• •
The radius of curvature of the bend is proportional to 2–3 times that of the channel width (rc ≈ 2 – 3 W). Meander amplitude is 5–7 times the channel width (MA ≈ 5 – 7 W).
Meanders over time Meanders constantly change and evolve. Whilst these changes may be relatively gradual, the curvature of a meander grows with time. As continued erosion occurs the river cliff will migrate back as deposition on the inside becomes more stabilised, leading to movement of the river across the landscape. Meander bends become more pronounced so that the path of the river no longer becomes the most efficient route. The river may continue to erode the outside of the bend before eroding a shortcut between meander bends, causing a temporary straightening of the channel. Where this occurs a bend may eventually become redundant. Isolated bends will become detached creating a feature known as an oxbow lake or cutoff, which, due to its lack of fluvial input, will dry up. Evidence of past meanders may be visible on the landscape as meander scars. A tributary that runs parallel to a river within the same valley for some distance before eventually joining it is known as a yazoo tributary.
meander scars
slip-off slope
fastest current
bank will eventually collapse
slowest current
deposition on the inside of the bend
lateral erosion moves the meander sideways
figure 1.23 Cross section of a meander showing its asymmetric shape.
meandering, graded stream meander scars oxbow lake
yazoo tributary cutoff point bar
bluffs
alluvial deposits natural levees backswamp figure 1.24 The middle course of a river highlighting the life cycle of a meander and oxbow lakes.
Rejuvination and sea level change The lowest point of a river’s course is known as its base level. In most cases this is the sea but on a localised scale it may be a pond, lake or reservoir. The river is constantly trying to produce the most efficient route to its base level whilst continually being influenced by the energy balance and outside factors. Changes in base level affect the energy balance and a river’s ability to erode. Over our history there have been many changes to our sea levels. During the last interglacial, 125 000 years ago, sea level was approximately 4 metres higher (eustatic rise) than the present day due to thermal expansion and ice melt. During the last ice age, 18 000 to 10 000 years ago, sea level was much lower (eustatic fall) due to thermal contraction and as water was trapped as ice on the land. Sea levels reduced by up to 120 metres on the west coast of England, which encouraged deep vertical erosion. As a result many parts of Britain have very deep estuaries known as rias that were scoured out when the sea level was much lower, such as at Dartmouth in Devon.
figure 1.25 Dartmouth Ria. A ria is a drowned river valley formed in glacial periods with characteristic deep channels.
Hydrology and fluvial geomorphology 27