Chapter 7 of 17 · Water and the Weather
Chapter 6. Rivers and catchments
How the waters of a territory gather into a single watercourse, where the water that flows in high summer comes from, and what a watershed is.
A river is a meeting place
Look at a river and ask yourself the only question that matters: where does all that water come from? It does not come out of a tank, nobody pours it in, and it is still flowing three weeks after the last rain. A watercourse is not a thing: it is the meeting place of everything that has fallen on a territory.
That territory has a name, and it is the central idea of this chapter. Eaufrance defines it thus: a catchment is a "clearly defined geographical territory: it corresponds to the whole of the surface receiving the waters that flow naturally towards one and the same watercourse or towards one and the same body of underground water". It is also called a drainage basin.
In other words: a river cannot be understood by looking at the river. It is understood by looking at everything that stands above it, right up to the ridges. This chapter therefore goes back up from the water to the territory, then comes back down from the territory to the water, and along the way it brings out the piece that was missing from the previous chapter: what becomes of groundwater.
The objectives of this chapter
Each one is observable. By the end of the chapter, either you do it, or you know which section to read again.
- Name the parts of a watercourse, from the source to the outlet, using the words tributary and confluence.
- Define a catchment, and say what marks out its boundary.
- Trace, on a description of a relief, the watershed between two neighbouring catchments.
- Cite the three inputs that feed a watercourse, and say which one makes it flow in a dry spell.
- Tell a flood apart from a low flow from an observation made in the field.
What this chapter assumes is already known
A single prerequisite, and it is installed by the previous chapter of this volume: runoff, infiltration, groundwater and the impermeable bedrock.
From the source to the outlet, and the words of the journey
A watercourse begins at its source, which is most often the place where groundwater comes back out into the open air. It ends at its outlet, that is to say at the place where it empties: into another watercourse, into a lake, or into the sea. Between the two, it receives other watercourses.
A watercourse that empties into another is its tributary, and the meeting point is the confluence. The words are simple, but using them accurately changes everything: to say that a river has many tributaries is to say that its catchment is vast, so that it gathers the rain of a great deal of ground, so that it reacts to showers that fell far away from it.
Eaufrance also describes the catchment in a more everyday sense: it is the "space that collects the water flowing through the various aquatic environments, watercourses, lakes, ponds, wetlands, estuaries or lagoons, from the sources as far as its outlet". That wording adds something important: a catchment does not contain only a river. It contains ponds, wetlands and pools, and those environments hold water back, slow it down and give it up again slowly.
The watershed
How do we know where a catchment stops? Eaufrance answers: "a catchment is bounded by watersheds between the various basins. These lines are natural boundaries drawn by the relief: they correspond to the ridge lines."
The same text gives the image that settles the matter for good: "the raindrops falling on one side or the other of this watershed will feed two catchments lying side by side". Two drops a few centimetres apart at the top of a ridge can therefore end up in two different seas.
This boundary has three properties that must be kept in mind. It is natural: nobody decided it, it is the relief that traces it. It practically never coincides with an administrative boundary: a catchment cheerfully crosses districts, regions and countries. And it is stable: a relief changes on the scale of millions of years, not of seasons.
It is for these three reasons that water is managed by catchment, and not by administrative division. A pollutant tipped in upstream does not stop at a district boundary; it goes down to the outlet, passing through the land of every downstream neighbour on the way.
The words to remember
Five words, of which three describe a journey and two describe a territory.
- The source
- The place where a watercourse begins, most often because underground water comes back out there into the open air.
- The tributary and the confluence
- A tributary is a watercourse that empties into another. The confluence is the exact point where the two join.
- The outlet
- The place where a watercourse, or a whole catchment, discharges its waters: another watercourse, a lake, or the sea.
- The catchment
- The territory corresponding to the whole of the surface receiving the waters that flow naturally towards one and the same watercourse or towards one and the same body of underground water. It is also called a drainage basin.
- The watershed
- The natural boundary between two neighbouring catchments, drawn by the relief and corresponding to the ridge lines.
What makes a river flow in high summer
Here is the piece the previous chapter left hanging. Eaufrance gives the complete list in one sentence: "From the source to the outlet, a watercourse is fed by other watercourses (which are its tributaries), by runoff water coming from precipitation, and by underground water." Three inputs, then, and they must be told apart because they have neither the same rhythm nor the same role.
The first input is runoff. It is immediate: a few hours after a shower, the surface water has arrived. It is also the most abrupt, and it is what makes the level rise suddenly.
The second input is the tributaries. A watercourse receives what its tributaries have themselves received, with the delay their journey imposes. A shower that fell fifty kilometres upstream can make a river rise a day later, when not a drop has fallen locally.
The third input is the quietest and the most decisive: the groundwater. Water that soaked in weeks or months earlier comes back out slowly into the bed of the watercourse. It is what explains why a river still flows after a month without rain, and it is what disappears last when a drought sets in.
Remember the consequence, because it ties this whole volume together: what the ground took in during the winter is what will flow during the summer. A territory that runs off a great deal and soaks in little has stronger floods AND more severe low flows, that is to say both drawbacks at once. The quantity of water received has not changed; it is its spread over time that has got worse.
| Input | Delay after the rain | What it brings about | What increases it |
|---|---|---|---|
| Direct runoff | From a few minutes to a few hours | The rapid rise of the level, and water turning brown because it takes up particles torn from the soil. | Slopes, impermeable soils, built-up surfaces, ground that is already saturated. |
| Input from tributaries | From a few hours to several days | A delayed rise, sometimes without any rain having fallen locally. | The extent of the catchment upstream and the number of tributaries. |
| Discharge from groundwater | From a few weeks to several months | The baseflow, the one that remains outside spells of rain. | Infiltration during the previous seasons and the capacity of the aquifer. |
Flood and low flow: the two extremes of one and the same watercourse
A flood is a marked rise in the discharge and the level of a watercourse. It can be recognised without any instrument: the water is high, it is brown because it is carrying particles torn from the soil, it flows fast, and it sweeps branches along. A flood is a natural and regular event; the fertile plains of most valleys were built by successive floods.
A low flow is the opposite: the time of year when the discharge of a watercourse is at its lowest. It too can be recognised: the water is clear because it comes mainly from groundwater, the level uncovers gravel banks, and the bed shrinks to a narrow channel.
Both are measured at the same place and with the same instruments: a gauging station records the water level continuously, and that level is converted into a discharge. Eaufrance states that water levels and discharges are recorded at about 5,000 gauging stations, and separately publishes low-flow data for the watercourses of mainland France in a dedicated observatory. It is that network which makes it possible to say, with figures to hand, that a river is lower this year than in previous years, instead of saying it from an impression.
This volume stops here on that subject, and it must say why. A flood can become dangerous, and how to behave in that case is not learnt from a manual: it is published by the competent authorities at the moment the danger exists, according to the place and the hour. This book explains what a flood is and where it comes from. It tells nobody what to do during one.
The trap of this chapter: believing that a catchment can be read on an administrative map. It never can. One district can belong to two catchments, and one catchment can cover several countries. The only thing that traces the boundary is the relief, and therefore the ridge lines. A concrete and often ignored consequence: the quality of the water of a river does not depend on what the district where we are looking at it does, but on what every district lying upstream in the same catchment does. That is exactly why water is managed by catchment, and not by administrative territory.
Worked example. Following two drops separated by a ridge
Question: "Two drops fall a metre apart at the top of a ridge. North of that ridge, the slopes go down towards a valley whose river joins a large river running west. To the south, the slopes go down towards a stream that joins a large river running south." Where do the two drops end up? Fully guided example.
- Identify the watershed
The question describes the top of a ridge with slopes on either side. Now watersheds correspond to ridge lines. The top described IS the watershed: the two drops are therefore on either side of a natural boundary. - Follow the first drop
The drop that fell on the north side follows the north slope. It runs off towards the valley, joins the river of that valley, which is a tributary of the large river running west. It will end at the outlet of that large river. - Follow the second drop
The drop that fell on the south side follows the south slope. It runs off towards the stream, which is a tributary of the large river running south. It will end at the outlet of that other large river. - Name the two territories
Each of the two drops belongs to a different catchment. The two catchments lie side by side and they touch exactly along the ridge, without overlapping anywhere. - Add the qualification about the real journey
Part of each drop does not run off: it soaks in, joins the groundwater, and comes back out later into the bed of the river of its catchment. The journey is then far slower, but the arrival is the same, because groundwater too flows towards the outlet of its catchment. - The result
Two drops a metre apart end up in two different large rivers, one to the west and the other to the south. The distance between them on arrival can be counted in hundreds of kilometres, and nothing but a metre of relief decided it.
Experiment 7. The catchment in crumpled paper
What you need: a large sheet of paper, washable felt pens in two colours, a water sprayer or a bottle pierced with a fine hole, a basin. SAFETY: this experiment is carried out with an adult's agreement, and the adult is present. It is done over a basin or a sink so as to wet nothing around it, and the felt pens must be washable: it is the adult who checks them before you start.
- Make a relief
Crumple the sheet of paper into a ball, then open it out again without flattening it. The folds make ridges and valleys: you have a miniature relief. Set it in the basin. - Mark the ridges
With the first felt pen, draw a line along the highest edges, the ones that visibly separate two hollows. You have just traced watersheds, working only from the relief, exactly as a hydrologist does. - Predict before watering
With the second felt pen, draw the paths the water ought to follow in your view, and ring the lowest hollow. Writing a prediction down BEFORE measuring is what separates an experiment from a demonstration. - Make it rain
Spray water gently and evenly over the whole relief, without aiming at any particular spot. The washable ink dilutes and follows the water: the real paths appear in colour. - Compare the prediction and the result
Compare the lines you had drawn with the coloured trails. The differences are the places where the relief was subtler than your reading of it, and they are the ones that teach you something. - Find the outlets
Look for the low points where the water builds up. Each one is the outlet of a catchment. Count them, then mark out with the felt pen the territory that feeds each one. - Lay a coin down and start again
Lay a coin or a piece of plastic somewhere on a slope, then spray again. The impermeable surface diverts the water and speeds up its arrival at the bottom: it is the effect of a car park, reduced to three centimetres.
What to remember from this chapter
- A catchment is the whole of the surface receiving the waters that flow naturally towards one and the same watercourse or one and the same body of groundwater.
- Its boundary is the watershed: a natural boundary drawn by the relief, which follows the ridge lines.
- A watercourse runs from its source to its outlet, receiving tributaries at confluences.
- Three inputs feed it: runoff within a few hours, tributaries within a few days, groundwater within a few weeks or months.
- It is the discharge from groundwater that makes a river flow outside spells of rain: what the ground took in during the winter flows during the summer.
- A territory that runs off a great deal and soaks in little has stronger floods and more severe low flows, without having received less water.
Self-check for chapter 6
Answer in writing, using only the knowledge given by this volume so far. The answers are gathered at the end of the volume.
- Define a catchment, then say what traces its boundary and why that boundary does not follow administrative borders.
- A river rises abruptly although it has not rained in the district. Put forward two explanations consistent with this chapter.
- In August, after six weeks without rain, a river is still flowing. Which input makes it flow, and where exactly does that water come from? Say also how one recognises in the field that a river is in low flow rather than in flood.
- One and the same catchment is built up over a third of its surface. Explain why this makes both the floods and the low flows worse, when the quantity of rain received has not changed.
- Two drops fall a metre apart on a ridge. Can they end up in two different seas? Justify your answer with the vocabulary of the chapter.