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After weathering liberates particles from bedrock, they are transported by flowing water, wind, glaciers, or gravity until energy decreases enough for deposition. Hjulström's diagram describes the critical velocity needed to erode, transport, or deposit particles of a given size: coarse gravel requires the fastest flows, while very fine clay also resists entrainment due to cohesion. During transport, sediments are sorted by size and density and rounded through abrasion, so grain size, sorting, and roundness record the transport history of a deposit. Depositional environments—river channels, deltas, beaches, deep-sea fans—each produce characteristic sediment assemblages that can be read from ancient rocks.
Flume experiments or stream table simulations where flow velocity is varied to show bedload vs. suspended load vs. solution load make the physics of transport intuitive. Interpreting the grain size distribution of a sand sample from a known environment reinforces how transport energy is encoded in sediment properties.
You know from studying weathering and erosion that physical and chemical processes break bedrock into particles of varying sizes — from clay-sized flakes to house-sized boulders. Sediment transport is what happens next: those particles are picked up, carried, and eventually dropped somewhere else, and the physics of how this works leaves a readable record in every sand grain, gravel bar, and mud flat on Earth.
The key concept is transport energy — the capacity of a moving fluid (water, wind, or ice) to carry particles. Hjulström's diagram captures the essential physics in a single graph: it plots flow velocity against grain size and shows three fields — erosion, transport, and deposition. For sand-sized particles (0.1–2 mm), the relationship is intuitive — faster water picks up bigger grains. But the diagram reveals two surprises. First, very coarse particles (gravel, cobbles) require enormous velocities to erode because they are simply heavy. Second, very fine particles (clay, silt) also resist erosion despite being tiny, because cohesive forces between clay minerals bind them together — it takes more energy to rip a clay particle off a muddy streambed than to pick up a loose sand grain. Once entrained, however, fine particles stay in suspension at much lower velocities than were needed to erode them, which is why rivers run muddy for days after a flood even as flow decreases.
Particles travel in three modes depending on their size and the flow conditions. Bedload consists of coarse grains that roll, slide, or bounce (saltate) along the bottom — gravel in a mountain stream is classic bedload. Suspended load consists of finer particles kept aloft by turbulent eddies in the flow — the brown color of a flooding river is suspended silt and clay. Dissolved load consists of ions in solution (calcium, sodium, silica) that are invisible and travel with the water itself until conditions change and minerals precipitate. During transport, particles undergo sorting (separation by size — faster flows carry bigger grains, so deposits at any location tend to have a characteristic size range) and rounding (abrasion knocks off corners, converting angular fragments into smooth, rounded grains). A well-rounded, well-sorted quartz sandstone has traveled a long way and been reworked by sustained, uniform currents; a poorly sorted, angular deposit like glacial till was dumped all at once without selective transport.
Deposition occurs wherever transport energy decreases — a river entering a lake, wind dying down behind a dune, a turbidity current losing speed on the ocean floor. Each depositional environment produces a characteristic assemblage of sediment properties. River channels deposit cross-bedded sands and gravels; floodplains accumulate fine silt and clay during overbank floods; deltas build outward with a predictable coarsening-upward sequence as the channel progrades over deeper-water muds; deep-sea fans receive graded beds from turbidity currents, with each bed recording a single catastrophic event. By examining grain size distribution, sorting, roundness, sedimentary structures, and fossil content, geologists can read ancient rocks and reconstruct the transport history and depositional environment — turning stone back into landscape.