Rivers can be divided into three stages, upper, middle and lower. The upper course contains the source which is where the river begins.

These are the lower slopes of Cuilcagh Mountain in County Fermanagh showing a river in its upper course. Rain falling in highland areas, flows downwards to collect in channels to form a stream. As the stream continues to run downhill it increases in size and speed as it is joined by other streams. You can see the small streams or tributaries flowing into the main river. The point at where these tributaries join the main river is called a confluence.

As rivers flow from the source they shape the land through a combination of erosion, transportation and deposition. This is the upper course of the Owenerk River in Raghtin More Mountain in County Donegal. In its upper course the river mainly erodes and transports material forming v-shaped steep-sided valleys. The sides of the valley are left exposed to weathering. The loose material is then washed into the river and transported downstream.

Steep sided valleys and heavy rainfall make the Mournes an ideal place to collect and store water. This is the Silent Valley Reservoir.

You can see the original stream between Silent Valley and Ben Crom Reservoir which is higher up the valley. Walls have been built across the river in its upper course to collect the water and prevent it flowing further downhill. The valley is narrower here which makes it easier to build a dam. The water from these reservoirs supplies the Greater Belfast area 30 miles to the North.

Rivers are an important part of the water cycle. When water falls on the land in the form of precipitation, it collects in rivers and returns to the sea.