Rivers drain the surrounding land, carrying water that falls as rain and snow down to the sea. As rivers flow, they erode (wear away) rock, breaking it into fragments, called sediments, that are carried downstream. Most erosion occurs when rivers flood after heavy rain or as mountain snows melt in spring. Over time, erosion creates valleys and waterfalls, and sediments form land areas called flood plains and deltas.
A river has three main stages. In the first, the river is steep and narrow and its flow is rapid and rough. In the second stage, it is wider, less steep and flows more smoothly through flat-bottomed valleys. In the third stage, the river is broad and flows placidly across flat coastal plains to the sea, where it drops its sediment.
When water flows over some rocks, such as limestone, caves may be formed by a process called chemical weathering. Water seeps into cracks and gradually dissolves the rock, widening the cracks until, over thousands of years, the limestone becomes riddled with caves and passageways. Water flowing through caves forms underground streams, rivers, and pools (such as this one in Mexico). Surface rivers disappear into sink holes and reappear many kilometres away. Eventually a cave roof may fall in, creating a gorge.
Groundwater is water under the Earth’s surface. Most groundwater is found in porous rocks, which have tiny holes in them. If a hole is bored straight down through the rock, groundwater is eventually found at a certain level. This level is called the water table, and it usually rises when rainwater soaks into the ground. A spring is a place where groundwater emerges from a hillside.
Even in arid (dry) places such as deserts, groundwater sometimes comes to the surface. These lush green areas are called oases. The water at an oasis may have travelled underground from mountains hundreds of kilometres away. Oases are an important source of water, and towns often grow up around them.
Lakes form where water fills hollows in the landscape. Some of these hollows are formed by glaciers gouging into the ground, and some are created when river valleys are blocked by dams. Other lakes are formed in volcanic craters, or when land sinks during earth movements. Most lakes contain freshwater, but there are some saltwater lakes, such as the Dead Sea between Israel and Jordan.
- Rivers can gradually produce lakes as they flow. The outside banks of the meanders (bends) erode and sediment builds up on the insides, making the meanders longer.
- Eventually two ends of a meander get so close to each other that the water breaks through. This often happens during a flood. Now most river water by-passes the bend.
- The water flowing into the bend slows down. It drops its sediment, which blocks the ends of the bend, leaving a crescent-shaped lake called an ox-bow lake.
These lakes on the Isle of Skye in Scotland were created by glaciers thousands of years ago. Glaciers begin to form high on mountain sides, from snow and ice which builds up and scours hollows in the rock called cirques. When the glaciers melt, these fill with melt water to form cirque lakes, which continue to be fed by rainwater flowing off the hillsides.