Once a stream leaves the mountains, what happens to its speed? Why does this happen?

Once a stream leaves the mountains, its speed generally *decreases*.<br /><br />This happens for several reasons:<br /><br />* **Gradient decreases:** Mountainous areas are characterized by steep slopes, creating a high gradient. This steep gradient causes gravity to pull the water downhill with greater force, resulting in higher speeds. As the stream leaves the mountains and enters flatter plains or valleys, the gradient decreases. The gentler slope means gravity has less influence on the stream's speed, causing it to slow down.<br /><br />* **Channel Widening and Deepening:** As a stream moves from mountains to plains, its channel typically widens and deepens. While the volume of water might increase, the increased cross-sectional area of the channel allows the water to spread out. This reduces friction against the banks and bed, but the overall effect is a decrease in velocity because the same volume of water is now occupying a larger space.<br /><br />* **Increased Friction:** Mountain streams often flow over bare rock and large boulders. As the stream moves downstream, the channel bed often becomes composed of smaller sediments like sand and gravel, and vegetation may grow along the banks. These factors increase friction, which slows the water down.<br /><br />* **Meandering:** In flatter terrain, streams tend to meander, or curve back and forth. This increases the overall distance the water travels to cover the same ground, effectively reducing its downstream velocity.<br />