How River Velocity Determines Sediment Transport: From Clay to Boulders
River speed is the master variable controlling what sediment a stream can erode, transport, and deposit. The Hjulström curve (1935) quantifies this relationship: particle size vs. critical erosion/transport/deposition velocities.
Sediment Classes & Transport Modes
| Sediment Class | Size Range | Critical Erosion Velocity | Transport Mode | Deposition Velocity |
|---|
| Clay | < 0.004 mm | ~0.2 m/s (cohesive) | Suspended load | < 0.01 m/s |
| Silt | 0.004–0.0625 mm | ~0.1 m/s | Suspended load | ~0.01 m/s |
| Sand | 0.0625–2 mm | 0.2–0.5 m/s | Suspended + bedload (saltation) | 0.1–0.2 m/s |
| Gravel | 2–64 mm | 0.5–1.5 m/s | Bedload (rolling, sliding) | 0.3–0.8 m/s |
| Cobbles | 64–256 mm | 1.5–3.0 m/s | Bedload (sliding, rare saltation) | 1.0–2.0 m/s |
| Boulders | > 256 mm | > 3.0 m/s (floods only) | Bedload (rolling, catastrophic) | > 2.0 m/s |
Transport Modes Explained
- Dissolved load: Ions in solution (invisible, ~10–50% of total flux)
- Suspended load: Fine particles held by turbulence (clay, silt, fine sand)—water appears muddy
- Bedload: Coarse particles rolling/sliding/saltating along bottom (sand, gravel, cobbles)—audible "bedload noise"
- Wash load: Finest fraction never deposits in channel; reaches ocean
Velocity Controls
- Gradient (slope): Steeper = faster = coarser transport
- Discharge (Q): Higher flow = deeper, faster, more competent
- Channel geometry: Narrow/deep = faster; wide/shallow = slower
- Roughness: Boulders, vegetation slow flow; smooth bed = faster
Practical Applications
- River engineering: Design stable channels (threshold velocity < critical erosion)
- Reservoir management: Predict delta progradation, turbine abrasion
- Habitat restoration: Match substrate to target species (salmon need gravel)
- Placer mining: Gold concentrates where velocity drops below ~0.5 m/s
- Paleohydrology: Ancient river deposits reveal paleo-velocities, climates
Key Insight: The Hjulström curve shows clay is harder to erode than sand (cohesion), but easier to transport once suspended. This paradox shapes delta formation, mudrock deposition, and contaminant transport.
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