Unit 3 — Rivers, sediment and floodplain evolution

How do water and sediment reorganize river networks?

Rivers couple catchment runoff to sediment delivery, channel form and downstream deposition. The water discharge passing a cross-section is not the same as the sediment discharge and neither alone determines channel geometry. Grain entrainment depends on boundary shear stress, grain properties and shielding; available transport capacity differs from the available supply. During floods, channels can erode banks, migrate or build bars while floodplains exchange water and fine sediment. In incising reaches, longitudinal profiles contain information about uplift, changing base level and rock strength, but the same knickpoint shape can reflect several histories. Channel width, depth, roughness, slope and velocity covary. Simplified formulas are appropriate for checking units and direction of change; robust inference depends on direct field sampling and hydraulic calibration. We also distinguish contemporary event sediment pulses from longer-term terrace records. Preservation of a floodplain surface depends on deposition and later erosion, so a stratigraphic sequence is a biased archive of earlier processes.

Learning outcomes and scale discipline

By the end of this unit, students should be able to describe at least five independent landform or sediment observations, demonstrate one dimensionally consistent quantitative calculation, design a comparative measurement, and distinguish an interpretation from the observations used to support it. Practice viewing the same landform in a map, a section and a material description. Keep an explicit column for phenomena that are absent, unobserved or only assumed. At the finest scale, a few grain contacts may govern transport; at larger scales, inherited valley geometry and event histories control the response. Switching scale is legitimate only after specifying which variables remain meaningful. At the start of the field exercise, make a drawing of a downstream profile, two bar maps and time-integrated transport samples; list which parts would be measured directly and which reconstructed from other evidence.

The unit's measurement choices should include surveyed discharge section, particle size and stage observations; the strongest report also checks bankfull stage and a consistent datum. Even a carefully derived rate does not establish its formative mechanism unless the sedimentary or stratigraphic evidence agrees. Interpret changes in the context of the process and observation interval, and use at least two sources of independent evidence. Most useful scientific uncertainties are not just numerical instrument errors: researchers must also consider incorrect boundaries, missing events, biased sample sites, mistaken ages and incomplete constitutive laws.

Process mechanism and scale

For a channel section, water discharge equals area times mean velocity; width and depth distributions matter when the cross section is irregular. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Current-meter measurements, stage records and surveyed transects constrain the hydrograph. Record surveyed discharge section, particle size and stage observations. In the diagram of a downstream profile, two bar maps and time-integrated transport samples, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A rapid change in water level can reflect downstream backwater rather than increased local discharge. Subdivide the section into measured velocity strips and compare their sum with a single average-velocity estimate. Consider changing roughness mistaken for a change in sediment supply. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Discharge from section geometry

Given. A rectangular approximation: channel width 12 m, mean depth 1.2 m, mean velocity 1.4 m/s. Calculation: 12 × 1.2 × 1.4. Result: 20.16 m³/s. Work through the units before using the output to discuss Earth-surface history. Velocity changes linearly affect the estimated discharge when area is held fixed. Actual discharge requires cross-sectional velocity integration. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document hydrograph timing; independently check cross-sectional integration. Assess sensitivity to supply/transport separation and verify stage comparability. Record each raw observation, units, uncertainty and assumptions before model fitting.

Surface velocity is not depth-averaged velocity and turbulence adds uncertainty. Report gauging method and section geometry with every discharge value. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for flow continuity and hydraulic geometry without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Fluid drag and submerged particle weight determine approximate entrainment thresholds, often summarized by dimensionless Shields stress. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Bed-surface grain size, riffle mobility, water-surface slope and transport samples constrain motion. Record surveyed discharge section, particle size and stage observations. In the diagram of a downstream profile, two bar maps and time-integrated transport samples, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A bed may remain immobile at high apparent stress if armoring or cohesion resists motion. Compute Shields stress for two grain sizes under an assumed hydraulic radius and slope; compare measured bed fractions. Consider changing roughness mistaken for a change in sediment supply. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Shields stress screening

Given. Water density 1000, grain density 2650 kg/m³, hydraulic radius 1.5 m, slope .006, grain diameter .020 m. Calculation: 1000×1.5×0.006 / [(2650−1000)×0.020]. Result: 0.27273 dimensionless. Work through the units before using the output to discuss Earth-surface history. Doubling the grain size halves the dimensionless stress in this simple fixed-flow comparison. Grain packing and cohesion alter real incipient-motion thresholds. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document hydrograph timing; independently check cross-sectional integration. Assess sensitivity to supply/transport separation and verify stage comparability. Record each raw observation, units, uncertainty and assumptions before model fitting.

Threshold values vary with grain Reynolds number, packing and mixture, so one constant is not universal. Call the outcome a screening calculation, not proof that grains moved. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for boundary shear and sediment entrainment without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Coarse particles may roll or hop along the bed while fine fractions travel in suspension; the delivered load also depends on sediment availability. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Turbidity, suspended-sediment samples, bar deposits and tracer stones reveal different pathways. Record surveyed discharge section, particle size and stage observations. In the diagram of a downstream profile, two bar maps and time-integrated transport samples, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

Water discharge peaks and sediment peaks can be asynchronous because a flood exhausts or reconnects sources. Sample sediment on rising and falling hydrograph limbs, then integrate loads with error brackets. Consider changing roughness mistaken for a change in sediment supply. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Suspended load

Given. Suspended sediment concentration .4 kg/m³ at a discharge of 40 m³/s. Calculation: 0.4 × 40. Result: 16 kg/s. Work through the units before using the output to discuss Earth-surface history. A daily load requires integrating concentration and discharge together over time. Instantaneous load is not a directly measured annual yield. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document hydrograph timing; independently check cross-sectional integration. Assess sensitivity to supply/transport separation and verify stage comparability. Record each raw observation, units, uncertainty and assumptions before model fitting.

A high turbidity reading is not itself a calibrated mass-concentration measurement. Keep supply limitation and transport capacity conceptually separate. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for bedload, suspension and sediment budgets without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

Curvature-driven secondary circulation, bank erodibility and sediment deposition influence migration, bar development and cutoffs. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Sequential mapped centerlines, bank pins, point-bar deposits and cutoff scars constrain movement. Record surveyed discharge section, particle size and stage observations. In the diagram of a downstream profile, two bar maps and time-integrated transport samples, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

A channel that changes shape after one flood may not follow its earlier average migration rate. Overlay photographs tied to common control points and compute positional uncertainty before estimating movement. Consider changing roughness mistaken for a change in sediment supply. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Lateral migration

Given. An 8 m bank displacement measured across a five-year map interval. Calculation: 8 / 5. Result: 1.6 m/yr. Work through the units before using the output to discuss Earth-surface history. One flood may account for most of the motion; a mean rate hides variability. Registration error must be smaller than the apparent movement. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document hydrograph timing; independently check cross-sectional integration. Assess sensitivity to supply/transport separation and verify stage comparability. Record each raw observation, units, uncertainty and assumptions before model fitting.

Bank-line definitions vary with flow stage and vegetation, which can produce apparent change. State whether migration refers to bank, centerline or channel-belt boundary. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for meanders, bars and channel migration without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Process mechanism and scale

A floodplain exchanges water and sediment with an active river while a terrace records a former surface disconnected by incision or uplift. Draw a control volume showing forcing, material transfers and storage. Name the scale and timescale of the proposed process; landform shape alone does not reveal either.

Observable evidence

Strath contacts, overbank silts, soil profiles, terrace elevations and dates distinguish depositional episodes. Record surveyed discharge section, particle size and stage observations. In the diagram of a downstream profile, two bar maps and time-integrated transport samples, label measured features separately from inferred histories. Provide survey dates, scale and a detection threshold.

Alternatives and field tests

Terraces at similar heights may form at different times in separate tributaries. Survey elevations relative to the modern channel and date material demonstrably tied to terrace formation. Consider changing roughness mistaken for a change in sediment supply. Identify one independently measurable outcome that would distinguish the two interpretations, and one negative result that would undermine the leading explanation.

Worked quantitative example — Terrace incision average

Given. 15 m difference between a terrace strath and modern channel dated to 6,000 years. Calculation: 15 / 6000 × 1000. Result: 2.5 mm/yr. Work through the units before using the output to discuss Earth-surface history. A younger valid terrace age implies a higher averaged incision rate. Incision rate is not automatically rock uplift rate. A correct result from an incorrect conceptual model is not evidence that the process operated. State which measured variable would need independent verification. If the answer is a rate, indicate the averaging interval; if it is a geometric ratio, identify the scale and measurement direction; if it is a budget, give the boundary and the sign convention.

Research procedure and critical interpretation

Document hydrograph timing; independently check cross-sectional integration. Assess sensitivity to supply/transport separation and verify stage comparability. Record each raw observation, units, uncertainty and assumptions before model fitting.

Reworked charcoal can be older than the floodplain deposit that encloses it. Use terrace height as geometry, not as an age measurement by itself. A high-resolution shape map without ages and a dated deposit without spatial coverage offer different constraints. Explain how they could be combined for terraces, floodplains and alluvial archives without overclaiming a process history. Identify one further test.

Problems with answer guidance

Problem 1. Recalculate the worked case after changing an input by 20% and show units. Problem 2. Contrast two possible landform origins using a specific field test. Answer guidance: correct units, explicit physical assumptions and a genuinely distinguishing observation are essential.

Use invented cross sections and sediment concentrations upstream and downstream of a bar complex. Compute water discharge, integrate a simple suspended-load time series, and carry a residual in the reach sediment budget. Explain two different histories consistent with the same net deposit thickness. Predict how an unusually sediment-poor flood could modify bed elevation.

Protocol and evidence allocation

Write a project proposal that names the study question, a defined sampling region, hypotheses H1 and H2 and three observations that could discriminate them. Build an input table with at least twelve synthetic measurements; record a unit, identifier, uncertainty class and data source for every row. Design one transect, one temporal repeat survey and one test using a different kind of evidence. State how the field team would avoid confirmation bias: preregister predicted outcomes, assign a second mapper to identify landforms independently and blind the initial data processing to the favored model. Any unmeasured quantity must be marked as an assumption. Base the arithmetic on the numerical cases in the unit, but do not present the invented data as actual measurements.

In the analysis, start with a mass or elevation balance rather than a purely visual explanation. If the balance does not close, label the residual and list processes that may lie outside the boundary, including sediment stored in a channel, transport at an unmonitored exit and error in estimated density or surface area. Separate interpretation of rate from reconstruction of age and chronology. An apparently consistent average rate can conceal long inactive periods punctuated by large storms, landslides or channel switching. Explore a parameter range wide enough to expose this sensitivity. Explain what observation would tell you whether the residual is an actual physical loss or a measurement artifact.

Reporting template and peer review

The final report should contain: a title that describes an inquiry rather than a conclusion; data source and acquisition notes; a map or section with scale; a table of raw observations; a short derivation with units; alternative interpretations; uncertainty and detection limits; and an honest conclusion identifying what the exercise cannot show. A peer reviewer should trace one plotted value back to an original input without assistance. Report uncertainty in both physical data and causal attribution. Describe how sampling locations could bias the result, and identify one independent validation datum that was not used when choosing the model. Treat a good fit as a starting point for further testing, not proof of a unique historical trajectory.

Assessment prompts and rubric

Interpretation. Provide two competing process histories consistent with the invented evidence, predict one differentiating field measurement, and identify which uncertainties could reverse the preferred interpretation. Quantitative reasoning. Recompute the selected unit case in SI units, vary two inputs independently, and interpret signs, magnitudes and nonlinearities. Evidence ethics. Identify all fabricated datasets and explain why they must never be described as real hazard, glacial or coastal monitoring records. Review standard. Full credit requires correct units, reproducible methods, a mass or geometry balance, explicit chronology limits and an alternative interpretation; numerical correctness alone is insufficient.

Return to the guiding question: How do water and sediment reorganize river networks? Make a process diagram contrasting an observable pattern with a process that could produce it. Now add a second process capable of leaving a similar pattern. Underline measurements that would distinguish the two. Ask whether the lesson's calculations predict the type of change measured by those instruments or only provide a scale for discussing it. The emphasis throughout geomorphology is not on naming spectacular landforms but on reconstructing defensible histories, with uncertainties, from repeatable measurements and physical consistency.