In the course of a river, in time, which one of the following features is formed when the old meander is closed off by deposition?

Correct — C, Oxbow lake. In its middle and lower course a river swings in loops called meanders, eroding the outer bank of each bend and depositing on the inner one. Over time the loop grows until the neck between the two ends of the bend is very narrow, and in a flood the river cuts straight across it. The abandoned loop is then sealed off at both ends by deposition, and the crescent of still water left behind is an oxbow lake — called a mortlake or cut-off lake in some texts, and a bayou along the lower Mississippi.

• (a)Pothole — A pothole is a cylindrical hollow drilled into the rocky bed of a river by pebbles swirling in an eddy. It belongs to the upper course and is purely erosional, and it has nothing to do with meanders.
• (b)Levee — A levee is the natural embankment of coarse sediment built along a river's banks when it overtops them in flood. It is depositional, but it lies alongside the channel rather than closing off a bend.
• (d)Flood plain — A flood plain is the broad flat area on either side of the channel built up by repeated flood deposition. The oxbow lake sits on the flood plain, but the flood plain is not itself the cut-off meander.

A river's work changes along its course. The upper course is dominated by vertical erosion, producing V-shaped valleys, gorges, waterfalls and potholes. The middle course is where lateral erosion and meandering take over, giving river cliffs, slip-off slopes and eventually cut-offs. The lower course is dominated by deposition, producing flood plains, levees, braided channels and deltas. The oxbow lake is the classic feature of the transition, since making it needs erosion to cut the neck and deposition to seal the loop.

All four options are genuine fluvial landforms, so the discriminator is the process named in the stem — deposition closing off an old meander. Sort the options by process and by course and only one survives: the pothole is erosional and upstream, and the levee and flood plain are depositional but are built beside the channel rather than across the mouth of a bend. The Ganga plain carries oxbow lakes in large numbers, and the older beds of the Kosi in Bihar are a well-known example of channels left behind as the river shifted.

• A meander is a loop in a river's course, eroded on the outer bank and deposited on the inner.
• When the narrow neck of a meander is cut through in flood and the ends silt up, an oxbow lake is left.
• Potholes are erosional hollows drilled in the bed of the upper course.
• Levees are natural embankments of coarse sediment built along the banks during floods.
• The flood plain is the depositional surface on which oxbow lakes and levees both sit.

• Choosing the levee because it is also depositional, without noticing that it lies along the bank.
• Placing the pothole in the middle course; it belongs to the rocky bed of the upper course.
• Treating the flood plain and the oxbow lake as the same feature because they occupy the same area.

A landform-identification item where every option is a real river feature; NDA has asked it with the identical option set, which makes the pairing worth rehearsing directly.

• practice — not a real PYQ

  A cylindrical hollow drilled into the rocky bed of a river by swirling pebbles is called

  • (a)a pothole
  • (b)an oxbow lake
  • (c)a levee
  • (d)a delta

  Answer(a) a pothole — an erosional feature of the upper course.

• practice — not a real PYQ

  Natural embankments of coarse sediment built along a river's banks during floods are

  • (a)meanders
  • (b)levees
  • (c)terraces
  • (d)distributaries

  Answer(b) levees — deposited when the river overtops its banks and loses velocity.