Geoscience Reference
In-Depth Information
of their artefacts, succeeded by widespread early to mid-
Holocene evidence of the first farmers. Since then,
extensive flood plains have become sites of intensive
human settlement, agriculture and industry. In exploiting
the fertile alluvium, sand and gravel resources, ready
access to water and sites suitable for urban, industrial
and communications infrastructure of river corridors
we also contend with the fickle behaviour of stream flow
and channel instability. Human actions manage and
reroute stream flow but often also alter channel dynamics
and sediment flux inadvertently. The relative safety of
higher river terraces contrasts with direct and human-
enhanced flood risks in the contemporary flood plain.
Our catchment occupation and actions require an
understanding of the dynamics of rivers and fluvial
landsystems.
KEY POINTS
1
Earth's land surface is divided into a hierarchical series of drainage basins or catchments which convert
precipitation to river flow. Each catchment contributes surface and subsurface water to a specific stream
or a major trunk river, separated from its neighbours by a watershed. At continental scales, the watershed
probably coincides with prominent morphotectonic features. Catchment topography, geology, vegetation
and land use systems retain, store and transfer water. They introduce delays and losses to onward water
transfer.
2
The fate of precipitation falling on the catchment can be quantified in a water balance equation, with the
volume of precipitation minus evapotranspiration generating river flow, known as the discharge. Discharge
volume and pattern over time are plotted on a hydrograph and reflect the contribution of the component
catchment stores.
3
Gravity-induced overland and subsurface flow is inefficient when diffuse, encountering high resistance
and friction loss. Channel flow is more efficient and is initiated where surface/subsurface flows converge,
initially as intermittent rills and gullies. Efficiency continues to develop downstream as river channels
enlarge with discharge. Channel forms compensate for falling gradients and potential energy. Channels
connect to form a catchment-wide network, with recognizable patterns and drainage densities determined
by catchment hydrometeorology and hydrogeology.
4
Flowing water plays an important role in continental denudation. Erosion occurs through fluid stressing
by water itself and/or by water movement of abrasive tools. Flow competence and sediment entrainment,
transport and deposition are a function of velocity and particle size, summarized by Hjulström's curve.
Stream flow creates distinctive landforms composed of straight, meandering and braided channels,
channel networks and flood plains.
5
Channel segments respond to flow regimes, sediment delivery, slope, etc., and undergo almost continuous
change. At whole-landscape scale, upper catchments are dominated by bedrock channels and deep, narrow
valleys, etc., with sediment pockets. Lower catchments tend to display extensive flood plains of primary
and reworked alluvial sediments. Quaternary sea-level change has driven repeated marine/landward
extensions of the flood plain and Holocene delta construction.
FURTHER READING
Bridge, J. S. (2003) Rivers and Floodplains: forms, processes and sedimentary record, Oxford: Blackwell. A richly
illustrated and comprehensive cover of fluvial processes and landforms, bridging the interface between geomorphology
with useful sections on sedimentology and fluvial stratigraphic records.
