River Processes and Landforms

How River Landforms Are Created: The Upper Course

In the upper course, the river is high above sea level with a steep gradient. Energy is primarily used for vertical erosion, creating distinctive features.

V-shaped valleys form through a combination of vertical erosion and weathering. The river erodes the bed downwards using its bedload (abrasion). Simultaneously, weathering (like freeze-thaw) and mass movement weaken the valley sides. This loose material eventually collapses or is washed into the river, leaving a steep-sided V-shape.

As the river winds through the landscape, it encounters obstacles of more resistant rock. Because the river lacks the energy for lateral erosion in the upper course, it cannot erode through these obstacles and is forced to flow around them. This creates interlocking spurs, which are "fingers" of land that project from alternate sides of the valley.

Waterfalls form via differential erosion where a horizontal band of resistant hard rock (such as Whinstone) overlays a band of softer rock (such as limestone). As water flows over the drop, hydraulic action and abrasion erode the softer rock much faster, undercutting the hard rock to form a notch.

The eroded soft rock is transported downstream via saltation and suspension. Meanwhile, the resistant hard rock is left protruding as an unsupported overhang, which eventually collapses under its own weight into the riverbed. The fallen rocks act as new "tools" for abrasion, swirling in the turbulent water to carve out a deep plunge pool. Over time, this cycle of undercutting and collapse repeats, causing the waterfall to retreat upstream and leave a steep-sided gorge behind it.

How River Landforms Are Created: The Middle and Lower Course

As the gradient flattens, the river gains more volume from tributaries and focuses its energy on lateral erosion and deposition.

Within a meander (a winding curve), the line of fastest flow, known as the thalweg, swings towards the outside bend. This high velocity leads to intense lateral erosion through hydraulic action and abrasion, carving out a steep river cliff. On the inside bend, velocity is lower and friction is higher, forcing the river to deposit sediment, creating a gently sloping slip-off slope.

Over time, meanders can evolve into an oxbow lake:

1. Continuous lateral erosion on the outside bends narrows the "swan neck" of land between two loops.
2. During a period of high discharge (flooding), the river breaches this narrow neck to take a straighter, more efficient route.
3. The river deposits heavy sediment at the ends of the old loop where velocity is now low.
4. This deposition eventually seals off the loop entirely.

Floodplains are the wide, flat areas of land on either side of the river. They are formed by meander migration, where lateral erosion widens the valley floor over time. When a river floods and overflows its banks, it spreads across the floodplain. The sudden increase in friction causes the river to lose energy and deposit fine silt and clay, known as alluvium, which builds up the height of the plain.

During these flood events, levees (natural raised banks) are formed along the edges of the channel. As the river overflows, its velocity drops immediately. It deposits its heaviest, coarsest sediment first at the channel's edge. Finer sediment is carried further onto the floodplain. After many floods, these sequential deposits build up to form prominent ridges.

Step-by-Step Guide: Identifying River Landforms on OS Maps

To recognise river features on 1:25,000 or 1:50,000 maps, follow this identification checklist:

Step 1: Identifying Upper Course Features

• V-shaped valleys: Locate closely packed contour lines forming a "V" shape. The apex (point) of the "V" points upstream.
• Interlocking spurs: Look for "V" shaped contours that point downslope from alternating sides of the river.
• Waterfalls: Search for the label "Waterfall" or "Wf". The contour lines here will be extremely close together, sometimes accompanied by a black cliff symbol.

Step 2: Identifying Middle Course Features

• Meanders: Identify these by a sinuous, winding blue line. Surrounding land will show widely spaced contours. Look for yellow or blue stippled dots on the inside of the bends, representing deposition on a slip-off slope.

Step 3: Identifying Lower Course Features

• Oxbow lakes: Look for crescent-shaped blue polygons detached from the main river channel.
• Floodplains: Large, white areas on either side of the river with an almost complete absence of contour lines.
• Levees: Identify these by the label "Embankment", a dashed line running parallel to the bank, or small hachured lines.

Calculating River Gradient on Maps

Gradient is calculated by dividing the change in height by the horizontal distance travelled.

A geography student is studying an OS map. A river drops from a spot height of $180$ m down to a contour line of $20$ m over a horizontal distance of $4,000$ m. Calculate the gradient.

Step 1: Identify the values.

• Vertical interval = $180 - 20 = 160$ m
• Horizontal distance = $4,000$ m

Step 2: Substitute into the gradient equation.

• $\text{Gradient} = \frac{\text{Vertical Interval}}{\text{Horizontal Distance}}$

• $\text{Gradient} = \frac{160}{4,000} = 0.04$

Step 3: Simplify the ratio.

• $0.04$ is the same as $1/25$. The ratio is $1:25$ (a $1$ m drop for every $25$ m horizontal distance).