The rivers are surface water characterized by having a channel more or less fixed and permanent flow. The contribution of groundwater is essential so that they have water throughout the year, although the flow also varies according to the season of the year and rainfall.
There are three sections in the course of a river:
- High course. It includes the headland and the first kilometers. The steep slopes through which the water circulates at high speed stand out. In this section, erosion predominates, which can generate deep gorges, with a V-shaped valley. If there are slopes and waterfalls form.
- Middle course. The slope is gentler, so the waters circulate more slowly, meandering. The valley widens, taking the shape of a trough. When a meander is abandoned by the wandering of the river, in Aragon it receives the name of galacho. Although erosion and sedimentation occurs in this section, the main process is transportation.
- Low course. The slope is very small, so the speed of the water is slow and the sedimentation of the materials that it carries in suspensionoccurs, which can form alluvial plains (or flood plains), suitable for agriculture.
At the mouth, the channel can be occupied by sea water that mixes with the river by the action of the tides, forming estuaries. They are frequent in rivers that contribute little sediment or that flow into seas with strong currents. For example, the estuaries of the rivers of the Cantabrian coast.
If the sea does not have very strong currents, the materials slowly transported by the river accumulate at the mouth to form deltas, so called because of their triangular shape, similar to that of the Greek letter Δ. For example, the Ebro delta.