How rivers work
A river is a flow, not a store. Understanding discharge, baseflow and the flow duration curve explains almost everything about what a river can and cannot supply.
Rivers hold about two ten-thousandths of one percent of Earth’s water at any moment, and supply most of what people abstract. The resolution of that apparent contradiction is that a river is a flow rather than a store: its water is replaced every few weeks, so a modest standing volume delivers an enormous annual throughput.
Where the water in a river comes from
- Overland flow
- Rain running across the surface. Dominant where soil is saturated, frozen, compacted or paved. Fast, and responsible for flood peaks.
- Throughflow
- Water moving laterally through soil above the water table. Intermediate in speed.
- Baseflow
- Groundwater seeping into the channel. Slow, steady, and the reason a river keeps flowing weeks after the last rain.
The proportions matter enormously. A chalk stream is almost entirely baseflow, so it runs clear and steady year round. A steep upland catchment on impermeable rock is mostly overland flow, so it rises and falls within hours of rainfall. The same amount of annual rain produces completely different rivers.
The flow duration curve
A river’s flow is not a single number. The flow duration curve plots discharge against the percentage of time it is exceeded, and it is the single most useful summary of a river’s behaviour.
| Statistic | What it means | What it is used for |
|---|---|---|
| Q5 | Flow exceeded 5% of the time — high flows | Flood and channel capacity assessment |
| Q50 | Median flow, exceeded half the time | General characterisation |
| Mean flow | Arithmetic average, usually well above the median | Total resource assessment |
| Q95 | Flow exceeded 95% of the time — low flows | Abstraction licensing and environmental flow |
| Q99 | Flow exceeded 99% of the time — extreme low flow | Drought planning |
Mean flow exceeds median flow on almost every river, because the distribution is skewed by floods. Quoting a mean without a Q95 says nothing about whether a river can be relied on in a dry summer.
Environmental flow
A river that is abstracted down to a trickle is not a functioning ecosystem. Environmental flow requirements set how much must be left in the channel, and they are increasingly defined not just as a minimum volume but as a regime — high flows to move gravel and trigger fish migration, low flows within a tolerable range, and seasonal variation that species are adapted to.
This is why abstraction licences are increasingly conditional on flow: a licence may allow abstraction only while flow exceeds a stated threshold, and require it to cease below that. It is also why the SDG 6.4.2 water stress indicator subtracts environmental flow from available resources before taking the ratio.
Sources
- UK Centre for Ecology and Hydrology — National River Flow Archive. Permission required · NRFA click-through licence for API access
- United States Geological Survey — National Water Information System. Open · US Government work — public domain
- Global Runoff Data Centre, Federal Institute of Hydrology (Germany) — Global Runoff Database. Permission required · Publisher agreement required