Page 238 - Fundamentals of Geomorphology

FLUVIAL LANDSCAPES 221

FLUVIAL ENVIRONMENTS FLUVIAL EROSIONAL LANDFORMS

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