Just like people have different types of belly buttons, rivers have different types of mouths. When a river flows into the sea, it can either stick out into the ocean or pull back from it. But unlike belly buttons, this isn’t about how they look; it’s about how they interact with the land and water.
Coasts are where land and sea meet, and they are constantly changing. For the ocean to move further onto the land, the sea level must rise, or the land must sink or wear away. On the other hand, for the land to push into the ocean, the sea level must drop, or the land must rise.
If the sea level goes up and down without really changing overall, things stay pretty much the same. But when a river is involved, it gets more interesting. During the last ice age, sea levels dropped a lot, and rivers carved out deeper paths to reach the ocean. About 18,000 years ago, as the ice started to melt, sea levels rose again, flooding river valleys and creating the coastlines we see today.
Around 7,000 years ago, the rising seas slowed down, and some rivers began to build new land at their mouths. This happened because rivers carry sediment, which is like tiny pieces of rock and soil. When the river reaches the sea, it slows down, and the sediment settles, forming new land. If there’s enough sediment and the ocean is calm, this new land can grow, creating a delta. This process splits the river into different channels and forms land that stretches into the sea.
Deltas have been crucial for human civilization. They provide fertile land for farming and are home to many people around the world. However, deltas are at risk from human activities like building dams, changing river paths, and rising sea levels due to climate change.
Organizations like the Belmont Forum, the Sustainable Deltas Initiative, and others are working hard to study and protect deltas. They focus on how these important landforms are threatened and what we can do to save them. If we don’t take care of our deltas, we might lose these vital areas that have supported human life for thousands of years.
Using a large tray, sand, water, and small rocks, create a model of a river delta. Pour water from one end to simulate a river and observe how the sediment settles to form a delta. Discuss with your classmates how this process mimics real-life delta formation.
Choose a famous river delta, such as the Nile or Mississippi, and research its formation, importance, and current environmental challenges. Present your findings to the class in a short presentation or poster.
Use online mapping tools to explore different river deltas around the world. Identify key features such as channels and sediment deposits. Share interesting facts about each delta with your classmates.
Participate in a debate about the impact of human activities on deltas. Take on roles such as environmentalists, farmers, and government officials to discuss how to balance development and conservation.
Work in groups to develop a plan to protect a specific delta from threats like pollution and rising sea levels. Present your plan to the class, highlighting innovative solutions and strategies for sustainable management.
The world is divided into two kinds of people: those with innie belly buttons and those with outies. Rivers also have innies and outies—not belly buttons, but mouths: where rivers flow into the sea, the land either protrudes or recedes. But rivers don’t have umbilical cords, so why do they have innies and outies?
Coasts are the front lines between two opposing forces—land and water. For the ocean to encroach upon the land, sea level must either rise, or the land must sink or erode. Conversely, for the land to extend into the ocean, sea level must drop, or the land must be elevated.
If sea level fluctuates without a net change, the situation remains stable. However, things become more complex when a river is involved. For instance, during the last ice age, sea levels fell by over 120 meters, and rivers carved deeper valleys to reach the receding seas. Approximately 18,000 years ago, warming temperatures began to melt the ice, causing rising seas to flood river valleys worldwide, creating vast estuaries and resulting in the diverse coastlines we see today.
When the gradual advance of the seas began to slow, around 7,000 years ago, some river mouths started to reclaim land. The key to this reversal was the sediment deposited by the river as its current slowed at the sea’s edge. Where sediment supply was sufficient and ocean conditions were calm, the accumulated material formed new land, extending the river and splitting it into separate channels. This process continued, creating new landforms that gradually advanced into the sea. Thus, many of the world’s prominent river deltas, which have supported human civilization since its inception, were formed during this time.
While the same cannot be said for all belly buttons, both innies and outies—whether in rivers or people—serve as a small record of our development.
A huge thank you to the organizations working toward sustainable deltas for sponsoring this video: the Belmont Forum, the Sustainable Deltas Initiative, the National Center for Earth-Surface Dynamics, the St. Anthony Falls Laboratory at the University of Minnesota, and the DELTAS project. These organizations study deltas globally, particularly how they are threatened by human activities such as dam construction, river channelization, and climate change-induced sea-level rise. If we do not pay attention, we risk losing the landforms that have allowed us to develop civilization in the first place.
Rivers – Large natural streams of water flowing in channels to the sea, a lake, or another river. – The Nile and the Amazon are two of the longest rivers in the world, providing water and fertile soil for agriculture.
Deltas – Landforms created by the deposition of sediment at the mouth of a river as it slows and enters a larger body of water. – The Mississippi River Delta is known for its rich soil and diverse wildlife habitats.
Sediment – Particles of rock, soil, and organic material that are transported and deposited by wind, water, or ice. – Over time, layers of sediment can build up and form sedimentary rock.
Sea – A large body of saltwater that is smaller than an ocean and is partly or completely surrounded by land. – The Mediterranean Sea has been a crucial route for trade and cultural exchange throughout history.
Land – The part of the Earth’s surface that is not covered by water. – Farmers cultivate the land to grow crops and raise livestock.
Ocean – A vast body of saltwater that covers almost three-quarters of the Earth’s surface. – The Pacific Ocean is the largest and deepest ocean on Earth.
Coastlines – The outlines or boundaries where the land meets the sea or ocean. – Erosion and weathering constantly change the shape of coastlines over time.
Climate – The long-term pattern of weather conditions in a particular area, including temperature, precipitation, and wind. – The climate in tropical regions is typically warm and humid throughout the year.
Valleys – Low areas of land between hills or mountains, often with a river running through them. – The Great Rift Valley in Africa is known for its stunning landscapes and diverse ecosystems.
Civilization – A complex society characterized by the development of cities, social classes, and cultural achievements. – Ancient civilizations like Mesopotamia and Egypt developed along fertile river valleys.
