Page 278 - Handbook of Gold Exploration and Evaluation

244 Handbook of gold exploration and evaluation

spacing of pools and riffles, meander wavelengths and average bank-full/bank-
width relationships. According to Leopold et al. (1964) each channel meander
wavelength contains two, pool-riffle sequences each being separated by six to
seven channel widths. Pools are located on the bends, riffles at the inflection
points and point bars form in successive stages on the inside bends of the
meanders in conformity with the shape of the meander, stream depth and
erodibility of the banks.
Low gradient streams typically assume a meandering pattern in areas of
moderate rainfall and moderate discharge. Current velocities are greatest along
the thalweg, which swings from one side of the channel to the other, even in
straight-sided channels. Material eroded from the banks by impingement of the
swifter current against the channel side is deposited in slacker water on the
opposite side. The end result is the development of point bars on the inside of
meanders, successive stages of growth comprising basal lag gravel overlaid by
sandy upward fining point bar deposits which, themselves are overlaid by silty
and muddy overbank deposits. The morphology of a meander system is
illustrated in Fig. 4.27.
Deeper streams and thicker deposits have larger meander traverses, and
incipient sidebars are common along the flanks of relatively straight sections.
Alluvial landforms in meandering channels extend downstream from the point
of maximum curvature of the meander belt. Meandering channel paystreaks,
which follow the courses of lag gravels in successive migrating meander, may
be reworked during intermittent pulses of uplift and erosion produced by minor
crustal adjustments. Meander scrolls migrate across the placer following each
adjustment and tend to disperse the concentrations of coarser gold according to
size into basal gravel and bedrock structures. Ultimately, the limits of economic
concentration are reached downstream when the gold grains are so reduced in

4.27 Morphological elements of a meandering river system.