Final answer:
Valleys and gullies are formed near the source of a river in the upper course, where steep slopes and processes like weathering, transportation, and erosion shape the landscape. These processes contribute to the river's characteristics and ecosystem as it flows downstream, where the water widens, sediment deposit increases, and currents slow down.
Step-by-step explanation:
Valleys and gullies are found in the upper course of the river near the source. Many streams flow down the steep slopes of the hillsides at the edge of the basin from about 600m from sea level. Abiotic features such as weathering (freeze-thaw), transportation (traction), and erosion (abrasion) have carved out steep valleys, for example, in the north-east Lake District. In these regions, the channel is narrow, the source water is usually cold, low in nutrients, and clear, which contributes to faster currents compared to downstream sections where the channel widens and currents slow down.
As a river flows away from its source, the processes of spreading and transporting of fragments, also known as erosion, help to shape the Earth's surface by eroding rocks and sediments, which are transported down the river and deposited in wider flood plains. This transport can also contribute to the increase of dissolved minerals in the water, benefiting the river's ecosystem. Additionally, steeper regions are prone to erosion, which results in the creation of characteristic valleys and gullies often seen in the upper reaches of a river system.