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Water in DesertsNumber 366

The action of flowing water, both in the present and past, is, and has been, important in shaping desert landscapes. Rainfall quantities in deserts are low overall but rainfall events that do occur can have a marked influence on the landscape. Many landforms in deserts are shaped by the action of rivers and the water that flows into and / or through the region. Figure 1 shows the global distribution of deserts.

Rainfall in desertsDesert regions can be classed as hyper-arid, arid or semi-arid depending on the average annual precipitation that they receive: Hyper-arid zones have a mean annual precipitation value of less than 100mm;Arid zones have an annual rainfall total of less than 250 mm;Semi-arid areas have an average annual precipitation of 250-500 mm.

Runoff in deserts Rainfall in deserts can have a significant impact on the landscape in a relatively short period of time because large amounts of runoff are generated in desert environments after episodes of rainfall. This is due to: ● Low levels of vegetation cover, which means that

absorption of the rainfall by vegetation is limited and theground saturates more rapidly, reducing the potential forinfiltration and increasing overland flow.

● The sparsity of vegetation cover means that there isan absence of plant roots. Humus in the soils is limited;existing soils are compact and infiltration is limited.

● Interception is minimal. The rainfall hits the groundand dislodges fine, loose particles, moving them by rainsplash. They can resettle and become lodged in any porespaces that did exist in the soil, acting to ‘plug’ the gapsand create a top soil layer which has reduced permeability.Such soils can only allow rainwater to infiltrate at a rateof around a few mm per hour, so any excess rainfall willbuild up on, and flow over, the ground surface.

Figure 1 The global distribution of deserts

Figure 2 How the hydrological cycle operates in deserts

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Much of the episodic surging river discharge experienced in hot deserts is therefore derived from infiltration-excess overland flow, which is when precipitation exceeds the infiltration capacity of the land and so flows over the surface. Overland flow may spread out and travel across the desert surface as a sheet flood as it moves towards river channels, or may be carried by relatively small ‘feeder’ channels called rills and gullies. Desert rivers, when in flow, carry out processes which play a significant part in shaping the desert landscape; eroding, transporting and depositing material.

Fluvial ErosionA flowing desert river may erode its channel through hydraulic action when the pressure and sheer force of the flowing water contacting the bed and banks of the river channel removes particles; abrasion when the large load carried by the river rubs and scrapes away material from the bed and banks, and solution (corrosion) when chemical processes dissolve the rock and remove small particles.

Fluvial TransportDue to the lack of vegetation in hot, arid environments, there is little to consolidate the ground, meaning that a large amount of loose weathered material is available. The fast-flowing, ephemeral rivers therefore pick up and transport much of this loose material – desert rivers are capable of carrying very large loads. The river’s load is transported either along the bed by traction or saltation, is moved as a suspended load or dissolved in solution.

Fluvial DepositionThe ephemeral nature of many desert rivers means that they have a discontinuous and fragmented flow. The water that feeds the rivers comes in short bursts during storm events and therefore river flow will increase and cease periodically. Velocities will fall as discharge levels decline after storm events, with water being lost from the channel through evaporation, infiltration, and percolation. This means that the river loses the ability to transport its load and will deposit it readily.

Rivers and drainage in hot desertsDesert rivers can be considered in different ways: ● Perennial rivers have a continuous flow all year round

in years providing that what is considered to be ‘normal’ (or in excess of normal) rainfall occurs for a given place.

● Perennial rivers that flow continuously are uncommonin arid zones due to the unreliable and infrequent natureof precipitation. Therefore any perennial desert riverstend to be exogenous, meaning that the river originatesfrom outside the desert. Examples of large perennialrivers are the Blue Nile (a major tributary of the Nile)which originates in the Ethiopian Highlands outside ofthe Sahara Desert region through which the Nile flows,and the Colorado River which originates in the RockyMountains outside of the Great Basin, Sonoran andMojave Deserts through which the river flows in theAmerican southwest.

● Rivers in the arid tropics are more likely to be ephemeralwhich means that they only flow occasionally (during andfollowing episodes of relatively heavy rainfall). The flowof such rivers is unpredictable, irregular and may not lastfor long, but when rivers do flow, they exhibit high levelsof discharge, meaning they possess a high level of erosivepower and the ability to entrain and transport large loads.

● Perennial or ephemeral rivers in the arid tropics may beendorheic, meaning that they have a centripetal inwardflowing pattern of drainage, with rivers flowing towardslarge inland lakes, basins, or salt pans. Examples of suchdrainage patterns include those where channels flow intoLake Chad and Chott el Djerid (Tunisia); both in theSahara region and the Great Salt Lake in Utah, USA (seesection on playas).

● In the great sand seas or ergs such as the Grand ErgOccidental, which is located in a hyper-arid zone inAlgeria (Sahara), drainage patterns are absent, meaningthat surface impressions left by flowing rivers do not exist.Water will flow in these regions, albeit rarely, but barelyleaves a mark due to the high porosity of the sandy material,through which water infiltrates, and is ‘lost’ underground.

Figure 3 Landforms created by fluvial erosion & deposition in deserts

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Fluvial landforms in hot desertsAlthough there is relatively little water in hot deserts, many of the erosional, and some of the depositional, landforms found there have been formed by the action of flowing water. Flowing water in deserts can act as an agent of erosion, transportation, and deposition of material and the landforms that these processes create are either erosional or depositional in nature. The erosional landforms created by flowing water in hot deserts include rills and gullies (sometimes creating badlands), wadis (also known as arroyos), canyons, mesas and buttes (which may be classed as inselbergs) and pediments; the depositional features include alluvial fans, bahadas and playas.

Desert landforms formed by water erosion1. Rills and gulliesSome of the infiltration excess overland flow, a common occurrence due to the nature of the climate and land in the arid tropics, may form features called rills and gullies. These small channels, carved out by the action of water running over the desert surface, can be found embroidering desert hillslopes. Rills are steep-sided, narrow watercourses which carry water during times of high precipitation. Rills are small features which can be 5-30 cm deep and 22-100 cm wide. Rills may be just temporary features, and can be destroyed during dry periods. Rills enlarge through a coalescent process called cross-grading. They are considered to be an intermediate stage between water being transported as merely overland flow and the development of a more considerably-sized gulley. Gullies are larger channels that are a more permanent feature on the landscape than rills (yet the flow of water within them is still ephemeral). Gullies are narrow yet relatively deep (between 0.5 and 30 metres), with steep sidewalls. They form when there is a sufficient gap in vegetation cover and where there is a small surface depression encouraging water to accumulate. This incises the land and a gully head forms (the start point of a gulley). Erosion continues, more water accumulates and the process continues through positive feedback to form a more permanent channel. Rill and gulley processes provide a starting point for the generation of channels carrying out fluvial processes, acting as tributary networks, feeding wadis and canyons (see below) during episodes of high precipitation. Rills and gullies may embroider soft, relatively impermeable rock formations with large numbers of small, ephemeral channels, usually in semi-arid environments, where rainfall is more plentiful, to form striking landscapes described as ‘badlands’. An example is the badlands topography in Death Valley, California in the southwest of the USA (Figure 4).

2. WadisWadis (also known as arroyos), such as Wadi Rum which is situated in Jordan, and Wadi Bani Khalid in Oman, both within the Arabian Desert, are dry river valleys which experience infrequent flows of water. They are deep, steep-sided ravines which vary in size from being just a few metres to several hundreds of kilometres long. Wadis are dry for most of the time but are filled with water derived from overland flow, which may arrive through rills and gullies, during irregular and infrequent flash flood events. When floodwaters subside due to evaporation and infiltration, thick layers of poorly sorted sediment are deposited, creating a flattened-out channel bed. The flow of water within the wadi often takes place in narrow, criss-crossing braided channels as the water navigates the wadi-floor deposits. In semi-arid desert fringes, there is an average of one flood a year, but in the more arid zones, wadis may go for ten years without carrying water. Due to the paucity of and infrequency of water entering the dry valley, it is suggested that wadis are relict features which could only have been formed at a time in which the climate of the place in which they are situated was wetter (see ‘pluvials’ below), with greater and more frequent rainfall events.

3. CanyonsCanyons, such as the Grand Canyon in the southwest of the USA (see Figure 5) are steep-sided, deeper versions of wadis. They are created by the incision (vertical/downward erosion) by the action of flowing water in deserts. Their often quite significant depth cannot be created by intermittent incision by ephemeral desert rivers and therefore it may be that they have been created by a perennial exogenous river, flowing into the arid region from elsewhere, or when the ephemeral yet powerful water flow is combined with tectonic uplift of land. This uplift raises the flowing river to a point where it is higher above base level (the lowest point to which a river can erode its valley), giving it the potential to incise down into the rock. It is uplift which helps to create the most pronounced canyons.

4. PedimentsPediments are gently sloping surfaces of between 1° and 7° which are located at the footslopes of receding desert mountain ranges. They may be totally exposed or partly / completely covered by debris or an alluvial fan and / or intersected by dry drainage channels, such as rills, gullies and wadis. Pediments may cover a distance of many miles and may join together to form the vast erosional plains known as pediplains. Isolated rock outcrops, such mesas and buttes, may arise from the pediplain. The exact mode of pediment formation is unknown, but it has been suggested that water plays a significant part in their development.

Figure 4 Badland topography in Death Valley, California Figure 5 The Grand Canyon

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Different theories exist on as to how pediments form: ● A first hypothesis suggests that they could have

been formed by sheet floods, which would haveplaned down the rock surface, or rill wash, whichis when there is removal of surface debris onlow-gradient slopes by raindrop impact, whichdislodges particles which are then carried away byoverland flow.

● A second theory suggests that pediments mighthave been shaped by lateral planation as rivers,flowing into the desert plains would swing fromside to side (similar to when lateral erosion bymeandering rivers carves out floodplains).

● A third hypothesis is suggested due to the greaterpresence of water which is likely to accumulate atthe foothills of a mountain, giving rise to greaterlevels of weathering, which would generate finematerial which could be washed away by sheetfloods or removed by the wind. This creates a knickwhich marks the sharp change in gradient.Note that pediments can be considered as a relictfeature formed from past pluvial action.

As mountain fronts retreat, pediments gradually advance into the upland area. The upland areas may eventually be destroyed and reduced to upstanding rock masses – inselbergs.

5. Mesas and buttesThe word inselberg, meaning ‘island hill’, is often used as an umbrella term to refer to all relic hills, including mesas and buttes, but is often used more specifically to described the rounded hills which are located in humid as well as arid areas. Mesas and buttes are relict hills which have been cut out and isolated by fluvial erosion, i.e. they are remnants of areas which were formerly more extensive plateaus or plains. They create distinctive desert topography such as that found in Monument Valley in Utah, in the Great Basin Desert of the USA in which plateaus, mesas, buttes and pinnacles can be found. Mesas are flat-topped table-like formations, usually wider than they are tall, and buttes are pillars of upstanding rock: a mesa can be considered as a small plateau and a butte as a very small plateau. Flat topped mesas may shrink and retreat on all sides to form smaller buttes (the maximum diameter of the flat top of a butte is less than its height from the plain below), then progressively smaller pillars (pillar-shaped rock landforms) and pinnacles (tall tower or spire-shaped rock formations), which one day will be completely eroded away.

Mesas and buttes are commonly created where sedimentary rocks are present. Such rocks have horizontal layers of compressed sediments which are of varying resistance to erosion and have weaknesses along any faults, joints and bedding planes that may be present. If there is a layer of more resistant rock, this acts as cap rock which protects a section of the rock mass from erosion. It is the differential erosion of a rock plateau that leaves mesas and buttes remaining. The edges of mesas are often marked by the imprints of wadis or canyons which have been left as river have ‘cut’ around the feature to create it. The lower parts of mesas and buttes may have slopes covered in screes (which can be seen in Figure 6) which have accumulated following the action of subaerial processes – mechanical weathering drives the fragmentation of the rocks and rock fall is the mass movement process which dominates.

Desert features formed primarily by water deposition1. Alluvial fans and bahadasWhere flash flood waters discharge from the wadis in upland areas and meet the flatter desert plains, alluvial fans may be formed. Alluvial fans are delta-like depositional features which are low-angled (less than 10°) and semi-conical in shape. They can be between a few metres and up to 20 km in length and 300 metres thick. Alluvial fans are common in semi-arid zones and are created where a wadi or smaller perennial stream which is loaded with sediment runs down from a mountain front onto a much gentler lowland slope (i.e. from the foothills of desert mountains to meet the pediment below). This quite sudden change in slope angle (called a knick or piedmont angle) results in a sudden loss in velocity and energy of the flowing water, leading to deposition of much of the river’s load into a fan shape, which lies between the mountain front and a low lying plain and may cover some or most of the pediment. Alluvial fans are dynamic features: because of the variability of river flow in desert regions, a fan’s channels are highly changeable, shifting laterally over wide expanses and repeatedly cutting and filling themselves. When a number of alluvial fans coalesce as a series of parallel wadis arrive at a mountain front, a bahada (or bajada) may be formed. Alluvial fans commonly occur in desert areas such as the basin and range province in the southwest of the USA where steep mountains fronts meet downfaulted basins, for example, where Death Valley meets the Sierra Nevada Mountains in California.

Figure 6 Monument Valley, USA

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2. PlayasA playa is a depression at the centre of a desert basin which provides a site for occasional temporary shallow lakes which form after episodes of rainfall. Playas, or salt lakes, exist in hot desert areas as evaporation rates are high and many rivers therefore do not reach the sea and instead centripetal drainage patterns exist through which ephemeral river drain towards the central playa. Playa lakes fluctuate considerably in size through wet and dry periods and will dry up completely when there is a lack of rainfall. They are therefore ephemeral features. Some desert lakes are more permanent features characterised by their high salt concentrations as water can only leave the lake site by evaporation, which leaves behind a concentration of salty water - if playa lake water does not drain away and instead evaporates slowly in the desert heat, evaporate minerals are deposited and a saltpan is formed. Playas are commonly found at the foot of alluvial fans or bahadas; the finest sediments of the fan, which are the carried furthest by the ephemeral channels, may play a part in infilling the lake. Desiccation cracks (cracks which form, usually creating polygonal patterns, on the bed of former water body as it is dried out by solar heating and shrinks) appear in playa lake deposits as sediments dry out between rainfall events.

Impact of pluvial periodsSome erosional desert landforms are being created at present, but many features created by the action of water in deserts were initiated and developed at times in the past when more water was available, during pluvials, which are lengthy periods of time, often lasting for decades or even up to thousands of years, when rainfall was considerably greater than preceding and succeeding stages and evaporation rates were lower. During such times, rivers and lakes would have been more permanent landscape features in comparison to the present condition of the same zones, which are currently arid and rivers and lakes within them are largely ephemeral. Many desert landforms of fluvial erosion are therefore considered to be ‘fossil’ or relict features.

ConclusionsHot deserts experience a paucity of rainfall, but the sporadic and sometimes intense rainfall events experienced, combined with a vegetation-poor landscape, can result in high volumes of surface runoff and flash flood events. The short-lived flow of water can carve out rills, gullies, wadis, canyons and pediments as well as isolated rock formations of mesas and buttes. While rivers are in flow, they can carry large loads, which are deposited as they flow out of hillslope areas into the flatter surrounds, creating alluvial fans, which may coalesce to form bahadas. Rivers may flow centripetally into playa lakes. Since flowing water can ‘disappear’ as quickly as it arrives in hot desert regions, features such as playas and wadis are often ‘dry’ despite their fluvial origins. Given the paucity of water and the infrequent nature of water flow in deserts, it is largely believed that many of the fluvial landforms which exist could only have been created during the wetter pluvials of the past which would have allowed fluvial processes to take place in a more intense and sustained fashion.

Bibliography and Further ReadingBishop, V. & Prosser, R. (2001) Landform Systems (Second Edition) Collins Clowes, A. & Comfort, P. (1987) Process and Landform: Conceptual Frameworks in Geography (Second Edition) Oliver and BoydCollard, R. (1988) The Physical Geography of Landscape Collins EducationalGoudie, A.S. (2013) Arid and Semi-Arid Geomorphology Cambridge University PressHill, M. (2002) Arid and Semi-Arid Environments Hodder Education

Acknowledgements: This Geo Factsheet was researched and written by Kate Cowan, a Teacher of Geography at King Edward VI High School for Girls, Birmingham, and published in September 2017 by Curriculum Press. Geography Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or by any other means, without the prior permission of the publisher. ISSN 1351-5136

Figure 7 Alluvial fan in Taklamakan Desert, Xinjiang Province