SAJC Cluster 2.2 Lecture 15 Notes (Fluvial Landforms(II); Deltas)

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 15_2025/pg1 Lecture 15 Fluvial Landforms (II): Deltas KEY QUESTIONS: ✓ What factors influence the formation of deltas in the humid tropics? ✓ What is the importance of deltas in the humid tropics? With the completion of this lecture, attached readings and tutorial, you should be able to: • Identify deltas as a fluvial landform • Discuss the features of deltas: delta plain, delta front, prodelta • Explain the role geomorphic processes, including the role of sediments removal and dispersal by waves and tides, in the formation of deltas in the humid tropics • Discuss the factors influencing the formation of deltas • Discuss the ecosystem services provided by deltas • Discuss the impact of human activities on ecosystem services provided by deltas Lecture Outline 15.1 Deltas 15.1.1 Features of a Delta 15.1.2 Conditions Necessary for the Formation of Deltas 15.1.3 Formation of Deltas (Geomorphic Processes including the Role of Sediments Removal and Dispersal by Waves and Tides in the Formation of Deltas) 15.2 Factors Influencing the Formation of Deltas 15.3 Ecosystem Services Provided by Deltas 15.4 Impact of Human Activities on Ecosystem Services Provided by Deltas 15.5 Conclusion

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 15_2025/pg2 15.1 Deltas • Deltas are partly subaerial and partly subaqueous accumulations of riverborne sediment deposited at the mouth of the river, with the sediment reorganised by tides, waves and currents. • Deltas are mainly formed by fluvial depositional process at the mouth of a river, where a river loses velocity and competence as it flows into an ocean. • Deltas are so called because their triangular shape which resembles the Greek Letter Δ. In fact, deltas vary greatly in shape and can be grouped into three basic forms: arcuate, cuspate and bird’s foot (see Fig. 1). Fig.2 shows examples of the different types of deltas. Fig. 1 Basic shapes of deltas Fig. 2 Examples of deltas • In shape, deltas are approximately triangular or lobate. The apex of the triangle starts where the main river first splits into more than one channel. Such channels which branch out from the main river are known as distributaries. The surface of the delta is marked by a number of distributaries and their number generally increases as the delta extends towards the sea or ocean. • In general, gradient or slope of delta is very low as these are nearly at sea level. • Deltas are mainly composed of fine-grained sediments like sand, silt and clay with sand overlying slit and clay. Sediments are reorganised by the tides, waves or currents depending on local conditions and delta-building continues under water by sedimentation in the depositional basin. Arcuate delta Cuspate delta Bird’s foot delta

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 15_2025/pg3 15.1.1 Features of a Delta • Deltas have three components: a low and flat delta plain forming the subaerial part of the delta; the seaward-dipping part which extends offshore beyond the delta plain, called the delta front; and the subaqueous low edge of the delta in front of and below the delta front, termed the prodelta (see Fig. 3). • When the delta advances towards the sea, these features move forward, overriding each other. Fig. 3 Features of a Delta a) Subaerial Delta: Delta Plain • The delta plain is the subaerial part of the delta which is formed above sea level. This is that part of the delta that is farthest inland i.e. nearest to the land. It includes the area near the river's mouth where the river splits into many channels (known as distributaries) that fan out across the delta. These distributaries spread the river's sediments over a wide area. Sediments here are coarse-grained, such as sand and silt, because of high-energy conditions. The delta plain is a highly fertile area due to the rich sediments deposited there and often features extensive wetlands, marshes, or swamps. • The delta plain is subdivided into two parts, upper delta plain and lower delta plain. o Upper delta plain containing river borne deposits of sand, silt and clay. It lies above high tide mark and is not affected by the actions or waves and tides. It is usually the area with least water and highest elevation. The river that forms the delta begins to divide in this area into smaller channels called distributaries, which carry river borne sediments towards the delta’s edge. o Lower delta plain lies immediately seawards of the upper deltaic plain, in the middle of the delta. It occupies the area between high and low tides and thus are periodically covered by tidal water (during high tide). Deposits in the lower delta plain are the result of both river borne and sea borne sediments. This part of the delta is affected by the actions of Ocean Delta Front Prodelta Distributaries Subaerial

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 15_2025/pg4 distributaries, tides and waves. It is a transition zone between comparatively dry upper delta and the wet subaqueous delta. b) Subaqueous Delta: Delta Front and Prodelta Delta Front • Below the delta plain, where the river water begins to meet the standing or slower-moving water of the ocean, is the delta front. The delta front is typically subaqueous which is formed below sea level and represents the transition zone between the delta plain and deeper water. • The delta front lies below the low-tide mark and extends seaward to that area actively receiving river borne sediment. This area ranges in water depth from 50 to 300 m and in width from a few kilometres to tens of kilometres. • The dominant sediment here includes fine sand and silt, as the energy decreases from the delta plain toward the water. The delta front is located between the zone of fine-grained prodelta deposition and the more landward located, coarser-grained distributary mouth deposits of a delta. • The delta front also represents a zone of transition between deposits representative of progradation (growth of a river delta into the sea over time) and aggradation (occurs in areas in which the supply of sediment is greater than the amount of material that the system is able to transport). As the delta front progrades (advances towards the ocean) horizontally, the delta plain aggrades (rises in height) vertically. • The delta front is also the location where the distributaries begin to deposit their sediment loads more heavily because the flow velocity decreases sharply. Prodelta • The prodelta is the deepest and outermost part of the delta system, found below the delta front. The prodelta is thus subaqueous i.e. formed below sea level. • It consists of the finest sediments, such as clay and silt, deposited in deeper waters due to further reduction in energy. • This zone is mainly influenced by marine currents and suspension settling rather than direct river action. • Prodelta deposits form a gentle seaward slope, gradually merging with the deeper ocean floor.

St Andrew’s Junior College H2 Cluster 2: Tropical Environments H2_Cluster 2 Lecture 15_2025/pg5 15.1.2 Conditions Necessary for the Formation of Deltas •