Erosional and mixed river landforms

Overview

Pools and riffles, cross-section of a meander bend and formation of an oxbow lake.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Under what conditions does a waterfall form?

   Model answer

   A waterfall forms where there is a sudden vertical drop in the long profile of the river. The commonest cause is a band of resistant rock lying over weaker rock, so the weaker rock downstream is worn down faster and a step develops. Waterfalls also form where a river crosses a fault line that has displaced the rocks, where a hanging valley left by a glacier joins a deeper main valley, and where a fall in sea level rejuvenates the river and creates a knickpoint.

2. 2

   Explain the sequence by which a waterfall retreats upstream and leaves a gorge.

   Model answer

   The river crosses a band of resistant rock overlying weaker rock, and erodes the weaker rock downstream more rapidly, so a step forms and the water begins to plunge over it. At the foot of the fall, hydraulic action and the abrasion of swirling rocks scour out a deep plunge pool. Splashback and turbulence undercut the weaker rock behind and beneath the fall, so the resistant cap rock is left projecting as an unsupported overhang. Eventually the overhang cannot support its own weight and collapses into the plunge pool, where the fallen blocks become tools for further abrasion. The waterfall is now positioned slightly further upstream, and repetition of this cycle over thousands of years makes it retreat progressively headwards, leaving behind a steep-sided, narrow gorge of recession marking the line of its former positions. Niagara Falls is a well-known example of this.

3. 3

   How do rapids differ from a waterfall, and how do they form?

   Model answer

   Rapids are a stretch of turbulent, broken, fast-flowing shallow water over a steepened section of channel, rather than a single vertical drop. They form where bands of harder and softer rock outcrop in the bed at a gentle angle or run roughly along the line of flow, so differential erosion produces a series of small irregular steps instead of one clean lip; where the gradient of the long profile steepens suddenly; or where large resistant boulders litter a shallow bed. Rapids may be an early stage in the development of a waterfall, or the worn-down remnant of one that has retreated and degraded.

4. 4

   What are pools and riffles, and how are they spaced?

   Model answer

   Pools and riffles are the alternating deeper and shallower sections found along even a fairly straight reach of a river. A pool is a deeper, scoured hollow with finer material on its bed, where the channel is more efficient at high flow. A riffle is a shallower section where coarser gravel has accumulated as a bar, creating friction and turbulent, broken water at low flow. They alternate regularly, with the spacing between successive riffles typically about five to six times the width of the channel.

5. 5

   How do pools and riffles begin the formation of meanders?

   Model answer

   Turbulent flow in the channel causes the coarsest bedload to be deposited unevenly, building riffles and scouring pools between them. As water passes over a riffle it becomes shallow and turbulent, and the greater friction deflects the main current, the thalweg, to one side of the channel. It then swings across to the other side at the next riffle. This alternating side-to-side flow concentrates energy against alternate banks, starting erosion there, and once the flow is curving, corkscrew helicoidal flow develops and the sinuosity is reinforced. What began as a straight channel with an uneven bed therefore becomes a meandering one.

6. 6

   Why is the water on the outside of a meander bend faster than the water on the inside?

   Model answer

   On the outer bend the channel is deeper and the flow is concentrated there, giving the water a greater hydraulic radius, so a smaller proportion of it is in contact with the bed and banks and less energy is lost to friction. The momentum of the water also carries it towards the outer bank as it turns. On the inner bend the water is shallow and spread over a wide, gently shelving bed, so friction is proportionally much greater and velocity is reduced.

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