The speed of water flow is connected to the size of material it can carry through the idea of "sediment transport capacity." Generally, when water flows faster, it becomes better at moving larger pieces of material. This happens because fast-moving water creates strong forces that can lift and shift bigger sediment particles.
The main thing to remember is that the force of the water needs to be strong enough to beat the pull of gravity on the sediment. If the water is too slow, it won't be able to move bigger particles, causing them to settle down. But when the water is flowing faster, it can lift and transport a wider range of sediment sizes, including the heavier ones.
This idea has real-life effects on things like river erosion, sediment settling, and how sedimentary structures form. It's important to know that other factors like how thick the water is, the shape of the sediment, and how the water flows also impact how fast water can carry sediment.