Features produced in the upper course of a river

At this stage the river is characterized by the following features: V-shaped valleys, potholes, interlocking spurs, plunge pools, waterfalls, gorges, canyons and rapids. These are described below.

V-shaped valley
In the upper stage the river flows with high speed due to the steep slope of the river bed. As the river flows, it performs vertical erosion which results into a steep sided V-shaped valley, as seen in Figure 4.5.

Figure 4.5: V-shaped valley

Interlocking spurs
As the river flows, it erodes the landscape in the upper course, winds and bends to avoid the areas of hard rock. This creates interlocking spurs which resemble interlocking parts of a zip. This often results into alternating projections of high land called spurs (Figure 4.6). Sometimes spurs are referred to as overlapping, intermediate or interlocking spurs.

Figure 4.6: Interlocking spurs

Rapids
Rapids are formed by a sudden steepening of the slope because of unequal resistance in the successive rocks in the river profile. It is where the river flows faster than its normal speed (Figure 4.7).

Figure 4.7: Rapids

Waterfalls
Waterfalls occur when there is a sharp break in a river bed. This is a result of erosion of soft rock which lies horizontally under hard rock. Waterfalls can also occur when a hard rock stands vertically along the edge of the steep sides of the river bed. The continuing corrosion causes the hard rock to hang and as a result, water drops down (Figure 4.8).

Potholes formed at the base of waterfalls. These are formed due / of falling water into the river bed (Figure 4.8).

Depressions on the river bed formed by the swirling action of ; pebbles carried in the water form a circular depression on the river e holes on it (Figure 4.9).

Figure 4.9: Pot holes

River gorges and canyons
Gorges are deep and narrow elongated steep sided valleys or troughs formed as a result of vertical erosion of a river. A canyon is a widened deep gorge. Conditions for formation of gorges include the following:

(a) It is formed where river erosion cuts down more rapidly than the forces of weathering. For
example, the Indus and Brahmaputra gorges were formed by powerful rivers cutting down at
the same rate in the Himalaya ranges. Other gorges include the Grand Canyon of the Colorado
River in U.S.A which is nearly 500km long and has a depth of 1.9 km.

(b) It is formed when a river cuts through limestone rocks in arid regions. An example is the
Grand Canyon of the Colorado River in the U.S.A, the largest and best- known canyon in the
world.

(c) When a waterfall is formed on a resistant rock layer that is horizontal or is gently dipping
upstream. Stiegler’s Gorge found in Rufiji river in Tanzania is an example for this condition.

(d) When the river flows in a faulted river bed which makes it erode more vertically than laterally.