Within a drainage basin, water collects in small depressions (rills) and these join to form larger streams. Eventually, these streams (tributaries) combine to form the main river. Water flows swiftly in the upper course, moving from higher to lower elevations. The faster a river flows, the more erosive power it has. Common features in the upper course include waterfalls, plunge pools and rapids.
SkillBuilder discussion Communication 1. What are a waterfall and a plunge pool? 2. Why do you think they develop in the upper course of a river? 3. Would it be safe to swim in this area? Discuss.
FIGURE5 A waterfall
Waterfall retreats.
Hard rock
Overhang
Ridges of hard rock create an uneven slope. This creates rapids.
Steep-sided gorge develops as waterfall retreats.
Plunge pool
Soft rock
Fallen rocks
Hard rock
Middle course Rivers follow the topography of the surrounding area. As the land flattens, the river forms long sweeping turns called meanders . The river’s energy slows as it carves a new path through the landscape. Over time, a meandering river will change its path, creating new bends and cutting off others. This can form an oxbow lake , referred to as a billabong in Australia. During times of high rainfall, the river may overflow its banks, flooding the adjacent land. This is referred to as a floodplain . As the floodwaters subside, they leave behind nutrient-rich sediment (alluvium) transported from the upper course, enriching the soil, which is often used for agriculture.
SkillBuilder discussion Communication 1. Why do you think a river slows in the middle course? 2. Why do you think the river changes its course and creates oxbow lakes? 3. Suggest a reason the middle course of a river is ideal for agriculture.
FIGURE6 The formation of a meander and oxbow lake
Meander
Deposition
Oxbow lake
Erosion
Soon-to-be oxbow lake with cut-off
302 Jacaranda Humanities Alive 8 Victorian Curriculum Third Edition
Made with FlippingBook interactive PDF creator