What does Hjülstrom curve show?

Hjülstrom curve is a logarithmic graph which shows the relationship between sediment size and velocity. The two curves on the graph show what happens to sediment in transit for various velocities. The first line (blue) is the critical erosion velocity curve, which show the minimum velocity required for various sediments to be picked up and transported within a river. For instance, the fine clay & silt fraction (0.001-0.01) require a velocity around 100-150 cm/s) to be transported. The cays & silts are harder to entrain despite there low mass, due to ability to bond together. The velocity needed then decreases with sediment size into the sand fraction. Fine sands are the easiest to entrain, needing a velocity of only 11 cm/s. As the sediment size increase into the coarser grained sand particles (1-2 mm), more energy is needed to transport them, with a velocity of around 70 cm/s . As the grain size increases from this point more energy is needed to entrain the sediment.
The second curve (red) shows the settling velocity curve, which is the velocity required for sediment to be deposited on the stream bed. So anything below this line will be deposited.
The second between the two lines defines the transport section. Towards the lower left, we'd expect to see transport in suspension section for the smaller partials (Find to medium sands), and the upper right side shows the transport as bed-load for the larger particles ( Gravels, pebbles, cobble and boulders).