SAJC Cluster 2.2 Lecture 14 Notes (Fluvial Landforms)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 14_2025/pg1 Lecture 14 Fluvial Landforms (I): Meanders and Braided Channels KEY QUESTIONS: ✓ How do natural processes and human activities influence landforms in the humid tropics? With the completion of this lecture, attached readings and tutorial, you should be able to understand: • the fluvial landforms found in the humid tropics like meanders and braided channels • the features of meanders and braided channels • the factors that influence the formation of meanders and braided channels in the humid tropics Lecture Outline 14.1 Introduction: Fluvial Landforms 14.2 Meanders 14.2.1 Features of a Meander 14.2.2 Conditions Necessary for the Formation of Meanders 14.2.3 Formation of Meanders 14.3 Braided Channels 14.3.1 Features of Braided Channels 14.3.2 Conditions Necessary for the Formation of Braided Channels 14.3.3 Formation of Braided Channels 14.4 Factors Influencing the Formation of Meanders and Braided Channels 14.5 Conclusion

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 14_2025/pg2 14.1 Introduction: Fluvial Landforms • Present-day landforms are shaped by processes that have operated over very long timescales. These processes work together to shape the earth’s surface, creating landforms that contribute to the splendor of the natural environment. As humans continue to build settlements in the natural environments, these landforms are impacted by human activities. • Although in our syllabus we have three fluvial landforms namely meanders, braided channels (see Lect.14) and deltas (see Lect.15), in this lecture, we will examine fluvial landforms in the humid tropics. Specifically, we will build our understanding of what meanders and braided channels are. We should be able to describe and sketch meanders and braided channels. We will also seek to determine the relative importance of natural and human factors in the formation of these two fluvial landforms. Box 1: Important Terms The course of a river is conceptually divided into 3 parts, upper, middle and lower. Although erosion, transportation and deposition happen throughout the course, their rates change over the course of a river. a) Long Profile of a River: The long profile shows how a river’s gradient changes as it flows from its source in its upper course to its mouth in the lower course. A long profile is usually concave and the slope becomes more gentle towards the mouth of the river. b) Cross Profile: A cross profile shows a cross-section of a river’s channel and valley at a certain point along the river’s course. o In the upper course, the river has less water and flows down the steep slope with high velocity. This causes more down-cutting or vertical corrasion. Thus, the channel becomes deep and narrow. o In the middle course, the river has more water, and the slope becomes gentler. Thus, the velocity of the river reduces, and the river does less vertical corrasion and more side-cutting or lateral corrasion, resulting in widening of the channel. o In the lower course, the slope is even gentler or flat and the river velocity is low. Thus, the river mainly deposits with very little erosion. The river channel becomes very wide. o Cross profile of the river is formed due to a combination of fluvial processes with some processes being dominant in certain stages of the course than others. Long Profile Cross Profile

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 14_2025/pg3 14.2 Meanders • A meander is a sinuous (having many curves and turns) river channel that migrate downstream owing to riverbank erosion on the outside of meander bends and deposition of bed material on the inner bank. Thus, meanders are the result of both fluvial erosional and depositional processes. • They are typically found in the middle and lower course of a river, because in these courses vertical corrasion is replaced by a lateral corrasion and corrosion, along with deposition. • Meanders are typically found in areas where river discharge is stable (e.g. areas with the Af climate) as it ensures that erosion and deposition processes remain balanced over time. • Example of a meandering river is Madre de Dios river in Peru (see Fig. 1). There are many rivers in the tropics which meanders such as, Amazon River in South America, Yangtze River in China, Orinoco River in Venezuela. Fig. 1 Madre de Dios river, Peru 14.2.1 Features of a Meander • A meandering river typically has a sinuous channel with a sinuosity index of more than 1.5 (see Fig. 2). In general, the meander length is on average 11 times, the bankfull channel width. Channel width remains same. Fig. 2 Sinuosity Index Sinuosity Sinuosity Sinuosity Index > 1.5

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 14_2025/pg4 • Each meander bend is asymmetrical in cross-section (see Fig. 3). • The outer bank has a steep slope called a river cliff, where the channel is the deepest forming a pool (accumulation of more water). • The inner bank has a gentle slip off slope, where the channel is very shallow forming a depositional feature called a point bar. Fig. 3 Cross-section of a Meander 14.2.2 Conditions Necessary for the Formation of Meanders • Stable discharge • Gentle slope in middle or lower course of the river • Presence of pools and riffles • Cohesive bank materials (clay)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 14_2025/pg5 14.2.3 Formation of Meanders The role of pools and riffles • In an initially straight river channel, the flow is disrupted by irregularities such as boulders, leading to variations in velocity. • These variations in velocity forms alternate pools and riffles in the river channel (see Fig. 4). • As the river flows over areas with boulders, the friction increases reducing river velocity. This encourages more deposition of sediments forming a shallow channel known as riffle. After depositing the sediments in the riffle, the river gains back energy and flows with higher velocity. This encourages fluvial erosion processes like corrasion and corrosion, forming a deep channel known as pool. • Pools experience greater erosion due to increased velocity and reduced friction, while riffles encourage deposition of coarse sediment. Fig. 4 Straight River channel with pools and riffles Riffles (low velocity) (deposition) Pools (high velocity) (erosion) River

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 14_2025/pg6 The role of swinging thalweg • The variations in velocity between pools and riffles causes the thalweg, which is the line of fastest velocity in the river, swings from bank to bank moving in a sinuous manner forming a meander bends which have an outer bank and an inner bank. • Meander starts to develop when there is erosion on the outer bank and deposition of bed material on the inner bank of the bend. • As the thalweg