The H₂O molecule plays a key role in shaping our planet's surface, especially as it transitions between its different states. Let's follow the journey of liquid water, starting with its transformation from water vapour in the atmosphere through precipitation and then ultimately reaching the oceans. When water vapour in the atmosphere cools, it condenses into tiny droplets that fall to the earth as rain. When they reach the ground, these raindrops either seep into the soil or flow across the surface, gradually forming small streams. As more droplets gather, these streams grow, merging into large rivers.

The great rivers we know today, such as the Nile, the Amazon, and the Mekong, are all formed by thousands of smaller streams coming together. Rivers carry both dissolved solids and suspended particles that slowly erode the Earth's surface through a process called weathering. Dissolved solids like minerals from rocks are invisible to the eye but still play a significant role in altering landscapes over time.

Suspended particles are tiny bits of silt, clay, and sand that we can actually see because they make the water look muddy or cloudy. The process of weathering and erosion transports sediments downstream, eventually leading to deposition as the river slows down. This can sometimes cause issues, like at the ancient city of Angkor Wat in present-day Cambodia. Here, deposition of silt in the city's channels made them unusable over time, contributing to its decline. But the same process can also be highly beneficial.

For example, the annual flooding of the River Nile in Egypt deposits nutrient-rich solids onto the floodplains, creating fertile soil that has sustained agriculture for thousands of years. Let's now focus on flooding. Flash floods occur when heavy rain falls quickly, causing rivers and streams to overflow. In places like the Amazon, flooding is a part of the natural cycle, and seasonal water levels can vary by 5 to 15 meters, supporting the unique ecosystems of this region.

But flooding can, of course, also lead to significant destruction, dramatically eroding landscapes and washing away soil and even human structures in their path. Especially in areas where the landscape is bare and lacks vegetation, flash floods after heavy rain can lead to severe erosion because there is nothing to slow down the flow of water or hold the soil in place.

Hard surfaces like this pavement I'm sitting on don't erode, and the rainwater flows over the ground instead of soaking in. When less water is dropping into the ground, it can lead to local water shortages. This is where groundwater plays a vital role. When rainwater can soak into the ground, it recharges water resources underground. This water then slowly seeps into rivers, helping to maintain their flow even when rainfall is scarce. This is known as baseflow, and globally, about 60% of the water in rivers passes through groundwater before it joins the river flow.

So, we see the journey of water from atmospheric vapour to rivers and seas is a powerful force that changes the appearance of our planet. In the next module, we'll explore how coastal processes shape our beaches and coastlines.