) is a basic requirement for a healthy aquatic ecosystem.
Most fish and aquatic insects “breathe” oxygen dissolved in the water column. Some fish and aquatic
organisms are adapted to low oxygen conditions, but most sport fish species, such as trout and salmon,