Upper and Middle Course River Landforms

The main landforms of a river’s upper and middle courses are V-shaped valleys with interlocking spurs, waterfalls and gorges, formed mainly by vertical erosion in the steep upper course, and meanders and oxbow lakes, formed by lateral erosion and deposition in the gentler middle course. In brief: a waterfall forms where a river flows over a band of hard rock lying over softer rock; the soft rock is eroded, undercutting the hard rock, which collapses, so the waterfall retreats upstream and leaves a gorge. A meander is a bend in the river with erosion on the outside and deposition on the inside; when the neck of a meander is cut through, the abandoned loop becomes an oxbow lake.

These features are among the most recognizable in physical geography and appear regularly in studies of rivers. This guide describes how each landform forms, compares them in a table, works through calculations of waterfall retreat and meander shape, outlines the history of their study and corrects common mistakes.

What landforms occur in the upper course?

In the upper course, the river has a steep gradient and cuts downward (vertical erosion), especially in times of flood, when it can carry large stones that grind the bed. Weathering and mass movement on the valley sides supply material, which the river removes. The result is a V-shaped valley: steep sides, a narrow floor filled by the channel. Because the river is not strong enough to erode the hillsides it winds around ridges of harder rock, creating interlocking spurs that overlap like the teeth of a zip when seen from above. Other features of the upper course are potholes in the bed, rapids (shallow, fast sections over hard rock) and the steep, narrow channels of mountain streams.

How do waterfalls and gorges form?

1. A river flows over a layer of resistant (hard) rock overlying a layer of less resistant (soft) rock.
2. The soft rock below is eroded faster by hydraulic action and abrasion, which forms a step.
3. The falling water forms a plunge pool at the base, where eddies of water and rock fragments erode the soft rock and undercut the hard rock above.
4. The overhanging hard rock loses its support and collapses.
5. The process repeats and the waterfall retreats upstream, leaving a steep-sided, deep, narrow valley called a gorge downstream of the fall.

Examples: Niagara Falls (the Niagara River over a layer of hard dolostone above softer shale, with a gorge of about 11 km downstream), Victoria Falls in Africa, and High Force in England (over a hard rock layer called the Whin Sill). Gorges can also be formed by the collapse of caves in limestone or by the melting of glaciers. Angel Falls in Venezuela, at about 979 m, is the highest uninterrupted waterfall.

How do meanders and oxbow lakes form?

As the river reaches the middle course, the gradient decreases and the river starts to swing from side to side in curves called meanders. In a bend, the water is faster and deeper on the outside, where it erodes the bank by hydraulic action and abrasion, creating a steep river cliff. On the inside of the bend the water is slow and shallow and deposits sand and gravel, forming a gently sloping point bar (slip-off slope). A secondary spiral flow (helicoidal flow) carries eroded material from the outside across the bed to the inside. Over time the meander moves sideways and downstream, widening the valley floor. As the outside banks of a meander approach each other, the narrow neck of land between them is eroded, and during a flood the river may break through and take the shorter course. The ends of the old loop are sealed with deposits, leaving a crescent-shaped oxbow lake, which fills slowly with sediment and plants and becomes a marsh (a meander scar).

Landforms compared

Worked example: retreat and sinuosity

1. Waterfall retreat: a gorge 11 km long (11,000 m) has been formed by a waterfall retreating upstream at an average rate of 0.5 m per year. The time is 11,000 ÷ 0.5 = 22,000 years. Real rates vary with the rock and the flow: if the 11 km gorge of Niagara began to form about 12,000 years ago, the average rate would be 11,000 ÷ 12,000 = about 0.9 m a year.
2. Sinuosity: sinuosity is the channel length divided by the straight-line distance. A river section with a channel length of 18 km between two points 12 km apart in a straight line has a sinuosity of 18 ÷ 12 = 1.5. A value of 1.5 or more is usually considered meandering; values near 1.0 mean a straight channel.
3. Meander wavelength: a rule of thumb is that the wavelength of meanders is about 10 to 14 times the width of the channel. For a channel 30 m wide: 10 × 30 = 300 m to 14 × 30 = 420 m.
4. Lateral migration: if the outer bank of a meander moves 2 m a year, then in 50 years it has moved 2 × 50 = 100 m, equal to more than three channel widths, so buildings on the outside of bends are at risk.
5. Neck cutoff: if the neck of a meander loop is 150 m wide, and the two banks approach each other at 2 m a year each (a closing speed of 4 m a year), the neck will be breached in 150 ÷ 4 = about 38 years, unless a flood cuts through earlier.

How were these landforms explained?

The idea that valleys are cut by the rivers that flow in them was argued by James Hutton and John Playfair in the late 18th and early 19th centuries, against the view that valleys were due to catastrophes. The retreat of Niagara Falls was studied in the 1840s by Charles Lyell, who estimated its rate of retreat. Meanders were described by geologists and engineers in the 19th century, and Albert Einstein wrote a paper on the formation of meanders and “Baer’s law” in 1926, pointing out the role of the secondary flow in bends, as in the tea-leaf paradox. Today, aerial photographs, lidar and computer models track meander migration.

Which mistakes are common?

Mistake 1: Thinking waterfalls always stay in the same place

They retreat upstream as the rock below the hard layer is undercut.

Mistake 2: Saying that the river erodes the inside of the bend

Erosion is on the outside of the bend, and deposition is on the inside.

Mistake 3: Believing that V-shaped valleys are formed by glaciers

V-shaped valleys are formed by rivers. Glaciers make U-shaped valleys.

Mistake 4: Describing the oxbow lake as formed by a single event only

The meander neck is narrowed gradually by erosion over many years. The cutoff happens in a flood.