The Present Day Drainage Patterns of the Congo River System and their Neogene Evolution - Geology and Resource Potential of the Congo Basin - page 314

Table 15.1 The description of the major drainage types found in the Congo Basin and their most probable controls
Drainage
type Description of drainage pattern Drainage pattern may indicate the following*

Dendritic Drainage is irregular, with channels branching in all directions

and tributaries joining the main channel at all angles. Drainage is
only truly dendritic if the main channels show no slope control. It
has the widest modifications of all the basic drainage types

Tectonic stability; regionally uniform uplift or subsidence; low,
uniform slopes; geology of uniform resistance; lack or weak
structural control; horizontal lithology; superimposition (if
occurring on folded or differentially resistant rocks or on a
massive igneous body). As this drainage may be the result of
several structural conditions, it is of limited use in determining
geologic structure compared to other drainage types

Sub-
dendritic

Deviation of dendritic Modification most commonly due to secondary regional controls

of structure and topography; transition phase of drainage

Parallel A number of rivers and streams flow parallel, or near parallel, to

one another over a considerable area, or in several successive
cases. Parallel drainage may occur as a transition phase of
dendritic or trellis drainage

Regional tilting; steep gradients (often on homogenous
lithology); parallel topographic features; parallel faulting;
transition phase of dendritic or trellis drainage. Parallel drainage
may also indicate recent tilting, especially on newly emerged
surfaces or may zones of parallel faulting. The parallel pattern
suggests homogeneous sediment, with river following the
greatest slope

Sub-parallel Lack regular pattern of parallel drainage. However the
distinction between sub-parallel and sub-trellis may be disputed

Slope changes; high relief slopes; mild structural control (i.e.
deformed strata of uniform resistance to erosion)

Trellis A trellis drainage is characterised by the occurrence of secondary

tributaries parallel to the major stream or other streams into
which primary tributaries enter. The secondary tributaries are
often elongated, orientated at right angles to the streams into
which they flow. Several modified types of trellis drainage exist,
but require relevant data exists to determine their cause. For
example fault and joint patterns appear similar. For this study
patterns were categorised as trellis although there may be
differences in control

Active faulting; the boundary of lithologies of varying resistance
(i.e. parallel belts of folded or tilted strata); faulting; lithological
jointing

Rectangular Rectangular drainage is characterised by right-angle bends in
both the main channel and its tributaries. It lacks the regular
pattern and shorter tributaries that defines trellis drainage

Right-angle channel bends may be due to orthogonal joint or
fault systems but lack regularity. Tectonic stability; regionally
uniform uplift or subsidence; highly complex fractured systems;
ongoing faulting

Rectangular-
angulate

A common modified, compound version rectangular drainage
but one where the pattern is dominated by non-right angle faults

Modified rectangular drainage where faulting is not dominantly
right angled; ongoing faulting

Contorted Deformed rocks (especially coarsely layered metamorphic

rocks) that contain resistant layers (i.e. dykes and migmatised
bands); stream and tributary length may indicate general
direction of dip, making it possible to distinguish between
plunging anticlines and synclines on the local level
Controlling factors are from Zernitz ( 1932 ) ; Howard ( 1967 ); Deffontaines and Chorowicz ( 1991 ) and are given in decreasing order of probability

Lack a regular pattern, with tributaries exhibiting a multitude of
flow directions

Table 15.2 Regional drainage pattern classification of the Congo Basin. See Fig. 15.3 for the location of each drainage type

Example of drainage
type from the Congo
Basin

Map
code Drainage type Location

Dendritic D Dominantly the (D1) Northern Oubangui, (D-Sd2) Cuvette Central and (D3) the Upper Malagarasi

systems

Figure 15.4A

Sub-dendritic Sd Three zones surrounding the Central Basin and parts of the (D-Sd2) Cuvette Central ; (Sd4) the

Western Congolain wetlands, and in the east the (Sd5) Lindi and (Sd6) Luvua systems

Figure 15.4B

Parallel P The (P7) Luangwe System of southern plateau lands of the basin including the middle Kasai Figure 15.4C

The (Sp8) Northern Bat ´ k ´ Plateau System

Sub-parallel Sp Figure 15.4D

Trellis T Five systems, three of which are form the western boundary of the basin, viz . (T9) southern Bat´k´

Plateau, (T10) Sembe-Ouesso and (T11) Kwango Valley systems. The (T12) Lufupa system forms
the extreme southern basin boundary and the (T13) Mid-Congo River zone in the central basin

Figure 15.4E

Rectangular R Only the (R14) Buhimay System is the only rectangular pattern, being found in the Katanga region Figure 15.4F

Rectangular-
angulate

RA The (RA15) Lulwango and (RA16) Ulindi systems are found on the western zone of influence by

the EARS

Figure 15.4G

Contorted C The (C17) Lower Congo in the extreme west of the basin and the (C18) Lake Tanganyika / EARS

in the eastern basin

Figure 15.4H