1.2+Drainage+basin

2. What are the features of a drainage basin? These are all the terms you need to know – The **drainage basin** is the area of land drained by the river and all its tributaries. Another name for it is the **catchment area**. The **watershed** is an area of higher land which is the boundary of the drainage basin. Any rainfall falling inside the watershed may end up in the river. Any rain falling outside the watershed cannot end up in that river, but it may run into the river on whose drainage basin it falls. Any small river joining the main one is called a **tributary**. The place where 2 rivers join is called the **confluence**. Where the river flows into a sea or sometimes a lake is called its **mouth**. The route through which a river flows is called the **river channel**. The whole collection of channels that make up the paths all the streams flowing in drainage basin are called the **channel network.** Some drainage basins are huge ( e.g. Mississippi which covers about 1/3 of the USA’s area) while one near you might be very small. But more important is the **drainage density**: Drainage density = sum of length of all the rivers ÷ area of drainage basin In the top half of the picture the drainage density is high because there are more rivers. This is because the rock underneath it is impermeable. **Impermeable rock** is one that does not allow water to soak into it. In the lower half the drainage density is low due to the permeable nature of the rock beneath it. **Permeable rock** does allow water to percolate through it. The type of rock and consequently the drainage density levels have an effect on how likely floods are to occur, as we will see in the next section.
 * Source i**s where the river starts. This maybe as water flowing out of the ground as **a spring**, or it may ouse out through a **boggy area**, with no definably obvious source, but it is a source nonetheless.

Here is an example of a drainage basin. It is of the River Tees. We will be back to see this river several times throughout this module.
 * Things to do**

[] choose right hand side after reading this section >
 * == What do you need to remember? == ||
 * * The meaning of all the words in BOLD
 * To be able to draw a sketch of a drainage basin and add the labels
 * To know that the River Tees forms a drainage basin that starts on Cross Fell at over 300 metres above Sea Level before flowing 132 km east to enter the North Sea at Middlesbrough ||