River Rejuvenation, Terraces, and Incised Meanders

River rejuvenation is the renewal of a river’s ability to erode downward, caused by a fall in base level or uplift of the land, and it leaves characteristic landforms: knickpoints, river terraces (old floodplains left as steps above the present river) and incised meanders (meanders cut deep into bedrock). In brief: when the base level falls, the river gains energy, cuts down into its valley floor and abandons its old floodplain, which stays as a terrace. If the river already had meanders, it keeps its winding course as it cuts down, forming an entrenched or ingrown meander. Terraces and incised valleys record past changes in sea level, tectonics and climate.

These landforms show that a river’s history can be read in the landscape, and terraces are valuable archives of past climate and archaeology. This guide explains what causes rejuvenation, how terraces and incised meanders form, how to measure incision rates, compares the landforms in a table, works through calculations, outlines the history and corrects common mistakes.

What causes rejuvenation?

A river is rejuvenated when its erosive power relative to the resistance of its bed increases. Causes include:

• Eustatic fall in sea level: a worldwide fall caused by the growth of ice sheets. At the peak of the last ice age, about 20,000 years ago, sea level was roughly 120 m lower than today, so rivers cut down to reach the lower base level.
• Tectonic or isostatic uplift: raising the land relative to sea level steepens the river’s gradient. After the removal of ice sheets, regions such as Scotland and Scandinavia rose by isostatic rebound.
• Increase in discharge or decrease in sediment load: a wetter climate, the capture of another river or a reduction in sediment (for example, after the end of glacial conditions) allows more incision.
• Local changes: removal of a dam, a drop in lake level or the cutting through a resistant rock barrier.

The response begins at the mouth, where the river is forced to cut down, and spreads upstream as a knickpoint, a sudden steepening in the long profile that migrates upstream, leaving a steeper, rejuvenated reach below it and the older, gentler profile above.

How do river terraces form?

When the river cuts down below its old floodplain, the previous floodplain surface is left as a flat, step-like feature on the side of the valley, called a terrace. Repeated episodes of incision produce a staircase of terraces, with the oldest highest and the youngest lowest. The edge of each terrace is a steep scarp.

• Paired terraces: matching terraces at the same height on both sides of the river, formed when the river cuts down without swinging sideways much.
• Unpaired terraces: terraces at different heights on each side, formed when the river migrates laterally as it incises.
• Rock-cut (strath) terraces and fill terraces: strath terraces have a bedrock surface covered by a thin layer of gravel, whereas fill terraces are made of thick deposits of sediment that the river later cut through.
• Climatic terraces: terraces formed by changes between aggradation (filling with sediment during cold, sediment-rich periods) and incision (during warmer periods), not only by base-level change.

The Thames in England has a well-known sequence of terraces that record cold stages and low sea levels of the last 2 million years, and in the Mississippi and Rhine valleys, terraces have been used to date deposits and archaeological sites.

How do incised meanders form?

When a river with meanders on a floodplain is rejuvenated, it cuts down vertically and keeps its winding pattern, so the meanders become incised into bedrock.

• Entrenched (symmetrical) meanders: formed when incision is rapid, so the river cuts down more than it moves sideways, resulting in a deep, steep-sided gorge with a symmetrical valley, as in the Goosenecks of the San Juan River in Utah and Horseshoe Bend on the Colorado River in Arizona.
• Ingrown (asymmetrical) meanders: formed when incision is slower and the river migrates sideways as it cuts down, so one side has a steep cliff (the outer bank) and the other side has a gentler slip-off slope, as in the valley of the Wye in Britain or the Moselle in Germany.

Incised meanders may also become abandoned when a river cuts through the narrow neck and leaves a dry loop with a hill in the middle. The size of meanders is related to the river’s discharge: the meander wavelength is usually about 10 to 14 times the channel width. If the wavelengths of incised meanders are much larger than the present channel suggests, the river was once much bigger.

What other features result?

Other features of a rejuvenated river include knickpoints and waterfalls, gorges, deep V-shaped valleys within broader older valleys, hanging tributaries that have not yet adjusted, and paired valley-side benches. In a landscape with a series of rejuvenation events, the valley cross-section shows a stepped “valley-in-valley” form. The Grand Canyon is a monumental example of incision by the Colorado River into the rising Colorado Plateau, with a maximum depth of about 1.8 km.

Rejuvenation landforms compared

Worked example: incision rates and sinuosity

1. Incision rate from a terrace: a terrace tread is 30 m above the present river and is dated to 120,000 years ago. The average incision rate is 30 ÷ 120,000 = 0.00025 m per year, which is 0.25 mm per year.
2. Several terraces: terraces at 12 m (dated 40,000 years), 30 m (120,000 years) and 55 m (240,000 years). The rates are 12 ÷ 40,000 = 0.30 mm per year, then (30 − 12) ÷ (120,000 − 40,000) = 18 ÷ 80,000 = 0.23 mm per year between the first and second terrace, and (55 − 30) ÷ (240,000 − 120,000) = 25 ÷ 120,000 = 0.21 mm per year between the second and third.
3. Grand Canyon: if the river cut about 1,800 m in about 6 million years, the average rate is 1,800 ÷ 6,000,000 = 0.0003 m per year, or 0.3 mm per year.
4. Sinuosity: sinuosity is the channel length divided by the straight-line valley length. A reach is 15 km along the channel and 10 km in a straight line, so the sinuosity is 15 ÷ 10 = 1.5. Values above 1.5 are typically called meandering.
5. Meander wavelength: a river with a channel width of 40 m is expected to have a meander wavelength of about 11 × 40 = 440 m. If the incised meanders in the valley have a wavelength of 4,400 m, the old river was about 10 times as wide, so its discharge was much larger.
6. Sea-level fall and incision: a river cuts down 120 m over the 100,000-year glacial cycle, with a net rate of 120 ÷ 100,000 = 1.2 mm per year, but most of the cutting occurs in a short period.

How were these landforms interpreted?

William Morris Davis considered rejuvenation a part of his cycle of erosion in the late 19th century, when uplift restarts the cycle. Geologists in the 19th century recognized river terraces in Europe and linked them to glaciations. Later, in the 20th century, scientists such as Luna Leopold and Gordon Wolman studied the relationship between meanders and discharge. Since the 1980s, methods such as optically stimulated luminescence, cosmogenic nuclides and radiocarbon dating have made it possible to date terrace deposits and to calculate incision rates, and show that many river terraces reflect climate cycles of 100,000 and 41,000 years. Studies of incision in the Grand Canyon, the Himalaya and the Alps use these methods to link river response to tectonics and climate.

Which mistakes are common?

Mistake 1: Thinking that terraces are always caused by a fall in sea level

Climate change, uplift, changes in sediment supply and local factors can also cause terraces, especially far from the coast.

Mistake 2: Confusing entrenched and ingrown meanders

Entrenched meanders are symmetrical, with steep walls on both sides. Ingrown meanders are asymmetrical, with a cliff on one side and a gentler slope on the other.

Mistake 3: Believing that incised meanders formed in their current shape in the gorge

They were inherited from a meandering river on a floodplain at a higher level, and then cut down into the rock.

Mistake 4: Assuming that the highest terrace is always the oldest at every site

This is usually true, but local tilting, tectonics or buried sequences can complicate the order, so dating is needed.