A river does different work in different places, so its landforms follow a sequence from source to mouth. To trace that sequence, name the landform, say where it is, and give the process that makes it.
This lesson builds on channel cross-sections and sits in rivers and drainage. The sequence also gives you the context you need to read a hydrograph later.
How does the sequence work?
Think of three stages. They are a useful guide, though real rivers do not switch from one stage to the next at a sharp line.
- Upper course: steep gradient, narrow channel, strong vertical erosion. Landforms include V-shaped valleys, interlocking spurs, rapids and waterfalls.
- Middle course: gentler gradient and a wider channel. Lateral erosion and deposition build meanders.
- Lower course: very gentle gradient and a wide, deep channel. Deposition dominates, giving floodplains, levées, oxbow lakes and, at the sea, a delta or an estuary.
How do you work with a long profile?
A long profile plots height against distance downstream. The gradient between two points is the fall in height divided by the distance.
Worked example
All figures are invented for practice. A student recorded four sample points on a river.
| Point | Distance from source | Height above sea level |
|---|---|---|
| 1 | 0 km | 900 m |
| 2 | 10 km | 400 m |
| 3 | 40 km | 100 m |
| 4 | 90 km | 20 m |
Step 1, gradient between points 1 and 2: fall = 900 − 400 = 500 m over 10 km, so 500 ÷ 10 = 50 m per km.
Step 2, points 2 and 3: fall = 400 − 100 = 300 m over 30 km, so 300 ÷ 30 = 10 m per km.
Step 3, points 3 and 4: fall = 100 − 20 = 80 m over 50 km, so 80 ÷ 50 = 1.6 m per km.
Check: total fall is 500 + 300 + 80 = 880 m, which equals 900 − 20. The distances add to 10 + 30 + 50 = 90 km. Both match the table.
Step 4, link to landforms: the gradient falls from 50 to 1.6 m per km. The steep section near points 1 and 2 matches strong vertical erosion, so we would expect a V-shaped valley and perhaps a waterfall. Between points 3 and 4 the very gentle gradient matches deposition and meandering, so a floodplain and possibly an oxbow lake are likely.
Step 5, write the sequence: V-shaped valley, then waterfall or rapids, then meanders, then floodplain with levées, then oxbow lake, ending in a delta or an estuary at the mouth.
Explain one link: a meander forms in the middle course because the river erodes the outside of a bend where flow is faster and deposits on the inside where it is slower. Repeated over time, the bend grows.
The mistake to watch for
Mistaken answer: “The oxbow lake forms first and then the river makes a meander.”
The order is the wrong way round. A meander must exist before a loop can be cut off. The correct order is: meander develops, the neck narrows, a flood breaks through the neck, deposition seals off the loop, and the abandoned loop becomes an oxbow lake.
Another slip is to place a waterfall in the lower course because “it is where the river falls”. Waterfalls form where a band of hard rock lies above softer rock, and they are most often found where the river is steep, in the upper course.
Check yourself
All data are invented.
1. A river falls from 600 m to 200 m over 20 km. What is the average gradient?
Show answer
Fall = 600 − 200 = 400 m. 400 ÷ 20 = 20 m per km.
2. Put in order from source to mouth: oxbow lake, V-shaped valley, delta, meander.
Show answer
V-shaped valley, meander, oxbow lake, delta.
3. Explain in two linked sentences why a floodplain is flat and fertile.
Show answer
When the river floods, it loses energy as water spreads out and slows. It deposits fine silt over the valley floor, which builds a flat surface of nutrient-rich alluvium.
Where this leads next
Next, read a hydrograph with time lag to see how a river responds to rainfall. Practise gradient and scale calculations with the map scale, contour and gradient practice, then try the mixed practice set.
Linking landforms into a clear sequence is much quicker with a teacher listening to your explanation, as in one-to-one Geography tuition.