Geology Reference
In-Depth Information
FLUVIAL ENVIRONMENTS
FLUVIAL EROSIONAL LANDFORMS
Running water dominates fluvial environments, which
are widespread except in frigid regions, where ice dom-
inates, and in dry regions, where wind tends to be
the main erosive agent. However, in arid and semi-arid
areas, fluvial activity can be instrumental in fashion-
ing landforms. Flash floods build alluvial fans and
run out on to desert floors. In the past, rivers once
flowed across many areas that today lack permanent
watercourses.
Water runs over hillslopes as overland flow and rushes
down gullies and river channels as streamflow. The
primary determinant of overland flow and streamflow
is runoff production.
Runoff
is a component of the
land-surface water balance. In brief, runoff is the dif-
ference between precipitation and evaporation rates,
assuming that soil water storage stays roughly con-
stant. In broad terms, fluvial environments dominate
where, over a year, precipitation exceeds evaporation
and the temperature regime does not favour persis-
tent ice formation. Those conditions cover a sizeable
portion of the land surface. The lowest annual runoff
rates, less than 5 cm, are found in deserts. Humid
climatic regions and mountains generate the most runoff,
upwards of 100 cm in places, and have the highest river
discharges.
Runoff is not produced evenly throughout the year.
Seasonal changes in precipitation and evaporation gen-
erate systematic patterns of runoff that are echoed
in
streamflow
. Streamflow tends to be highest dur-
ing wet seasons and lowest during dry seasons. The
changes of streamflow through a year define a river
regime. Each climatic type fosters a distinct river regime.
In monsoon climates, for example, river discharge swings
from high to low with the shift from the wet sea-
son to the dry season. Humid climates tend to sus-
tain a year-round flow of water in
perennial streams
.
Some climates do not sustain a year-round river dis-
charge.
Intermittent streams
flow for at least one
month a year when runoff is produced.
Ephemeral
streams
, which are common in arid environments,
flow after occasional storms but are dry the rest of
the time.
The action of flowing water cuts rills, gullies, and river
channels into the land surface.
Rills and gullies
Rills
are tiny hillside channels a few centimetres wide
and deep that are cut by ephemeral rivulets. They grade
into gullies. An arbitrary upper limit for rills is less than
a third of a metre wide and two-thirds of a metre deep.
Any fluvial hillside channel larger than that is a gully.
Gullies
are intermediate between rills and arroyos, which
are larger incised stream beds. They tend to be deep
and long and narrow, and continuous or discontinu-
ous. They are not as long as valleys but are too deep
to be crossed by wheeled vehicles or to be 'ironed out' by
ploughing. They often start at a head-scarp or waterfall.
Gullies bear many local names, including dongas,
vocaro-
cas
, ramps, and
lavakas
. Much current gullying appears to
result from human modification of the land surface lead-
ing to disequilibrium in the hillslope system.
Arroyos
,
which are also called
wadis
,
washes
,
dry washes
, and
coulees
, are ephemeral stream channels in arid and semi-
arid regions. They often have steep or vertical walls and
flat, sandy floors. Flash floods course down normally dry
arroyos during seasonal or irregular rainstorms, causing
considerable erosion, transport, and deposition.
Bedrock channels
River channels may cut into rock and sediment. It is
common to distinguish alluvial and bedrock channels,
but many river channels form in a combination of allu-
vium and bedrock. Bedrock may alternate with thick
alluvial fills, or bedrock may lie below a thin veneer
of alluvium. The three chief types of river channel are
bedrock channels, alluvial channels, and semi-controlled
or channelized channels.
Bedrock channels
are eroded into rock. They are
resistant to erosion and tend to persist for long peri-
ods. They may move laterally in rock that is less resistant
to erosion. Most rivers cut into bedrock in their upper
reaches, where gradients are steep and their loads coarser.
