Final answer:
The slowest stream velocity needed to maintain 1cm particles moving downstream depends on factors like particle size, shape, and density. Fine sand particles may require a velocity of around 30 cm/s to be transported, while larger gravel particles may require higher velocities.
Step-by-step explanation:
The slowest stream velocity needed to maintain 1cm particles moving downstream can be determined by considering the critical velocity for sediment transport. Sediment transport occurs when the velocity of the water is high enough to start moving particles from the bed of the stream. The critical velocity depends on factors like particle size, shape, and density. For example, fine sand particles may require a velocity of around 30 cm/s to be transported, while larger gravel particles may require velocities over 100 cm/s.