Final answer:
The stream gradient varies due to the interspersion of weak and strong rocks, influencing the river or stream's flow speed and sediment transport.
Step-by-step explanation:
The gradient of the stream changes between steep and gentle in response to the alternating weak and strong rocks. As the river or stream flows from its source, various abiotic factors influence its characteristics. Where the stream flows over weaker rocks, erosion is more intensive leading to a steeper gradient.
Conversely, over stronger rocks, erosion is less effective and the gradient tends to be gentler. These changes in gradient impact the speed of the water and the nature of the sediment transported and deposited along the course of the stream.