Which of the following statements regarding river meanders is/are correct? 1. It is a U-shaped bend in a river. 2. An extremely tight meander is called a gooseneck; it is likely to become a cutoff, or oxbow lake, after a flood. 3. Tectonic uplift can cause a river to continue downcutting along its meandering course, producing incised or entrenched meanders. Select the correct answer using the code given below.
- (a)1 only
- (b)1 and 2 only
- (c)2 and 3 only
- (d)1, 2 and 3
Correct — D, 1, 2 and 3. All three statements hold. A meander is the looping, U-shaped bend a river cuts as it swings across its valley floor, eroding the outer bank where the current is fast and depositing on the inner bank where it is slow. A very tight loop leaves only a narrow neck of land, and geographers call such a loop a gooseneck; during a big flood the river can cut straight across the neck, abandoning the loop, which is then sealed off as a crescent-shaped oxbow lake. And where the land beneath a meandering river is pushed up by tectonic uplift, or where sea level falls, the river keeps its bends but starts cutting downwards, sinking the loops into rock as incised or entrenched meanders. Nothing here is false, so the answer is all three.
- (a)1 only — Statements 2 and 3 are both standard textbook geomorphology — the gooseneck-to-oxbow sequence and the incised meanders of an uplifted terrain.
- (b)1 and 2 only — Drops statement 3, but incised or entrenched meanders formed by downcutting after uplift are a well-established landform, seen for example in the goosenecks of the San Juan River in Utah.
- (c)2 and 3 only — Drops statement 1, but the U-shaped or looping bend is the very definition of a meander.
A meander is one of a series of sinuous bends in a river's course. The current runs fastest on the outside of a bend, so the outer bank is undercut while sediment is laid down as a point bar on the inside. Bends therefore migrate sideways and grow tighter over time, until a flood breaches the neck and the loop is cut off as an oxbow lake. If instead the land rises or base level falls, the river cuts down while keeping its bends, and the meanders become incised into the bedrock.
A three-statement item where none of the statements is planted false is unusual, and it rewards a student who can picture the whole life cycle of a bend rather than one stage of it. The trap is the word gooseneck in statement 2, which reads like an invented term to a candidate who has not met it; it is genuine, and the goosenecks of the San Juan River in Utah are the classic photograph of the landform. Statement 3 is the other place candidates hesitate, because meanders are associated with flat floodplains — but uplift lets a river keep its meandering plan while it cuts down, which is exactly how entrenched meanders form.
- Erosion on the outer bank of a bend and deposition on the inner bank make meanders migrate sideways and grow tighter.
- A neck cutoff usually happens during a major flood, when the river is out of its banks and can take the shorter route across the neck.
- The abandoned loop, sealed off from the channel, becomes a crescent-shaped oxbow lake.
- Incised meanders form when base level falls, whether from a change in sea level or from isostatic or tectonic uplift, so the river cuts downward along its existing meandering course.
- A gentle bend forms; the current runs fastest along the outer bank
- The outer bank is eroded, sediment builds up on the inner bank, and the bend grows
- The loop tightens into a gooseneck, leaving only a narrow neck of land
- A flood cuts through the neck; the river takes the short route
- The abandoned loop is sealed off as a crescent-shaped oxbow lake
Statement 2 of the question describes stages three to five of this sequence; statement 3 describes the different path a meandering river takes when the land beneath it is uplifted.
- Rejecting statement 2 because gooseneck sounds like an invented term — it is a real name for an extremely tight meander loop.
- Assuming meanders belong only to flat plains, and so rejecting the incised meanders of statement 3.
- Confusing an oxbow lake, which is a cut-off river loop, with a lagoon or a playa.
Asked as a statement-verification item that runs through the whole life of a meander — definition, cutoff to oxbow, and incision after uplift.
Which one of the following features is the result of erosion and deposition work of a river?
- (a) Pothole
- (b) Oxbow lake
- (c) Levee
- (d) Rapid
Answer(b) Oxbow lake
Same landform, tested on the process instead of the sequence. An oxbow lake is the end point of statement 2 of this CDS item, and that NDA item makes the reason explicit — it takes both erosion on the outer bank and deposition on the inner bank to produce it, unlike a pothole or rapid (erosion alone) or a levee (deposition alone).
- practice — not a real PYQ
An oxbow lake is formed when
- (a)a glacier scours a hollow in bedrock
- (b)a river cuts across the neck of a tight meander and abandons the loop
- (c)a coastal bar seals off a bay
- (d)a landslide dams a valley
Answer(b) a river cuts across the neck of a tight meander and abandons the loop — the sealed-off loop becomes a crescent-shaped lake.
- practice — not a real PYQ
Incised or entrenched meanders indicate that the region has most likely experienced
- (a)subsidence of the land or a rise in base level
- (b)uplift of the land or a fall in base level
- (c)an increase in the river's sediment load only
- (d)a shift from a humid to an arid climate only
Answer(b) uplift of the land or a fall in base level — the river keeps its meandering plan but begins to cut downward into rock.