A2.2SL1Surface Processes and LandformsWEEK 3FLUVIAL PROCESSES
Outline • The drainage basin • Processes and channel form • Single channels • Braided channels • Distributary channels and deltas • Changes over time • Channel migration • River terraces
Flowing water is the principal geomorphic agent in the mid latitudes. • It is estimated that the world’s rivers discharge around 30 GT of material annually, about half of which is in solution. • This is equivalent to about 0.25mm of erosion per year, or 250m per million years.
The drainage basin is often regarded as the fundamental unit of landscape. • Within the basin, the drainage pattern is controlled by • geological structure • bedrock type • rainfall intensity • Several simplified geometric patterns have been proposed. Real drainage networks are often a composite of these patterns.
The network is often analysed in terms of stream order (or number). • This describes the hierarchy of streams and is related to the overall length, discharge and hydrograph pattern, amongst other things.
It is also possible to analyse the extent of erosion within a basin by examining the relative area in relation to relative elevation. • This leads to a hypsometric curve and to concepts of erosional stage. • Such concepts are not universally accepted.
This general approach is termed drainage basin morphometry. • It can form a useful tool for the comparison of different areas but can also become rather pointless and sterile!
PROCESSES AND CHANNEL FORM Introduction
Flowing water moves in channels - sheet flow is limited to very shallow flows. • Channel patterns may be: • Single (meandering) • Simply divided • Braided • Distributary
The channel form is controlled by the energy of the flow. • High energy (=steep slopes, high discharge) produces low sinuosity meanders or braids. • Low energy (=shallow slopes, low discharge) produces high sinuosity meanders.
PROCESSES AND CHANNEL FORM Single Channels
Single channels are almost always meandering. Straight channels are inherently unstable on an erodible bed. • The sinuosity of the meanders is controlled by the discharge and the sediment load.
Meanders will develop on any erodible bed. • Thus they are known from areas of solid bedrock, where they are incised into the rock.
The meandering pattern introduces a series alternating curves into the flow, linked by crossovers. • This in turn creates a sequence of deeper pools on the curves and shallower riffles on the crossovers. • The curvature also introduces a secondary cross-flow pattern on the outer bend.
The curvature introduces a characteristic pattern of erosion and deposition • Erosion occurs on the outer curve of the bed, deposition occurs on the inner curve. • This creates a structure termed a point bar, which is the fundamental sedimentary unit that builds the floodplain.
PROCESSES AND CHANNEL FORM Braided Channels
Braided patterns are characteristic of high sediment loads and erodible beds. • These are met by many rivers in arid or cold-climate regions. • The key feature is a lack of vegetation to stabilise the banks and a large supply of sediment (from flash floods or glaciers )
Braids form when the channel divides and reunites in an anastomosing pattern. • Braided rivers are subject to large variations in discharge, often over a short time. • Under most flow conditions, not all the channels are occupied. The ‘extra’ channels are a response to the peak flow.
Markarfljot Iceland Photo: J.W.Merritt
The combination of variable flow, abundant sediment and frequent channel switching produces a characteristic architecture of braid stream sediments. • These include braid bars, pond deposits and occasional mass flows. • These elements may replace one another very rapidly both vertically and laterally.
Classical braided model • Peak flows build gravel bars • Declining flows allow upwards fining, exposure cuts secondary channels in bar surface • Low flows deposit sand units in main channels • Very low flows allows ponding in which mud drapes are deposited.
Breidamerkursandur Iceland Photo: M.A.Paul
On the larger scale, there is a systematic change in architecture along a braided river system. • The upper (proximal) end is dominated by flash flood or mass flow deposits. • The lower (distal) end is dominated by bar or ponded water deposits. • A nominal sequence has been proposed based on a series of type rivers.