Figure 31 Longshore drift can carry sediment great distances along the shore, creating long sandbars or spits that grow parallel to the shoreline.
Sand is continuously eroded and deposited along the shore by waves and currents. Longshore drift is the process that moves sand along a shore. Look at Figure 31 to see how longshore drift occurs. Waves carrying sediment approach the shore at an angle. As the waves break, they carry sand up the beach at that angle. The water and sand then flow directly back toward the water. In this way, longshore drift produces a zigzag motion that moves sand great distances along a shore.
Deposition by Currents and Waves
The sediment that is eroded from a coastline or brought to the ocean by rivers is also deposited. Currents and waves deposit sediment in bays and inlets as the water slows down. Over time, sediment can collect along the shore. A shoreline covered with sand or other sediment is a beach. In many areas, rivers that discharge into the ocean are the main source of the sand on ocean beaches.
Currents move large amounts of sediment parallel to the shore. When these currents meet a bend in the shoreline, they slow down and deposit the sediment. This deposit is called a spit.
Section 23.5 Assessment
Reviewing Concepts
How do temperature and pressure change with depth within the ocean?- Compare and contrast how surface currents and density currents form.
- What causes upwelling?
- What happens to rock as waves hit the shore?
What process moves sand down a coastline?
Critical Thinking
Predicting El Niño is a weather event in the Pacific Ocean. It pushes warm water over the cold current that flows along the western coast of South America, where upwelling normally occurs. How might El Niño affect the fishing industry in this area?
Developing Hypotheses A dam is built on a major river that flows into the ocean. Several years later, people observe that nearby ocean beaches have less sand than before. Develop a hypothesis to explain this phenomenon.
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