SCIENCE · INVESTIGATE AND EXPLAIN

Separate suspended load from catchment erosion

Trace sediment production, transport and storage without attributing an outlet total to an unmeasured source.

Learning outcome: State what a gauge load measures and identify additional evidence needed for erosion or intervention claims.

Starting recap

Weathering alters material in place; erosion removes it; transport moves it and deposition stores it. Outlet suspended load captures only part of that history.

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Try

A 151.2-tonne suspended load at the modeled gauge does not show that 151.2 tonnes of new hillslope erosion occurred during the six-hour event.

Open the experiment

Predict

Can two catchments deliver the same outlet suspended load while having different erosion and storage histories?

Your prediction and reason

Experiment

Compare the aligned and late schedules and state the measured-section quantity each total represents. Draw alternative upstream source-and-storage histories consistent with one total.

Integrate a storm suspended-sediment load

Worked teaching model · synthetic values · Six fictional one-hour intervals with constant values within each interval. Base streamflow is 10, 20, 40, 40, 20, 10 m³/s. Concentration is assumed representative of the whole flowing cross-section, not a point sample or turbidity reading. Changing timing rearranges the same six concentration values. Flow scaling holds concentration fixed as a controlled accounting exercise, not a causal runoff or erosion model. The paired sum is exact only for these supplied piecewise-constant records; bedload and dissolved material are excluded.

Streamflow multiplier: 1 × Concentration timing: Aligned concentration peak

Reset keeps your prediction, captures and practice history.

Paired hourly records and integrated mass
Hours	Q (m³/s)	C (mg/L)	Rate (kg/s)	Hour mass (kg)	Cumulative (kg)
0–1	10	100	1.00	3,600	3,600
1–2	20	200	4.00	14,400	18,000
2–3	40	400	16.00	57,600	75,600
3–4	40	400	16.00	57,600	133,200
4–5	20	200	4.00	14,400	147,600
5–6	10	100	1.00	3,600	151,200

Compare shortcuts with the supplied paired records
Method	Mass (kg)	Error: estimate minus paired sum (kg)	Relative error (%)
Paired hourly integration	151,200	0	0.00
Product of separate time means	117,600	-33,600	-22.22
First-bin concentration for all water	50,400	-100,800	-66.67

Time-mean Q: 23.333 m³/s; time-mean C: 233.333 mg/L. Water volume: 504,000 m³. Flow-weighted C: 300.000 mg/L. Equal separate means do not preserve the time pairing needed for load. A shortcut that matches one case can still be unreliable in another.

These are fictional interval records, not storm measurements. A section's suspended load omits bedload and dissolved material and cannot locate sediment sources or separate fresh erosion from remobilized storage. Real sampling must address timing and cross-section representativeness.

Six-hour suspended mass: 151,200 kg (151.2 metric tonnes). Water volume: 504,000 m³.

Rate = Q × C × 0.001 kg/s; each hourly mass is rate × 3,600 s. Flow-weighted concentration = 300.00 mg/L; separate time-mean concentration = 233.33 mg/L. Suspended mass at this section is not whole-watershed gross erosion.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Keep the location and sediment fraction in the claim

The aligned case at multiplier 1 transports 151.2 metric tonnes of suspended sediment past the modeled stream section during six hours. That sentence states the quantity, location, fraction and interval. Changing it to “the watershed eroded by 151.2 tonnes” adds an inference that the supplied calculation does not establish.

The gauge calculation counts particles carried in suspension at its section. It omits bedload moving along or near the bed and dissolved material carried in solution. It also does not measure every place where sediment was detached or deposited upstream. An outlet transport total is therefore neither total sediment transport across all modes nor gross erosion throughout the catchment.

Distinguish the processes in a sediment history

Weathering breaks down or chemically alters material in place. Erosion removes material from where it was. Transport moves it, and deposition places it into storage. A particle can experience several episodes of movement and storage before passing a gauge.

Sediment deposited in a channel or floodplain can later be remobilized. Thus water passing the gauge during a storm can carry material detached earlier, not only newly eroded hillslope soil. USGS describes sediment settling, accumulating and being picked up again as river conditions change. USGS explanation of suspended sediment and storage.

Draw four boxes: hillslope material, channel storage, floodplain storage and the gauged outlet. Use arrows to show possible transfers. Put the browser’s calculated suspended load on the outlet arrow only. Leave the unmeasured internal arrows labeled unknown rather than assigning invented fractions to them.

Build two histories that fit one outlet total

Construct one hypothetical history in which much of the transported material was newly detached during the event. Construct another in which much came from remobilized channel deposits. Both can be compatible with the same outlet suspended mass unless additional evidence distinguishes their sources.

These are alternative explanations, not two measured histories. The browser contains no source tracers, repeat topographic surveys or internal sediment-storage observations. It cannot decide between them from Q and C alone. Nor can it calculate a catchment sediment-delivery ratio without a suitable estimate of sediment production and a consistent accounting interval.

A paper sediment budget can help expose the missing terms. External export depends on upstream supply and changes in internal storage. If some eroded material remains stored, gross erosion can exceed export. If previously stored sediment is released, export can include material not newly eroded during the selected interval. Neither condition contradicts the gauge calculation.

Compare timing without inventing an intervention

Switch from aligned to late concentration timing at multiplier 1. The calculated load falls from 151.2 to 100.8 tonnes. The cause within this constructed comparison is the changed pairing of supplied concentrations with discharge. Nothing in the controls represents vegetation removal, a restoration project, a road or a particular land-use change.

A claim that vegetation caused the difference would need an appropriate study design and observations. Write an evidence plan that distinguishes the intervention from other differences in rainfall, discharge, sediment availability and previous storage. Consider comparable observation intervals and how measurement uncertainty would affect the comparison. Do not treat two manually chosen schedules as a field experiment.

Make the conclusion proportional to the evidence

Create a ledger with three columns: supported result, additional interpretation and missing evidence. Put the six-hour suspended load in the first. Put source attribution, gross catchment erosion or a treatment effect in the second. Match each interpretation to a specific kind of evidence in the third.

The same discipline applies to high concentration or muddy appearance. Neither alone quantifies mass export; flow and time matter. An accurate load estimate can be valuable without answering every upstream process question. Preserve its scope instead of either overstating it or dismissing it.

Transfer: review a source claim

A report states, “The outlet carried 100.8 tonnes, so exactly 100.8 tonnes of soil were newly eroded from farms during this storm.” Identify at least three missing links and write a defensible replacement.

Check the erosion-evidence transfer

The calculation does not establish new erosion timing, farm origin or equality between gross erosion and outlet suspended export. Storage, remobilization and excluded transport fractions are unresolved. A defensible statement is that 100.8 metric tonnes of suspended sediment passed the modeled section during the six-hour interval under the supplied discharge and representative-concentration assumptions.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Separate suspended load from bedload and dissolved transport, name unmeasured storage changes, and explain what evidence would be needed to assess a vegetation intervention.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Test how sampling timing changes a sediment estimate Next: Connect air pressure to the mass above

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