Fluvial systems: catchments and rivers - Fundamentals of the Physical Environment - page 329

(a)

and boundary shear stress , o , is defined as:

o = gRS

where

gravitational acceleration) and Q , R and S are standard
parameters used above. Boundary shear stress multiplied
by mean velocity gives the specific stream power:

g is the specific weight of water (density

= o v mean

(b)

and is high (over 1 kW m -2 ) in steep, high-discharge
rivers and low (under 100 W m -2 ) in gentle, low-discharge
rivers. This confirms the importance of channel efficiency
in establishing a power threshold , at which available power
is just sufficient to overcome friction resistance to mean
water and sediment discharge. Below this threshold, the
stream deposits a proportion of its sediment load to
restore efficiency. Above the threshold, the stream actively
erodes its channel.

Channel networks

Groundwater flow lines
Lines of equipotential gravity flow

Principles of channel process and form extend from
individual segments to the entire river and, indeed, the
drainage basin. Downstream changes discussed so far are
applied later to the geomorphological development of
fluvial landsystems, after first reviewing their significance
for the basin-wide network . Drainage networks are
organized systems of channels which transfer water and
sediment, in incremental amounts, through the catchment
via a definite sequence (e.g. rills qgullies qtributary
river channels qtrunk rivers). Networks possess measur-
able order (hierarchy), density and pattern, and reflect
the principal catchment attributes, including stage of
development and catchment shape.
Stream order recognizes the nature and development
of channel hierarchy as more aggressive channels 'capture'
neighbours. Having looked earlier at how rills focus flow
in a downslope manner, upslope and lateral development
extend this simple model. Local convergence of water in
the channel, from (ideally) isotropic or equipotential
throughflow or overland flow, causes spring sapping and
headward retreat of the point of initiation. This may
continue via branching until the remaining catchment
area is too small to feed any additional channels ( Figure
14.16 ). At its simplest, stream ordering identifies all
streams lacking tributaries as first-order streams and each
sequential stream by either arithmetic progression (2, 3,
etc.) or as the sum of feeder streams. Shrève's scheme is
preferred to Strahler's, which fails to recognize the scale

Figure 14.16 Spring sapping and drainage network devel-
opment. Chance disturbance of drainage on an otherwise
uniform land surface initiates local convergence of flow lines
(a), spring sapping and headward retreat (b).
Source: After Knighton (1984)

of increasing discharge, although it is thought to generate
a number of useful correlations between river connectivity
and the catchment ( Figure 14.17 ).
Drainage density , D d , refers to the channel length ( L )
draining a unit area ( A ):

D d = L / A

and reflects catchment variables, channel network
efficiency and maturity. In hydrometeorological terms,
higher drainage densities are associated with higher mean
annual precipitation, or strongly seasonal regimes, but
correlations are far from simple. High densities may also
be found in semi-arid areas where lack of vegetation cover
compensates for lower rainfall. Drainage density increases
during wet spells, as the permanent network expands to
incorporate intermittent channels. Geology influences
drainage density, which is inversely proportional to