Page 250 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 221

4.15 Geometric elements of stream channels.

Although natural channels cannot be measured as precisely as can artificial
channels, the application of hydraulic theory to artificial channels will usually
provide approximate values that are reasonably consistent with actual
observations and experience. Channel shape is influenced strongly by the inter-
action of gravitational forces pulling the water downslope, frictional resistance,
and the volume of water available for discharge. It is no coincidence that the
approximate shape developed by narrow stream channels is semi-circular, since
this profile offers the smallest possible shear surface for a given cross-sectional
area in both artificial and natural stream channels.

Longitudinal profile

The plan view of the longitudinal section of a river exhibits individual channels
and channel sections of varying configuration in terms of straightness,
anastomosing, braiding and meandering. The longitudinal profile is described
in terms of the continued fall in elevation of the streambed and the horizontal
distance between source and mouth. It is characterised by steep gradients in the
upper reaches, which become flatter in the lower, wider valley sections where
the river flows in its own alluvium. Gravitation forces put pressure on the
confining channel walls as they pull the water downslope; frictional forces
oppose its movement along the channel bed and walls. Sediment loading and the
varying degrees of susceptibility of the bed and the banks to erosion determine
both the cross-sectional shape and the slope of the stream. If the sediments are
fine-grained and the banks more resistant to erosion than the bed, the cross-
section will be narrow and deep. If the sediments are coarse and the banks more
resistant than the bed, the stream will tend to widen and shoal.

4.3.4 Stream power

The effective power of a stream is a function of the amount of sediment it
transports from one place to another and the rate at which it does this work.