The Core Idea
Three Ways a Stream Moves Material
Streams transport sediment through three distinct modes. Bedload consists of coarse particles that roll, slide, and saltate (bounce) along the channel bottom, typically making up only 10 to 20% of a stream's total sediment load by mass, despite being the most visually obvious. Suspended load consists of fine sand, silt, and clay carried within the water column itself, representing the majority of a stream's total transported mass. Dissolved load consists of ions carried in solution — completely invisible, yet a genuine and sometimes significant component of what a stream actually transports downstream.
Two related but distinct concepts describe a stream's overall sediment-moving ability: competence, the largest particle size a stream is capable of moving (which scales with velocity squared), and capacity, the total volume of sediment a stream can carry (which scales with velocity to the fifth power) — meaning capacity increases far more dramatically than competence as velocity rises even modestly.
💡 Memory Trick
Picture three ways to move furniture out of a house: Bedload is like rolling a heavy couch along the floor — slow, difficult, and it stays in contact with the ground the whole time. Suspended load is like carrying boxes of books through the air — lighter, faster, and making up most of the total stuff being moved. Dissolved load is like carrying away invisible steam or gas from the house — you can't see it, but it's still genuinely leaving. And remember: capacity (total volume moved) scales with velocity to the FIFTH power, while competence (largest single item movable) only scales with velocity SQUARED — meaning speeding up the moving crew even a little dramatically increases how MUCH they can carry, more than it increases how big a single item they can lift.
Erosion, Transport, and Deposition
The Hjulström Curve
1
Erosion vs. Deposition Velocity Thresholds
The Hjulström curve shows that a higher velocity is generally required to erode a particle from the channel bed than to keep it moving once it's already in transport — meaning particles tend to stay in motion at lower velocities than were originally needed to dislodge them.
Example: this asymmetry explains why sediment, once picked up by a fast flow, can continue traveling even as the stream's velocity gradually decreases somewhat.
2
The Fine Silt Paradox
Extremely fine silt particles are actually more difficult to erode than sand, despite being smaller, because cohesive forces between fine particles resist being picked up by flowing water.
Example: this counterintuitive result means the easiest particles to erode aren't the smallest ones, but rather medium-sized sand grains that are both loose enough to be picked up and heavy enough to lack strong cohesive bonding.
3
Graded Bedding From Decreasing Velocity
As stream velocity decreases (such as when a flood recedes, or when a river enters a lake or ocean), coarser particles settle out first, producing graded bedding — a pattern also covered as a sedimentary structure in the Sedimentary Structures lesson from Rocks.
Example: this same graded bedding principle explains why gravel deposits typically sit closer to a mountain source than fine mud, which settles out much farther downstream.
Human Impact on Sediment Transport
Dams and Downstream Channel Incision
Stream power — calculated as ω = ρgQS (density times gravity times discharge times slope) — controls a stream's overall erosion and transport ability. Dams dramatically disrupt natural sediment transport by trapping sediment upstream, releasing unusually clear ('sediment-starved') water immediately downstream. Since this clear water still retains significant erosive energy but now carries almost no sediment, it frequently causes channel incision (deepening) downstream of dams, as the stream compensates for its now-empty sediment load by eroding its own bed instead.
🖥️ Applied Scenario
An engineer studies channel changes downstream of a newly constructed dam over a period of several years.
1
The engineer observes that the river downstream of the dam now carries much clearer water, since the dam traps most of the sediment that would naturally be transported downstream.
2
Despite carrying less sediment, the river retains significant erosive energy, leading the engineer to observe channel incision — the streambed itself deepening as the river erodes its own bed to compensate for the missing sediment supply.
3
The engineer concludes this incision is a direct, predictable consequence of disrupting the river's natural sediment transport balance, and recommends monitoring for potential impacts on bridges, infrastructure, and habitat that depend on a stable channel bed elevation.
📌 Exam Application
Exams frequently ask you to distinguish bedload, suspended load, and dissolved load, or to explain the difference between competence and capacity — always remember competence scales with velocity squared while capacity scales with velocity to the fifth power, since this specific mathematical relationship is a commonly tested detail.
⚠️ Most Common Sediment Transport Mistakes
Don't assume the smallest sediment particles are always the easiest to erode — the fine silt paradox shows that very fine, cohesive silt actually requires higher velocity to erode than sand, despite being smaller. Also remember dams can cause channel incision DOWNSTREAM specifically because they release sediment-starved (clear) water that still has erosive energy but nothing to deposit — don't assume dams simply reduce all downstream geomorphic activity uniformly.
✓ Quick Self-Test
1) Name the three modes of sediment transport and roughly what fraction of total load bedload typically represents. 2) Explain the difference between competence and capacity, including how each scales with velocity. 3) Why do dams often cause channel incision downstream?
Next Lesson
Snow Hydrology
→
← All Hydrology Lessons