This shows the relationship between transportation, erosion and deposition, with different particle sizes and velocity. The Hjulstrom Curve plots two curves representing 1) the minimum stream velocity required to erode sediments of varying sizes from the stream bed, and 2) the minimum velocity required to continue to transport sediments of varying sizes. Notice that for coarser sediments (sand and gravel) it takes just a little higher velocity to initially erode particles than it takes to continue to transport them. For small particles (clay and silt) considerably higher velocities are required for erosion than for transportation because these finer particles have cohesion resulting from electrostatic attractions.

Entrainment Velocity - The speed/velocity of the river needed to pick up the particles.