Page 247 - Handbook of Gold Exploration and Evaluation

218 Handbook of gold exploration and evaluation

channel per unit area. Stream frequency is the number of channels making up the
total length.

Drainage density
The manner of dissection of a drainage basin is quantified by the expression:

Dd ˆ L=A 4.16
where Dd is the drainage density, L is the total length of streams in the
drainage net and A is the area of the basin. The rate of dissection is strongly
affected by climate and the intensity of individual rainstorms may be more
significant than annual rates of precipitation. Run-off from violent rainfall is
more erosive and can support more channels per unit area than run-off from
persistent light rain that never reaches a peak. Altitude affects the rate of
accumulation of water equivalent (snow and ice) and its seasonal rate of dis-
charge. In dry climates run-off is ephemeral and drainage density is determined
principally by the physical properties of the surface material.

Stream frequency
Quantitative assessment of individual components of a network of streams
forming a single pattern involves their classification within a graded system of
ordering. At least two streams of a given order are needed to form a stream of
the next highest order. Streams having no tributaries (e.g., fingertip channels)
are classified as first-order streams; two or more first-order streams join together
to form second-order streams; these in turn unite to form third-order streams and
so on until the main trunk stream reaches the valley outlet.
Figure 4.13 is a conceptual diagram of fluvial distribution systems numbered
according to this system of ordering. Features outlined by dashes are
depositional features that could be alluvial fans; in this case the feature shown
at the bottom of the diagram is a modern fan within the detritus of a favourable
source; the other two fans are dissected ancient fans. Two systems of drainage
are shown which separately contribute gold-bearing and barren sediment to a
fourth-order main trunk stream gravel.

4.3.2 Channel styles

As represented by the plan view of a river, channel trace patterns are tradi-
tionally described as straight, braided, meandering or anastomosing. Miall
(1985) suggests separating channels into `fixed', `mobile' and `sheet-like' types
(according to width/depth ratios) as modified from Etheridge (1985). In this
concept, fixed channels are narrow with ratios less than 15:1; mobile channels
are broad and shallow with ratios between 15:1 and 100:1 and sheet-like