B4 · Publication Volume 9

Rivers, Slopes and Sediment Transport

drainage, colluvium, alluvium and sediment pathways

Connected hillslope, colluvial, alluvial and channel sediment pathways
Connected hillslope, colluvial, alluvial and channel sediment pathways

Learning objectives

After this lesson, you should be able to distinguish weathering supply from sediment transport; identify colluvial, alluvial and aeolian pathway evidence; calculate a simple sediment budget; explain why hillslope flux may become nonlinear near a threshold gradient; trace source-to-sink connectivity; and predict how sorting, abrasion, storage and remobilisation alter mineralogical and geochemical signals.

Start with a field problem

A metal anomaly occurs in fine sediment along one tributary but not in coarse gravel from the same channel. Upstream, a thin soil mantles bedrock on convex slopes, while footslopes contain matrix-rich colluvium and the valley floor contains several inset alluvial packages. Which medium is closest to the target source, and which has integrated the largest contributing area?

The answer depends on grain size, transport competence, mineral hosts, storage time and tributary mixing. Fine suspended or wash load may travel farther than bed material. Dense or resistant grains may be mechanically concentrated. Reactive elements may leave grains, adsorb to iron oxides or precipitate at a redox boundary. An anomaly is therefore a property of a medium and pathway, not simply a point upstream from a source.

Core process model

Weathering produces transportable material; transport requires a force and pathway. On hillslopes, creep, bioturbation, rainsplash, overland flow, shallow landsliding and debris flow operate at different rates and thresholds. Colluvium is transported mainly by gravity-assisted slope processes and commonly has short, poorly sorted pathways. Alluvium is deposited by running water and may show channel geometry, imbrication, stratification, sorting or floodplain organisation. These are process interpretations supported by evidence, not labels assigned from map position alone.

A nonlinear hillslope-flux model can be written


q_s=-K_s\frac{\partial z/\partial x}{1-\left(|\partial z/\partial x|/S_c\right)^2},

where q_s is sediment flux per unit contour width, K_s is a transport coefficient and S_c is a critical-gradient parameter. At low gradient it resembles linear diffusion; as gradient approaches S_c, predicted flux rises strongly. Real slopes can fail before the mathematical singularity, and S_c is an empirical model parameter rather than a universal angle.

Sediment continuity is


\frac{\partial h}{\partial t}=-\frac{1}{1-\phi}\nabla\cdot\mathbf{q}_s+P-W,

where h is sediment thickness, \phi is porosity, P is production into the mobile layer and W is weathering or loss within it. A positive input-output difference increases storage. Connectivity determines whether a local source reaches the sampled store; a fan, floodplain, reservoir or dune can interrupt and later reactivate that connection.

Transport transforms evidence. Hydraulic sorting responds to size, density and shape. Abrasion changes surface area and mineral exposure. Repeated storage mixes events. Dissolution removes unstable phases. Fine particles and coatings carry high surface-area chemical signals. Consequently, a downstream concentration trend may reflect changing host fraction rather than changing source strength.

Evidence and measurement

At slopes, record gradient, curvature, soil thickness, clast fabric, stone lines, basal shear surfaces, lobate fronts, sorting and relation to bedrock. In channels, distinguish bed, bar, bank, floodplain and abandoned-channel stores. Measure channel width and depth at a stated flow condition and survey cross-sections against stable control. Sediment provenance can use lithology, mineral assemblage, grain shape, geochemistry, isotopes or age distributions, each with its own mixing and preservation bias.

Sampling must control fraction and geomorphic unit. A composite of active fine sediment is not comparable with one gravel clast or an overbank core. Report wet or dry mass, sieve limits, replicate design, collection area and whether coatings were retained. Map artificial disturbance separately; road aggregate, drains and historical workings can create highly connected false pathways.

Connectivity should be stated for an event and grain-size class. A dry swale may disconnect coarse bed load but transmit dissolved material during an exceptional storm. A terrace is disconnected from ordinary floods yet may contribute sediment by bank erosion.

Worked example

Consider a synthetic 5-hectare footslope store. Annual hillslope and gully inputs are estimated as 1,650 t, while channel export is 1,000 t. The storage increase is 650 t/yr. With bulk density 1.6 t/m³, the volume increase is


650/1.6=406.25\ \text{m}^3/\text{yr}.

Distributed across 50,000 m², the mean aggradation rate is about 0.0081 m/yr, or 8.1 mm/yr. This synthetic result should trigger checks: are the fluxes averaged over matching periods, is compaction ignored, did the survey include all lobes, and could episodic storms dominate the apparent annual rate?

For a tracer, suppose source A supplies 60% of the fine sediment at 100 mg/kg and source B supplies 40% at 20 mg/kg. Conservative mixing predicts 68 mg/kg. A measured 45 mg/kg could indicate a different mixture, selective loss, dilution by a third source or analytical/support mismatch. The concentration alone cannot identify which process occurred.

Misinterpretations and uncertainty

Do not use angularity alone to infer short transport; inherited jointing, mineral durability and source texture matter. Do not assume all matrix-rich deposits are debris flows or all sorted deposits are fluvial. Post-depositional soil formation can obscure primary structures, while winnowing can imitate original sorting.

Sediment budgets often compare incompatible time scales: a rare-event deposit, a short monitoring interval and a millennial average. Density, porosity and unmeasured dissolved load add uncertainty. Source apportionment is non-unique when end members overlap or change through weathering. Treat zero detection as a censored measurement, not proof of absence.

Practical investigation

Draw a source-to-sink network for a small catchment. Nodes are residual source, convex slope, hollow, fan, active channel, floodplain and outlet; edges are process-specific transfers. Create separate networks for coarse particles, fine particles and dissolved load. Identify where each network disconnects during ordinary conditions and where an extreme event could reconnect it.

Collect or use a supplied set of paired grain-size data from at least three geomorphic units. Plot concentration against fraction, mineral host and distance. Test conservative mixing before invoking reaction. Produce a sediment budget with low, central and high estimates and list the term that most controls closure.

Mastery check

  1. Why is a sampling medium inseparable from its transport pathway?
  2. What changes as a nonlinear hillslope model approaches its critical gradient?
  3. How can a terrace be both a store and a later source?
  4. Which observations distinguish colluvium from alluvium?
  5. Why can downstream dilution occur without a weaker source?

Sources and further reading