Page 143 - Sedimentology and Stratigraphy

130 Rivers and Alluvial Fans

Fig. 9.1 The geomorphological zones in
alluvial and fluvial systems: in general
braided rivers tend to occur in more
proximal areas and meandering rivers
occur further downstream.

Water flow in rivers and streams is normally con- capacity for storing water in the soil and as ground-
fined to channels, which are depressions or scours in water than a large catchment area. The second factor
the land surface that contain the flow. The overbank is the climate: catchment areas in temperate or tropi-
area or floodplain is the area of land between or cal regions where there is regular rainfall remain wet
beyond the channels that (apart from rain) receives throughout the year and keep the river supplied with
water only when the river is in flood. Together the water.
channel and overbank settings comprise the fluvial A large river system with a catchment area that
environment. Alluvial is a more general term for experiences year-round rainfall is constantly supplied
land surface processes that involve the flow of water. with water and the discharge (the volume of water
It includes features such as a water-lain fan of detritus flowing in a river in a time period) shows only a
(an alluvial fan – 9.5) that are not necessarily related moderate variation through the year. These are called
to rivers. An alluvial plain is a general term for a perennial fluvial systems. In contrast, rivers that
low-relief continental area where sediment is accu- have much smaller drainage areas and/or seasonal
mulating, which may include the floodplains of indi- rainfall may have highly variable discharge. If the
vidual rivers. rivers are dry for long periods of time and only experi-
ence flow after there has been sufficient rain in the
catchment area they are considered to be ephemeral
9.1.1 Catchment and discharge rivers.

The area of ground that supplies water to a river
system is the catchment area (sometimes also 9.1.2 Flow in channels
referred to as the drainage basin). Rivers and
streams are mainly fed by surface run-off and ground- The main characteristic of a fluvial system is that
water from subsurface aquifers in the catchment area most of the time the flow is concentrated within
following periods of rain. Soils act as a sponge soaking channels. When the water level is well below the
up moisture and gradually releasing it out into the level of the channel banks it is at low flow stage.A
streams. A continuous supply of water can be pro- river with water flowing close to or at the level of the
vided if rainfall is frequent enough to stop the soils bank is at high flow stage or bank-full flow.At
drying out. Two factors are important in controlling times when the volume of water being supplied to a
the supply of water to a river system. First, the size of particular section of the river exceeds the volume that
the catchment area: a small area has a more limited can be contained within the channel, the river floods