Technische Universität München

Lehrstuhl für Hydrologie und Flussgebietsmanagement

Study Project

Planform patterns of arid land rivers on earth - Spatial distribution and potential causes

Iris Konnerth

München, 26.07.2015

Study Project

Technische Universität München

Lehrstuhl für Hydrologie und Flussgebietsmanagement

Planform patterns of arid land rivers on earth - Spatial distribution and potential causes

Prüfer: Prof. Dr. Markus Disse

Betreuer: Guo-An Yu, PhD eingereicht am: von: Iris Konnerth Kraillinger Weg 10 81241 München

Abstract In this study, exemplary dryland rivers from several of the world’s main arid regions are analyzed regarding the spatial distribution of channel patterns along their alluvial reaches. Satellite images and several geometrical parameters of the rivers such as gradient, channel width and sinuosity were evaluated using Google Earth to classify the channel patterns. In addition hydrological data and the sediment regime of each river as well as other process- related factors such as riparian vegetation and human impact are discussed to determine potential cause and effect relationships. It was found that within the group of analyzed rivers there is a pronounced difference between large perennial streams and smaller ephermal streams: the large streams mainly exhibit meandering and anastomosing patterns owing mainly to their very low gradients. The smaller ephermal streams show a larger variety of patterns that change frequently. It is proposed that the planform of those smaller ephermal rivers is more likely to be influenced by local changes along their course and by the spatial and temporal variability of flood events.

Outline 1. Introduction ...... 1 2. Theory and Classification of Channel Patterns ...... 4 2.1. Classification of Channel Patterns...... 4 2.2. Formation of Channel Patterns ...... 5 3. Methodology ...... 7 3.1 Study Areas ...... 7 3.1.1 Drylands of Central Asia: Tarim River ...... 7 3.1.2. Drylands of Southwest Asia: Euphrates River ...... 10 3.1.6. Drylands of Southern Europe: Guadalquivir River ...... 13 3.1.4. Drylands of Australia: Murray River, Plenty River ...... 15 3.1.5. Drylands of Australia: Plenty River ...... 17 3.1.3. Drylands of North America: Little Colorado River ...... 19 3.2 Methods ...... 23 3.2.1. Elevation profile ...... 23 3.2.2. Satellite Images Analysis in Google Earth ...... 24 3.3.3. Measurement of Geometric Parameters ...... 24 4. Results ...... 27 4.1. Tarim River ...... 27 4.3. Euphrates River ...... 29 4.6. Guadalquivir River ...... 31 4.2. Murray River ...... 32 4.5. Plenty River ...... 34 4.4. Little Colorado River ...... 36 5. Analysis ...... 38 5.1. Natural Impact Factors ...... 38 5.2. Human Impact Factors ...... 39 6. Conclusion ...... 42 List of Figures ...... 43 List of Tables ...... 45 List of References ...... 45 1. Introduction The term “arid land river” or “dryland river” comprises a large variety of streams, rivers and creeks which are located in many different climatic, geological and vegetative settings. Since rivers are a product of their environment, forms and processes in dryland rivers are also highly diverse. Nonetheless, some characteristic forms and processes have been identified that may however not be generalized for all dryland rivers (Tooth & Nanson, 2011). Characteristics of dryland rivers and their environment An important characteristic of an arid environment is its degree of aridity. It is defined by the moisture index Ih: the quotient of the average annual precipitation 푃 and the potential evapotranspiration 푃퐸푇. Four aridity classes can be distinguished (source EEA): - Hyper-arid zones: true desert (P/PET: < 0.03) - Arid zones: semi-deserts (P/PET: 0.03 - 0.2) - Semi-arid zones: steppes, prairies, certain types of savannah and a large part of the Mediterranean vegetation (P/PET: 0.2 - 0.5) - Sub-humid zones (P/PET: 0.5 - 0.75) This definition includes cold drylands such as the polar regions and high altitude mountain ranges. Most drylands are however located in the tropics and subtropics and are qualified as warm drylands (see Figure 1).

Figure 1: the world’s warm drylands (Stephen Tooth, 2000b)

Two main types of dryland rivers have to be distinguished: Firstly, exotic streams that collect runoff outside the arid land from rain and snowmelt in more humid, often mountainous areas. These rivers include some of the world’s largest rivers that are of national if not continental importance and that have supported the evolution of entire cultures, such as the River Nile and the Euphrates Tigris River Network. Secondly, endogenic rivers, which collect their runoff within the arid region and which typically have much smaller basins. In contrast to exotic streams, which are usually perennial, meaning that they carry water all year, endogenic rivers are usually ephermal, which means they stay dry during most of the year and only carry water during the infrequent but typically heavy rain events within the arid region. Rivers that have a strongly seasonal and little to no flow during the dry season are also called intermitted streams. There is of course a whole spectrum in between those definitions of perennial and ephermal flow (Tooth & Nanson, 2011). According to Tooth (2000a), dryland rivers more often undergo significant change in downstream direction compared to rivers in more humid regions. The relatively high 1 evaporation in warm arid regions, along with losses due to infiltration, lead to significant water losses in downstream direction. These so called transmission losses cause many rivers not to reach the sea but to end in salt lakes or inland deltas. Furthermore, apart from the headwaters, the rivers often only have few tributaries, which fortifies this phenomenon. Other factors causing changes in downstream direction of dryland rivers can be changes in sediment type and transport, vegetation or slope (Tooth, 2000a). Rain events in drylands often differ greatly from those in more humid regions. They are often caused by discrete convection cells, which generally have a small diameter and result in heavy and thunderstorm events which can lead to overland flow being generated within minutes in arid environments. The high intensity and spatial variability of those rain events results in very variable hydrographs for one catchment and in high, sometimes multiple, peaks in the flood hydrograph of its streams (see Figure 2) (Reid & Frostick, 2011). According to Tooth (2000a), those so called flash floods have a large impact on the channel morphology. In contrast, the flood hydrographs of exotic perennial streams affected by widespread frontal rain are more predictable and less concentrated, spatially and temporally (Reid & Frostick, 2011).

Figure 2: flood hydrographs of emphermal streams in desert regions (Reid & Frostick, 2011)

Dryland rivers are in general of enormous importance to sustain the local biosphere as well as to provide water to human settlements. At the same time, irrigation is necessary to grow most common crops in arid and semi-arid environments and water from dryland rivers, which are often the only controllable source of water in these environments, is needed to support it. The vast increase of irrigation agricultures in the past decades, along with an increased domestic and industrial demand for water and hydropower, has led many dryland rivers to become highly modified. Along the Colorado River for example, which used to show extreme flood variations, 29 large dams are constructed and 1,6 million hectares of agricultural land are irrigated with the rivers water making it now one of the world’s most controlled dryland rivers (Walker et al., 1995). When analyzing channel patterns of dryland rivers it is thus generally necessary to consider human impact. At the same time the research on dryland rivers is very relevant since the livelihood of so many people depends on them.

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Study Aims and Research Questions Dryland rivers have so far not been as much in the focus of researchers as their humid counter parts and, according to Tooth and Nanson (2011), research has so far mostly been conducted on small endogenic rivers and thus in a “limited range of dryland settings” which has led to an overemphasis of the distinctiveness of dryland rivers. The authors claim that, “when assessing dryland river characteristics,[…] there is a need to step back and look at the ‘big picture’ across drylands as a whole”. In this study, a broad range of types of dryland rivers in several of the world’s main arid regions is inspected regarding the channel pattern development and its change along the alluvial reaches of rivers. Information on the specific environmental settings of the rivers was gathered so that both ways of analyzing the characteristics of channel pattern evolution of dryland rivers are possible: one being to look at “the big picture” and the other to discuss common features specifically for a certain type of dryland river. The main questions asked in this project are: 1. How are channel patterns distributed along the selected dryland rivers and which patterns dominate? 2. How do the identified patterns relate to environmental settings and human impact? 3. Do dryland rivers show a characteristic spatial distribution of channel patterns?

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2. Theory and Classification of Channel Patterns 2.1. Classification of Channel Patterns One of the first widely noted classifications of river channel patterns was established by Leopold and Wolman (1957) who define a channel pattern as being “[..]the plan view of a reach of river as seen from an airplane[..]” (Leopold & Wolman, 1957) The authors differentiate between three patterns: straight, meandering and braided. Leopold and Wolman (1957) also recognized that “Channel patterns do not fall easily into well-defined categories for, as will be discussed, there is a gradual merging of one pattern into another.” (Leopold & Wolman, 1957) This might be a reason why today there is still no universally accepted classification. The variety of channel patterns discovered in the past decades lead to new terms that especially recognize the shortcomings of the braided channel pattern defined by Leopold and Wolman, which comprised all patterns “characterized by channel division around alluvial islands“ (Leopold & Wolman, 1957) Chitale (1970) proposed a classification which at first differentiates between single-thread and multi-thread channels. Single-thread channels are either meandering or straight. Multi-thread channels can be subdivided into braided, anabranching and anastomosing channels. Today, the following definition of a braided channel is generally accepted: It is a river characterized by flow separated by mobile bars that are located within the channel and which are submerged under water at high flow stages (Huang & Nanson, 2007). Since some authors use the terms anabranching and anastomosing as synonyms or anastomosing as a subgroup of anabranching and their definition of the terms often varies greatly (Makaske, 2001), those terms specifically need to be defined in order to avoid any confusion. For anastomosis the definition proposed by Makaske (2001) is adopted: “An anastomosing river is composed of two or more interconnected channels that enclose flood basins.”

Figure 3: channel patterns, modified from Alabyan and Chalov (1998)

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This means, that anastomosing is a higher order of channel pattern than braided, straight and meandering and that the channels of anastomosing rivers can each be classified according to those categories separately (Makaske, 2001). An anabranching river is defined by Nanson and Knighton (1996) as having semi-permanent alluvial islands that have either been excised from a floodplain or have formed within the channel. This definition includes anastomosing channels as defined by Makaske (2001) as one of six types of anabranching patterns identified by the authors. For this study, only the types that describe rivers with vegetated in-channel islands that divide the flow up till nearly bankfull, in an unconfined alluvial environment, are recognized. The main apparent difference to a braided pattern is the higher stability of the island formed within the channel which is caused by vegetation cover. Figure 3 shows an overview over the defined patterns and some characteristics in their appearance on different structural levels. In the following section some background on the development of the five channel patterns will be presented. 2.2. Formation of Channel Patterns Knighton and Nanson (1993) state: “Channel pattern is one of the means whereby a natural river can adjust its channel form to imposed flow and sediment conditions.” The flow conditions or the river discharge and the slope of the river determine the stream power or shear stress, which has a high impact on the channel pattern development (Alabyan & Chalov, 1998). The discharge is determined by the climatic conditions and the runoff formation in the basin. The sediment conditions can be described by the sediment load and the grain size. According to Huang and Nanson (2007), most alluvial rivers exist in stable energetic conditions, where the river has the same energy available it needs to transport its water and sediment. According to this theory, the channel slope determines the available potential energy of the river and the discharge and sediment load represent the work the river has to do. Nanson and Huang (2008) also state that rivers can achieve this energetic balance by changing their channel slope (accretion or incision, sinuosity), or shape (width depth ratio, number of branches) if they have surplus or insufficient energy. In those cases the valley slope 푆푉 is higher or lower than the minimum channel energy slope 푆푓푚푖푛 that would be necessary to transport the sediment load and water in a straight single thread channel. This means that the channel planform of a river is a product of its energetic conditions. There are further factors that influence the channel development which the authors call externally imposed factors that interact with the named main factors: amongst others riparian vegetation, bedrock, peat, colluvium, glacial or lacustrine deposits. Since the slope of the alluvium might have been formed by a previous regime it can be considered an externally imposed factor. Regarding the energetic balance Nanson and Huang (2008) found three possible cases:

푆푉 = 푆푓푚푖푛 : The valley slope matches the minimum required energy slope. The result is a single thread straight channel with a uniform channel geometry that uses the energy in the most efficient way.

푆푉 > 푆푓푚푖푛 : The valley slope is higher than the required energy slope and the river needs to expend the excess energy. There are three ways of achieving that and creating a stable state in which the excess energy is transferred by increased friction: in steep narrow valleys by creating step-

5 pools, in wider less steep valleys by widening the channel and creating braid bars, and in low gradient environments by creating meanders.

푆푉 < 푆푓푚푖푛 : The valley slope is insufficient to transport the river sediment without deposition. One way to achieve equilibrium is by adjusting the channel slope by aggradation. This leads to the formation of unstable aggrading braided, meandering, anastomosing and wandering channels. Another way to increase the flow efficiency is to reduce the width depth ratio 푊/퐷 in cases when 푆푉 is only to some extent below 푆푓푚푖푛 . This can be achieved by anabranching: creating stable vegetated banks and the formation of stable vegetated islands or ridges. The same principle might be relevant for the formation of anastomosing patterns. Figure 4 shows a channel pattern classification by Schumm (1985). The patterns are organized according to bed stability, gradient and other variables. This graph matches the theory of Nanson and Huang (2008) in the sense that high energy systems, which transport high loads of large sized sediments and have a high stream power, form braided patterns and have a higher width depth ratio than lower energy system, which form meanders or straight channels. When the flow or sediment conditions in a river change in a way that it has insufficient energy the river can adjust by changing from a braided towards a meandering order from a meandering towards a straight river.

Figure 4: channel classification based on pattern and type of sediment load, showing types of channels, their relative stability, and some associated variables (Schumm, 1985).

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3. Methodology Overall eight dryland rivers were examined. In this chapter their climatic and environmental settings as well as changes the rivers experienced in the recent past will be discussed and the approach to the classification will be described. 3.1 Study Areas 3.1.1 Drylands of Central Asia: Tarim River

Figure 5: Taklamakan Desert (left) (Adams et al., 1979) and Tarim River (right) (Yu et al., 2015)

General Information and Climate

Tarim River, one of the largest inland rivers in China with a total length of 1321 km, is located in the north of the Taklimakan Desert, which forms part of the Xinjiang Uygur Autonomous Region. The Basin is located between the Kunlun, Pamir and Tian Shan Mountains, which, by retaining humid air from the Atlantic and the Indian Ocean, causes the extreme aridity of the basin (Thevs, 2011). The runoff of Tarim River is generated in the Himalayan Mountains where the three main tributaries of the river originate. The average annual precipitation in the mountains is between 200-500 mm and the melting of glaciers and snowpack creates runoff during the summer. The annual precipitation in the main stem area ranges between 50-80 mm. Tarim River, which also doesn’t receive any mentionable groundwater inflow, is thus an allogenic river. The mean annual potential evaporation of over 2500 mm and the average air temperature range of -9 °C in January and 25 °C in July are representative of the highly continental climate in the basin (Thevs et al., 2008). In the past decades the basin has undergone significant hydrological and environmental degradation mainly caused by increased utilization of the river water (Feng et al., 2001). Historical Changes in the River Basin Since 1949 Xinjiang has experienced massive immigration from other parts of China and the population has since then increased from 4.3 to 17.5 million people in 1998 (Shen & Lein, 2005). Along with the population growth large areas of desert, pastureland and cropland have been converted into cropland, which, due to the aridity of the region, depends entirely on

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Figure 6: decreasing annual discharge at Alar Station (left) (Thevs, 2011) and at Yingbazha Station (right) (Pang et al., 2010). irrigation with river water (Shen & Lein, 2005). Among the three tributaries only Aksu River discharges into Tarim River over the entire year and provides over 70 per cent of the annual discharge, whereas, owing to river regulation, Hotan and Yarkand River presently only recharge the Tarim River during large floods (Pang et al., 2010). The discharge at Alar station, the starting point of Tarim River, has constantly decreased over the past decades even though the runoff into Aksu River has increased because global warming has led to enhanced glacier melt. The reduced discharge at Alar station is caused by the increased water extraction for irrigation agricultural (Thevs, 2011). In the middle reaches at Yingbazha station the annual average runoff has also declined continuously (Figure 6) (Pang et al., 2010). The increased water diversion and the construction of reservoirs have caused the ending point of the river to move upstream from Lop Nor to Daxihaizi Reservoir and even further upstream during dry seasons. Furthermore, the formerly branched middle and lower reaches of the river which used to be interconnected with several other tributaries have developed into one single stream. At the same time dike construction has reduced the rivers ability to form its own channel and the meandering reaches of the river show a lower sinuosity then the abandoned channels from former times (Yu et al., 2015). Figure 7 shows as an example the changes of the river network near Iminqäk over the past century.

Figure 7: changes of Tarim River course along the middle reaches of the river (Thevs et al., 2008).

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Hydrology The runoff of Tarim River is maximal in July and August, which is when 75% of the annual discharge occurs. During the resulting large summer floods the river banks are flooded and

Figure 8: monthly runoff at Yengi Bazar (Thevs et al., 2008). the groundwater is recharged (Thevs, 2011). Figure 8 shows the strongly concentrated monthly runoff measured at Yengi Bazar in the middle reaches of the river. It can be observed from Figure 6 that the annual discharge is highly variable and decreases strongly from Alar Station to Yingbazha Station: Alar ca. 2.5-7 km3/a, Yingbazha ca. 1-5 km3/a. Sediment The sediment of the river consists mostly of coarse silt and very fine sand. According to Yu et al. (2015), over the past decades, the sediment load has continuously decreased along with the average discharge, whilst siltation processes prevail mobilization of sediment. Furthermore an increase of channel bed elevations was documented for Alar and Xinquman stations showing clear signs of siltation. The annual sediment load comes close to zero at Qiala Station because all sediment has deposited in the channel, the floodplains or been diverted between Alar and Quiala (Yu et al., 2015).

Figure 9: decrease of annual runoff and sediment load (Yu et al., 2015).

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Vegetation The riparian vegetation in the Tarim Basin is called Tugai, which is typical for the flood plains of the continental, winter cold deserts of Central Asia. The tree and shrub species associated

Figure 10: area of Tugai forest in the Basin (Thevs, 2007) with the Tugai vegetation depend on groundwater along riverbanks, springs and lakes. In the middle and lower reaches of Tarim River the Tugai forest consist almost exclusively of Populus euphratica and Tamarix shrub communities, which must both have continuous access to groundwater to survive (Thevs, 2007). Also grasslands are found along the river. The forests provide multiple ecosystem services, not only as a natural resource, but also as wind protection by stabilizing sand, the soil and the riverbanks. The area covered by Tugai forests in the Tarim River Basin has however shrunk down to less than half within 20 years declining to 200’000 ha in 1978. The causes for the destruction where firstly the opening of land for farming and secondly the declining groundwater tables caused be the excessive water use for irrigation (Thevs et al., 2008). Since in the 1950s and 1960s land reclamation along the Tarim River was focused on the more easily accessible upper and lower reaches of the river, the forests in the middle reaches where less affected (Figure 10) (Thevs, 2007). 3.1.2. Drylands of Southwest Asia: Euphrates River

Figure 11: Euphrates River. Left (Adams et al., 1979), right (Flint et al., 2011)

General Information and Climate

The Euphrates River Basin has a total area of 444 000 km2 and stretches over four countries. The headwaters of the two rivers are located in the mountains of Eastern Turkey and they confluence in the Shatt Al-Arab River ca. 200 km before reaching the Persian Gulf. Euphrates River has a length of 2781 km. Two thirds of the river flow through the highlands of Anatolia 10 and the valleys of Syrian and Iraqi plateaus before it reaches the Mesopotamian Plain (Frenken, 2009). The climate in these areas is semi-arid, Mediterranean with wet winters and dry summers. The largest part of the runoff is generated during the seasonal rain between December and March and by snow melt during spring and it peaks between March and May. While the yearly precipitation is over 1000 mm in some areas of the basin, it is only 50 mm in some parts of the arid Mesopotamian Plain (Figure 12). The lower reaches can thus be considered exotic. The potential evaporation in the basin ranges between 800-2000 mm/yr which causes very dry summers. The northern parts of the basin experience cold winters with near freezing temperatures while the winters in the southern parts have mean minimum temperatures of 4-5 °C. The temperatures in southern parts of the basin are much higher especially in the summer with mean maximum temperatures of over 40 °(Flint et al., 2011).

Figure 12: left: average yearly precipitation. Right: potential Evaporation (Flint et al. 2011)

Hydrology ) The flow of the rivers varies strongly from year to year which is why the average annual discharge is difficult to determine. Records from the past century show combined annual discharges of the Euphrates and Tigris rivers of 30-84 km3. Today, the Euphrates River’s annual flow from Turkey to Syria is 26.9 km3 and 30 km3 from Syria to Iraq (Frenken, 2009). In Iraq the river loses a lot of water to irrigation and evaporation. In Nasiriya 150 km upstream of the confluence with the Tigris River the discharge is only 37 % of the discharge in Hit 750 km upstream of the confluence.

Figure 13: discharge at Hit and Haditha cities 1948-2007(Al-Ansari & Knutsson, 2011)

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Historical Changes in the River Basin The basin, which had served as “Breadbasket of the Mideast” for ancient cultures, is still an important region for agricultural production. In order to maintain production in the drylands a dense network of irrigation and drainage canals was established connecting the Euphrates to several lakes and reservoirs as well as the to Tigris River. Furthermore, the widespread natural network of channels of the Euphrates is created by channel avulsion, the partial or complete abandonment of a channel and the choosing of a new path, which made it possible to irrigate large areas of land in the “avulsion-plain” of lower Mesopotamia (Heyvaert & Baeteman, 2008). In 2006, 25% of Iraq was irrigated land (Flint et al., 2011). In order to control the seasonal, potentially destructive flooding, for water storage purposes and for hydro power production many large dams were built in Turkey, Syria and Iraq during the past century which had a high impact on the river discharge. Figure 13 shows the changes in discharge in Hit and at the Hadita Dam which are less than 100 km apart. The average discharge declined by 43% from 967m3/s to 553 m3/ s after 1985 and the flood peaks are much lower due to the flow regulation (Al-Ansari & Knutsson, 2011). Sediment The sediment load of the Euphrates is generally low in the alluvial reaches in Syria and Iraq due to the strong regulation of discharge and the many barrages in which sediment is being caught (Al-Ansari et al., 1988). The maximum sediment load, 25% of the annual load, occurs in March during the flood season. The sediment type is dominantly silt (>60%) and the rest consists of clay and sand (Aqrawi & Evans, 1994). At Hadhita, which is located downstream of the last reservoir in the reservoirs chain, a maximum suspended sediment load of 2.1*107 t/yr was measured in 1970 and a minimum suspended load of 1.9*106 t/yr in 1985 (Al-Ansari et al., 1988). Vegetation The desert steppe vegetation in Iraq and Syria is very minimal. Between the Euphrates and the Tigris, large wetlands exist, which, before being reduced to less than 15% of the original area during the 1990’s by a governmental drainage project, used to be a species-rich densely vegetated area. This led to severe ecological damages including the desertification of 19000 km2 of former marshland which are currently in the process of being partially restored (Hamdan et al., 2010). The natural aquatic vegetation of the marshlands includes reeds, rushes, and papyrus (WWF, 2015).

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3.1.6. Drylands of Southern Europe: Guadalquivir River

Figure 14: drylands of Europe, in most parts of southern Spain the moisture index is 0.2-5 (left) (Estrela et al. 1996), watershed of Guadalquivir River (right) (Wikipedia, 2015a)

General Information and Climate With a length of 657 km Guadalquivir River is the fourth longest river in Spain and the river basin measures 58,000 km2. It is located in Andalusia, which is one of the driest regions in Europe. The yearly average temperature in the semi-arid basin is 16.8°C with an average annual precipitation of 617 mm and a potential evaporation of 951 mm/yr (Estrela et al., 1996). Rainfalls in the Mediterranean region occur mainly during wintertime and the completely dry season during the summer lasts several months. The size of storm cells is often small and very heavy rain events often occur within a small radius. This can lead to high peaked flash floods with a high potential for soil erosion (Estrela et al., 1996). The perennial river begins its course in the Cazorla Mountain Range and then runs through a very fertile area of Spain which contains 25% of Spain’s irrigated land (Berbel et al., 2012). After reaching Seville the river is canalized and serves as a major water route to the large port of Seville which runs through extensive marshlands, in which rice is cultivated (Columbia, 2013). Historical Changes in the River Basin The river basin is highly regulated with more than 60 dams spread over the Guadalquivir and its tributaries. According to Bohorquez et al. (2014), the river has experienced significant changes due to flow regulation and water diversion: by analyzing aerial images the authors found that the channel has narrowed and is in a state of aggradation. This evolution is caused by increased erosion rates in the basin which are a consequence of the intensive agricultural production and the reduced plant coverage. The channel narrowing combined with the loss of reservoir capacity caused the flood events in the recent years to be more damaging than in prior, more natural states of the river system.

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Hydrology

Figure 15: daily average water flow discharge since October 2009 and peak annual water flow discharge during the last century at Marmolejo dam in the Upper Guadalquivir River (Bohorquez et al., 2014).

3 The long term annual average flow into the Gulf of Cadiz is 7.2 km . Annual average discharges and peak discharges vary strongly in the basin and droughts and floods are very common: Figure 15 shows the highly variable peak discharges within the last century as well as exceptionally high flood events that occurred in 2009, 2010 and 2012 (1500-2000 m3/s). Overall the average discharges in the basin have however decreased due to increased water diversion (Figure 16) (Droogers & Immerzeel, 2008).

Figure 16: annual average discharge since 1912 (Droogers & Immerzeel, 2008)

Sediment The sediment load in basin is estimated the 70 × 106 m3 per year and is dominated by fine sized sediments (Bohorquez et al., 2014). Vegetation The riparian vegetation of the Guadalquivir consists mostly of white poplar trees along with shrub communities, reeds and grasslands. Sediment deposits near the riverbanks and lateral bars are also vegetated (Bohorquez et al., 2014).

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3.1.4. Drylands of Australia: Murray River, Plenty River

Figure 17: left: drylands of Australia (Adams et al., 1979); Right: River Murray in Southeast Australia (Wikipedia, 2015b)

General Information and Climate

River Murray is a largely exotic stream of great regional importance that originates in the

Australian Alps in southeastern Victoria, then flows westwards and is joined by its largest tributary, the Darling River, before reaching the sea near Adelaide. The river has a length of 2375 km and usually flows perennially. Even though the river is one of the world’s longest rivers its annual discharge is rather low. The climate in the Murray River Basin is increasingly arid from east to west with an average annual precipitation of 469 mm in the entire Murray-

Figure 18: annual runoff and annual potential evaporation in the Murray-Darling River Basin (Murray-Darling-Basin- Authority, 2010). 15

Darling-Basin and a potential evaporation of ca. 1250 mm in the arid reaches of River Murray (see Figure 18) (Murray-Darling-Basin-Authority, 2010). Hydrology The natural regime of the river has a strongly seasonal pattern with peaks in winter and spring and the lowest flows occur in late summer and autumn. The annual flow varied between 16 and 54 km3 in the past century, with a mean of 10 km3 (Maheshwari et al., 1995). Under current regulated conditions average annual outflows of the basin are ca. 5 km3 (Murray-Darling-Basin-Authority, 2010). Historical Changes in the River Basin The river is of large importance as a freshwater supply for the region: 3.4 million people living inside and outside the Murray-Darling Basin rely on the supply and 40% of Australia’s farms are located in the Basin of which many depend on the rivers supply of irrigation water. The water consumption increased constantly within the 20th century and large scale storage and diversion infrastructure such as the Hume Dam in the river’s headwaters and the river’s largest diversion weir at Yarrawonga were constructed. Annual flows have thereby been reduced by over 50% and remain constant along the river even though it receives water from several larger tributaries along its course (see Figure 19). The regulation of the river reduced the seasonal variability of discharge especially in the lower reaches of the river but large floods and droughts still strongly affect the regime: due to reduced annual flows, water diversion for agricultural use has declined since 2002 and severe droughts have led the river to nearly dry up several times in that period (Murray-Darling-Basin-Authority, 2010).

Figure 19: left: effect of regulation on the annual flows at eight stations along the Murray from Albury to the Murray Mouth. Right: variation in average monthly flow at the mouth under natural and present conditions (Maheshwari et al., 1995).

Sediment The sediment load in most parts of the basin can be estimated to be rather low and grain size of the transported sediment to be rather fine sized since the gradients in the basin are very low. DeRose and Prosser (2003) modelled the sediment transport in the basin and found that the suspended sediment load especially in the upland areas of the basin is much higher (up to 20 times) in the current regulated state of the river than under natural conditions. Furthermore, their results suggest that only 1% of the Murray-Darling River Basin reaches the coast, which

16 indicates that sedimentation in the river and the flood plains has increased as well. Other model studies support these results (e.g. Thoms et al. (2000)). Vegetation The flood-plains in the lowlands of the Murray stretch up to 40 km wide in unconfined reaches and 2-5 km in confined reaches. The riparian vegetation in the river’s wide-reaching wetlands consists of the so called red gum trees, some shrubs and grasslands. The recent millennium drought had detrimental effects on the red gum population and the vegetation cover has severely declined (Doody et al., 2014). 3.1.5. Drylands of Australia: Plenty River

Figure 20: Plenty River in Northern Territory (Tooth and Nanson, 2004)

General Information and Climate Plenty River is located in the remote unpopulated arid central Australian desert in Northern Territory. The riverbed has a length of approximately 400 km and ends in the Hay River in the Simpson Desert. The stream however usually runs dry ca 60 km before the confluence. Combined with the Marshall River, which also drains into the Hay River, the Plenty’s catchment is ca 18000 km2.The climate in the region is characterized by average annual precipitation of ca. 300 mm and a high potential evaporation of 3000 mm/yr. The river has a strongly ephermal discharge pattern and is only charged during occasional heavy rainfalls (> 100 mm/d) that result from moist tropical airflows or passing frontal weather systems (Tooth & Nanson, 2004). Hydrologic Regime Figure 21 shows the high variability in the flood records of the Plenty River. It can also be observed that the river rarely exceeds its bankfull stage, which is caused by dunes that confine the channel and allow a high water depth during floods. Discharges usually decrease along the course of the river due to transmission losses. Since rainfalls sometimes only occur within a small radius, runoff might however only enter the river in its middle or lower reaches while the upper reaches stay dry (Tooth & Nanson, 2004).

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Figure 21: continuous flood records of the Plenty River 1975-1986 (Tooth & Nanson, 2004)

Sediment The rivers sediment is dominated by medium and coarse sand. There are no records of the river’s sediment transport (Tooth & Nanson, 2004). Vegetation The channel banks especially in the upper reaches are well vegetated with trees, shrubs and perennial grasses and large river red gum trees grow in the channel bed (see Figure 22). The density of in-channel trees varies along the course of the river (Tooth & Nanson, 2004). Tooth and Nanson (2004) took some measurements of cross-sectional profiles in the middle reaches of the Plenty River. They show how the in-channel trees function as stabilizers for the formation of ridges in the channel and for the river banks (see Figure 22).

Figure 22: Plenty River. Left: cross-sectional profiles. Right: aerial photographs (Tooth & Nanson, 2004).

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3.1.3. Drylands of North America: Little Colorado River

Figure 23: Little Colorado River. Left (Adams et al., 1979), right modified: (Wikipedia, 2015c)

General Information and Climate The Little Colorado River is a major tributary to the Colorado River in southeastern Arizona, located in the so called Colorado Plateau. This plateau is known for its famous canyon landscapes. The Little Colorado is however one of the Colorado tributaries with a largely unconfined course which is why it was selected for this study. Only the last 90 km of the river have carved a narrow canyon in the plateau with up to 980 m depth which then opens into the Grand Canyon. The total length of the river is 544 km and the basin has a size of 68,635 km2. With an average precipitation of 373 mm in south eastern Arizona and high evaporation rates in the basin’s hot summers (annual average temperature 16 °C) the climate in the basin is arid to semi-arid and precipitation varies strongly in short and long timescales (years, decades) as well as spatially. 52% of the rain falls in summer during the monsoon season and 35% arises from frontal storm systems during winter (ADWR, 2015). Hydrology The river’s headwaters in the White Mountains in southeastern Arizona are perennial and collect snowmelt and highland rain. After leaving the mountains the river turns into an ephermal stream which peaks during the spring from snowmelt and during the monsoon season (see Figure 24). After it reaches the canyons the river flows perennially again as it receives groundwater inflow. Little Colorado is thus an endogenic stream. The annual discharge at

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Figure 24: mean monthly discharge of the Little Colorado River (1948-1992) at a station near Cameron (Birkeland, 1996).

Cameron station, upstream of the canyon reach, the average discharge was 10.40 m3/s in the past 70 years. Historical Changes in the River Basin The river has, like other rivers in the Colorado Plateau, undergone significant changes in the past century. The channel bed has migrated severely within 74 years up to 2 km in some areas, the channel width is constantly decreasing and the channel length has increased along with sinuosity (see Figure 28). At the same time, several environmental factors have shifted: temperatures are constantly rising due to climate change and potentially due to loss of vegetation cover. Precipitation during the summer monsoon and the rivers average annual peak discharge have decreased continuously (see Figure 29). Vegetation According to Block (2014), the Little Colorado River had already lost most of its original riparian vegetation of cottonwood and willows and the basin had undergone desertification, when in the 1930’s alien species tamarisk (mostly tamarix chinesis) began to spread and change the distribution and composition of riparian vegetation in the whole region (Block, 2014). Tamarisk is a fast growing woody shrub or tree which quickly colonizes open sandbanks of rivers (Birkeland, 1996). The effect of Tamarisk populations on fluvial geomorphology has been analyzed and discussed in several studies, amongst others by Graf (1978). He found, that by stabilizing the river banks and anchoring material, the plant can trigger a substantial decrease in bank-to-bank width and a formation of stable vegetated islands (see Figure 25).

Figure 25: changes due to the spread of tamarisk in a cross section on the Green River (Utah) (Graf, 1978).

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Figure 26: morphological changes of the Little Colorado near Winslow and the spread of tamarisk (Block, 2014).

In case of the Little Colorado investigations where undertaken by Birkeland (1996), Hereford (1984) and Block (2014). Contrary to the findings of Graf (1978), Hereford found that the narrowing of the channel of the Little Colorado was not caused by the spread of tamarisk but by the changes in the hydrologic regime. Birkeland showed, that the spread of the plant does not affect the morphology of the confined canyon reaches of the river. Block found that in an analyzed area near Winslow erosion processes were not influenced by the spatial distribution of the plants and that the cover with tamarisk erodes even during low to moderate floods and thus is not able to act as a stabilizing agent. It was concluded that the spreading of the plant was a consequence of hydrological and morphological changes of the river environment. Sediment The alluvial deposits in the basin consist of an easily erodible mixture of sand, silt and clay, (Hains et al., 1952). Figure 27 shows the high suspended sediment load in the river during a flash flood.

Figure 27: Little Colorado River: Grand Falls near Flagstaff during a flash flood (http://i.ytimg.com/vi/rDSmxbMWKNg/maxresdefault.jpg).

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Figure 28: channel migration of Little Colorado River (Block, 2014)

Figure 29: changes in peak discharge, precipitation and temperature in the Little Colorado Basin during the past century (Block, 2014).

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3.2 Methods In this section, the methods that were used to classify the selected rivers from upstream to downstream according to their channel patterns are described. The main tool used in this project was Google Earth. Satellite images of the study areas where analyzed visually and several geometrical parameters of the rivers were measured using Google Earth in order to obtain a more detailed overview of the morphological changes along the rivers. The data were then processed and visualized with Microsoft Excel. In a first step, homogenous reaches were identified in which the channel planform and the channel width do not change significantly. 3.2.1. Elevation profile As explained in section 2.2, the gradient of a river channel has a strong impact on the channel pattern evolution. To determine the average gradients for the defined homogeneous reaches an elevation profile was produced using the elevation data provided by Google Earth. At the starting point of each reach the average channel bed elevation was determined by extracting the elevation of several equally spread points along the cross section of the river and an average was formed (see Figure 30). The average gradient was then calculated from the elevation difference over the reach and the reach length which was also measured on Google Earth by drawing a polyline along the river. The DEM supplied by Google Earth, which, according to Wikipedia (2015a), is based on SRTM raster data collected by NASA, is not equally precise all over the globe and in flat and mountainous terrain. Reliable information on the accuracy of the DEM could be found as Google Earth does not provide such information or publishes the sources of its data. As the measured average gradients are mostly calculated for river reaches of >20 km length, the results are expected be precise enough to give sufficient qualitative information about the river gradients, which however might not be perfectly exact regarding their quantitative value.

Figure 30: measurement of average channel bed elevation.

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3.2.2. Satellite Images Analysis in Google Earth For many locations, Google Earth gives access to several images taken at different times. Mostly the latest pictures were used for the measurements and the classification, because this way, recent changes of the riverbed would not be neglected. Having several pictures available has various advantages: In order to have comparable results for each river, the season the images were chosen from could be fixed. For the measurement of channel width and braiding index, pictures taken during flood and dry season bring different results as the river banks and braid bars might be submerged and thus invisible on the images (see Figure 31). Furthermore, the comparison of several images of a river reach is helpful in classifying its channel pattern and to see whether the river has recently changed its course or its pattern.

Figure 31: braided reach of Tarim River during flood season (left) and dry season (right).

3.3.3. Measurement of Geometric Parameters In this section, the measured parameters will be explained and the approach that was chosen for the measurements will be described. Active Channel Width The active channel of a river is determined by its bankfull stage, which is defined the following: “The bankfull stage corresponds to the discharge at which channel maintenance is the most effective, that is, the discharge at which moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work results in the average morphologic characteristics of channels.” (Dunne & Leopold, 1978) To identify the active channel from aerial photographs, several indicators could be used that indicate recent river activity in a dry part of the riverbed which sets it apart from the rivers floodplains (see Figure 32): - No signs of land-use. - None or little vegetation cover. - Changes in the sediment color; Flood plain deposits are associated with finer material than active channel deposits which may lead to a different color (EPA, 2012). For anastomosing channels, the width was defined as the summed width of all channels within a cross-section of the channel network. This way the total channel width could be compared to other reaches with a different number of channels.

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Figure 32: measurement of the active channel width. Two exaples from Tarim River. Braiding Index

To measure the braiding intensity of a river reach there are several methods proposed by different authors. The method chosen here was the channel count method as it is easily applicable and gives precise results (Egozi & Ashmore, 2008). The braiding index is defined here as the average number of active channels for several cross-sections within a river reach (see Figure 33). Egozi and Ashmore (2008) evaluated the number of cross-sections that is necessary to have reliable results and found a minimum number of 10 above which the results do not change. Furthermore, the distance of the cross-sections should not be larger than the average wetted width of the river. Their findings were applied in this study and the channel count was performed over 10 cross-sections at the beginning of a homogenous reach spaced accordingly.

Figure 33: the channel count method for measuring the braiding intensity (Egozi & Ashmore, 2008).

Sinuosity Index The sinuosity index of a river reach is defined as the ratio of the channel length to the valley length and ranges from 1.0 to 3.0 (Schumm, 1985). As a threshold to distinguish between a straight and a meandering pattern a sinuosity index of 1.5 was used. The sinuosity index was measured over the whole length of each defined homogenous reach. The length of the downvalley path was determined, equally to the channel length, by drawing a polyline in

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Google Earth following the valley slope. In flat terrain with low gradients, the downvalley path could not be determined from the Google Earth elevation data because it is too imprecise. In that case, small changes in the river course were classified as meanders (which are not following the downvalley path) and large changes in the course of the rivers were assumed to be caused by changes in the direction of the valley slope. One exception is the occurrence of so-called macromeanders which have a meandering pattern superimposed over another meandering pattern were larger changes of the river course are caused by the meandering process (Figure 35).

Figure 34: measuring the sinuosity index (here: Euphrates River).

Figure 35: macromeanders in Murray River.

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4. Results In the following section, the results of the classification will be discussed in relation to the background information that was collected regarding the rivers’ environmental settings and history. 4.1. Tarim River

Figure 36: gradient, elevation profile and distribution of channel patterns along Tarim River.

The channel pattern distribution was analyzed starting in Alar station where the three tributaries of the Tarim River confluence and ending 900 km downstream in Daxihaizi Reservoir where the river currently ends. Starting in Alar, the stream shows a braided pattern for approximately 200 km and then gradually changes into a single thread meandering channel (see orthophoto Figure 38). Figure 36 shows the elevation profile of the river measured every 50 km and the according average gradient, furthermore the identified channel patterns along the course are listed. It can be observed, that the gradient over the whole reach is very low, ranging between 0.05 and 0.3 ‰, and is decreasing strongly in the middle reaches while the river starts to form meanders and turns into a single thread channel with a vastly reduced channel width (reduction from up to 2000 m width to a width < 50 m), a braiding index equal to 1 and an increased sinuosity of up to 2.1 (see Figure 37). The lower gradient in the middle reaches of the river, which is partly a consequence of sedimentation processes, and the decrease in discharge over the length of the river leads to a significantly reduced stream power which causes the shift in the channel pattern. Even further downstream the river is straightening as the stream power continues to decline due to decreasing flows.

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Figure 37: Tarim River. Left: Active channel width. Rigth: braiding index and sinuosity index

Figure 38: left: braided pattern of Tarim River downstream of Alar (Google Earth, 2014). Right: high sinuosity meanders in the middle reaches of Tarim River (Google Earth, 2012). Both photographs are taken in the dry season (frame width: 5.5 km)

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4.3. Euphrates River

Figure 39: gradient, elevation profile and distribution of channel patterns along Euphrates River.

The channel planforms of Euphrates River were analyzed over a reach of 1700 km starting downstream of the large reservoir Lake Assad in Syria, upstream of which the river comes closer to being a chain of reservoirs, than a natural river. Within the 1700 km long reach ending in the Persian Gulf, the river is dammed one more time in Iraq and forms the Haditha Dam Reservoir. Furthermore, multiple diversion reservoirs exist along its course. A dominant characteristic of the entire reach is the extremely low slope of the river except for the area of the reservoir: the gradients are < 0.3 ‰ upstream of the reservoir and < 0.15 ‰ downstream of the reservoir. For the first 400 km downstream of Lake Assad the river is anastomosing and has on average less than 5 branches with a sinuosity < 1.5. Many meander cutoffs still remain as unconnected stream branches. The added width of the channels is around 200 m and they are mostly spread over a width of 0.5-1.5 km. Above the Haditha Dam Reservoir the channel becomes single- threaded meandering. The gradient is a little higher (ca. 0.35 ‰), the channel is somewhat incised in the desert land (which might be due to the higher gradient) and the floodplains are quite narrow. Below the reservoir, a similar planform pattern continues for 450 km with some rather straight and some meandering reaches. Around 600 km downstream of the reservoir, the river reaches the wide alluvial plains of lower Mesopotamia, where a complex network of anastomosing natural and artificial channels, connected with the Tigris River, is spread over a width of >150 km. The anastomosing pattern in this area is quite different to the pattern downstream of Lake Assad: it stretches over a much wider area and has more resemblance to an inland delta. Extremely low gradients in this area, the occurrence of extreme floods that break up natural levees formed by flood deposits and the silting up of former channels have been identified as factors that promote the channel avulsion and thus the formation of the 29 anastomosing planform (Heyvaert & Baeteman, 2008). The total channel width is smaller in the anastomosing reaches than in the single thread reaches (see Figure 40, left). This might indicate that the flow efficiency in those reaches is increased by a decreased W/D ratio. The 200 km long Al-Arab River that discharges the Euphrates and Tigris River to the sea is a straight channel and nearly at sea level.

Figure 40: Euphrates River. Left: total channel width and number of channels. Right: sinuositiy index and number of channels.

A B

C

Figure 41: Euphrates River. A: anastomosing reach downstream of Lake Assad. B: straight and meandering reach above the Haditha Dam. C: lower Mesopotamian Plain with widespread river-channel network and the confluence with the Tigris upstream of Basra

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4.6. Guadalquivir River

Figure 43: gradient, elevation profile and distribution of channel patterns along Guadalquivir River

Figure 42: Guadalquivir River. Left: active channel width. Rigth: sinuosity index.

The channel of the Guadalquivir was analyzed over a length of 560 km starting in El Cerillo where the river begins to be less confined and ending at the river mouth in the Gulf of Cadiz. The gradient of the river’s middle reaches is around 1‰ and then ca. 0.5‰ in the tidal channel starting in Seville. The course of the river generally rests against the foothills of the Sierra Morena, which partly confines the channel (see Figure 44). The channel varies between a straight and a meandering pattern, which appears to depend mostly on the width of the alluvium. In entirely unconfined reaches sinuosity indexes >1.5 prevail. The lower reaches of the river were straightened artificially but shapes of old meanders are still visible in the structure of the land parcels and irrigation channels (see Figure 44). Since the river receives inflows from numerous tributaries from the close by mountain ranges, its discharge does not decline over its length as it is common for dryland rivers and the channel width increases accordingly over its length.

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Figure 44: left: Guadalquivir, middle reaches near the foothills of the Sierra Morena and some tributaries with reservoirs. Right: lower reaches, channelized, with still recognizable old meanders.

4.2. Murray River

Figure 45: gradient, elevation profile and distribution of channel patterns along Murray River.

The Murray River’s channel patterns were analyzed starting at the large headwater reservoir Lake Hume to its mouth ca. 2000 km downstream. The river’s slope quickly declines after it leaves the mountains and is then below 0.1 ‰ and declining. The channel is for the largest part characterized by high sinuosity meanders (SI up to 2.5, see Figure 46) that, in the upper reaches, form a nearly anastomosing network of channels with still partly active meander cutoffs. On a larger scale, macromeanders can be identified. As the river travels westward into more arid regions the abandoned channels are less connected to the main channel and often dried up. The lower ca 500 km long reach of the Murray shows a decreasing sinuosity and the width of the floodplains is strongly confined by dikes (see Figure 47). The channel width ranges between 60 and 180 m and shows an increase along the river course (Figure 46). About the causes for the straightening of the river and the increase in channel width in the lower reaches can only be speculated since no older aerial photographs or maps of those reaches could be found that would allow an assessment of the human influence in that question. It might have been caused by the formerly higher flood peaks in the lower Murray compared to the upper and middle reaches.

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Figure 46: Murray River. Left: channel Width. Right: sinuosity index.

A B

C D

Figure 47: Murray River. A: meander cutoffs.: macromeander. C: dry floodplains. D: confined floodplains.

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4.5. Plenty River

Figure 48: gradient, elevation profile and distribution of channel patterns along Plenty River

The Plenty River was analyzed from its starting point where several smaller creeks confluence to the lower reaches where channel becomes nearly unrecognizable and spreads out in what might be a so called flood-out as described by Tooth (2000a) 320 km downstream (Figure 49 F). The channel patterns of the river are at parts hard to determine since there are no aerial photographs available of the river carrying water, so conclusions had to be drawn from pictures of the dry riverbed. Several features could however be identified: The low to intermediate gradient channel (upstream ca. 1.6‰, downstream ca. 0.8 ‰) is largely straight with a sinuosity around 1.1. The channel bed widens quickly along the first 100 km and reaches average bed widths > 300 m, then declines again to values < 200 m and then increases again > 300 m (see Figure 50). In-channel trees are found in most parts of the river and they have partly formed what appear to be stable islands or ridges, which classifies those reaches as anabranching (Figure 49 B). The large width and the appearance of the channel bed indicate a braided pattern for some of the upper and middle reaches of the river (Figure 49 D); however, braiding indexes could not be determined from the aerial images. Furthermore a tendency of the channel to separate into a anastomosing network of channels is visible in the middle reaches of the Plenty (Figure 49) and becomes more pronounced further downstream when the river splits up over a length of 60 km and then reunites (Figure 49 E). Before finally losing its course after 320 km, the river develops some meanders. Overall it can be stated that the Plenty River cannot be characterized as having a clear evolution of channel patterns from upstream to downstream but changes its form frequently. This might be caused by the high variability in the magnitude and frequency of floods, the occurrence of localized rain events that only affect

34 parts of the river and the interaction with the riverine vegetation. Each flood can change the riverbed drastically and the somewhat coincidental manifestation of in-channel trees might have unforeseeable effects, such as the splitting of the channel and the formation of islands. The increasing tendency of the river to form anastomosing channels can however be attributed to the decrease in stream power and a way of the river to increase the flow efficiency by increasing the W/D ratio (as mentioned in section 2.2).

Figure 50: Plenty River. Left: active channel width. Right: sinuosity Index.

A B

C D

E F

Figure 49: Plenty River from upstream to downstream. A: upper reach, highly vegetated straight channel. B: stable vegetated islands. C: smaller channel separation. D: braided appearance of the river. E: anastomosing reach. F: flood- out. 35

4.4. Little Colorado River

Figure 51: gradient, elevation profile and distribution of channel patterns along Little Colorado River.

The channel patterns of the Little Colorado River were analyzed starting downstream of St. Johns below the White Mountains over a length of 450 km up to the river’s inflow into the Colorado River. The first 100 km of its course the river has a rather small channel bed (width < 10 m) that gradually shifts from a straight to a meandering stream. The riverbanks are mostly vegetated with what might be mostly tamarisk shrubs and trees (as described in section 3.1.3.). Before the inflow of its largest tributary, the Puerco River, the Little Colorado River flows straight through a smaller ca. 30 m deep canyon of ca. 30 km length. After the inflow of the Puerco the channel widens rapidly (> 50 m) and the stream shifts several times between a

Figure 52: channel width and sinuosity index Little Colorado River.

36 wider channel bed with a braided to transitional pattern and a narrow meandering channel (see Figure 52). Braiding indexes could not be measured, since only images of the dry channel bed were available. As described in section 3.1.3. the channel shows signs of migration: Recently abandoned stream courses can still be recognized since they have not yet been covered by the fast spreading tamarisk. Gradients of the river are intermediate to low (< 2 ‰) for the largest part and then quickly increase as it carves its way into the plateau to reach the deep canyon of the Colorado River (see Figure 52). The first 40 km the canyon is shaped in, partly quiet symmetrical, meanders that straighten as the gradient increases towards the river’s mouth. As a consequence of the reduced average flows and peak flows, the river is in the process of adapting to the new energetic conditions and is shifting from a formerly largely braided stream to an unstable wandering meandering stream.

A B

C

Figure 53: Little Colorado River. A: meanders before inflow of the Puerco River. B: wandering channel bed near Winslow. C: transitional channel pattern braided/meandering.

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5. Analysis

Table 1: spatial distribution of planform patters – comparison of the analyzed rivers. River Distribution of channel patterns (distance downstream in km)

Euphrates River

Tarim River

Murray River

Guadalquivir

Little Colorado River

Plenty River

When comparing the spatial distributions of channel patterns in the analyzed river (see Table 1), at first a large diversity can be recognized. All of the known channel patterns were found in the rivers and along each of the rivers, they change multiple times. The most dominant channel patterns of the analyzed large, perennial streams are meandering and anastomosing. In the smaller, ephermal rivers, no clearly dominating patterns could be identified and the channel pattern change a lot more frequently along the much shorter course of the rivers. Straight and braided are the only patterns both rivers show in significantly long reaches. In order to find an explanation for this difference, the natural and human impact factors for those two groups will be compared briefly (see Table 2). 5.1. Natural Impact Factors Perennial Streams: Tarim River, Euphrates River, Murray River, Guadalquivir River The four named rivers all flow perennially and have an annual average discharge of 5-30 km3. Except Guadalquivir River, which in comparison has a smaller basin, they are very long exotic rivers (>1000 km). The gradients in the alluvial reaches of the rivers are generally very low and they transport fine sediments. Their hydrologic regime is characterized by large seasonal floods. Tarim River and Euphrates River lose a lot of water along their course, annual flows of

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Murray River decrease slightly in the lower reaches and Guadalquivir River which receives inflow from numerous tributaries gains water over its course. In all four rivers, the low gradients promote their meandering, straight and anastomosing patters. While the loss in stream power of Tarim River, due to the declining discharges along its course, cause the planform to change from braided to meandering to straight, the constant/increasing discharge of Guadalquivir and Murray River result in channel patterns that largely remain the same over their course (except for the reaches that are naturally or artificially confined in both rivers). The spatial distribution of channel patterns along Euphrates River is more complex but can be explained mostly from the changes in average gradients: In the upper reaches (of the analyzed part of the river), they are low and channel avulsion processes have created an anastomosing planform. In the middle reaches, they are higher and the meandering channel is carved in the desert soil. The extremely low gradients in the lower Mesopotamian plane cause the river to spread out in an anastomosing network resembling an inland delta. Smaller Ephermal Rivers: Little Colorado River, Plenty River The two ephermal rivers have much smaller basins than the analyzed perennial rivers (except Guadalquivir River) and the average annual discharge of Little Colorado River is only 0.3 km3. The discharge is however strongly concentrated and can reach high peaks during flood events. The flashfloods that arise from heavy local rainfalls cause water levels to rise fast and mobilize a lot of sediment. Furthermore, the gradients of both rivers mostly ranged between 1-2 ‰ which is significantly higher than in the long perennial rivers which generates a higher stream power per unit discharge. The rivers, therefore, become very powerful during the floods and each flood event can change the channel morphology by eroding the riverbanks, creating a new channel or removing vegetation on the banks or in the channel. For both rivers, there is some evidence that vegetation is impacting the channel morphology. The in-channel eucalyptus trees in the Plenty River stabilize the soil and debris and sediment is likely to be caught by the trees, which leads to the formation of ridges and islands. The spread of tamarisk in the Little Colorado River might have supported the channel narrowing the river has experienced in the recent century by stabilizing the river banks. Another factor that was found to be impactful was the inflow of tributaries. In the Little Colorado River, the inflow of Puerco River causes the channel pattern to instantly change from meandering to braided. Generally it can be observed that small local environmental changes in slope, vegetation or discharge (from tributary inflow) seem to affect the channel planform of the smaller ephermal streams more than the planform of the large perennial streams. This leads to frequent planform changes along the course of these rivers. Additionally, the temporal and spatial variability of flood events in the basins and the possibly strong effect of one single event on the channel morphology increase the diversity in planforms of these rivers. 5.2. Human Impact Factors On the analyzed dryland rivers, human influence is very pronounced in all of the larger perennial rivers. The main impact factors were water diversion and flow regulation. Damming not only changes the flow regime of the rivers but also causes additional water losses due to evaporation in the reservoirs. Water diversion mainly for irrigation agriculture has caused discharges to be reduced by up to 50% in these rivers, which causes a loss in stream power that leads to aggradation. In Tarim River a decreased sinuosity and a strongly reduced length of the river is the consequence. In the Guadalquivir River, sedimentation, which is further

39 reinforced by high erosion rates in the basin, has caused channel narrowing. Another strong human impact on the channel morphology comes from embankment, which prevents the channel to choose its path freely as in the lower Murray River, where the formerly anastomosing planform has been destroyed. The two ephermal rivers are located in rather remote regions and are less impacted by human activity. Little Colorado River which is only slightly modified has been mostly influenced by climate change which has caused annual flows to constantly decline and consequently led to changes in the channel planform.

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Table 2: natural and human impact factors - comparison.

Major Main Planform Dominant Channel River Discharge Sediment Human Characteristics Changes Planforms Impact Decreased Straight 79% Exogenic ~ 30 km3/a avulsion Meandering 15% Perennial Regulation Euphrates River Seasonal Floods Silt (dikes) Large river Diversion Partly Anastomosing 36% Very low gradient straightened Exogenic Upstream: Silt and fine Meandering 68% Perennial Decreasing Tarim River ~ 5 km3/a Seasonal sand Diversion Braided 24% Larger river sinuosity Floods Aggradation Straight 7% Very low gradient Exogenic Meandering 87% Decreased Perennial ~ 5 km3/a Seasonal Fine sized Regulation Straight 13% Murray River avulsion Larger river Floods Aggradation Diversion (dikes) Anastomosing 77% Very low gradient Perennial Downstream: Larger river Fine sized Regulation Partly Straight 73% Guadalquivir ~ 7 km3/a Low gradient Aggradation Diversion straightened Meandering 27% Seasonal Floods

Decreasing Braided to Ephermal Mixture of Meandering 41% Little Colorado ~ 0,3 km3/a Discharges meandering Smaller river sand silt and Straight 33% River Flash Floods (Climate Channel Higher gradient clay Braided 26% Change) migration Straight 52% Ephermal Anabranching 27% Small river ? Medium and Plenty River - - Braided 20% Intermediate Flash Floods coarse sand gradient Anastomosing 23%

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6. Conclusion In this study, six dryland rivers were analyzed regarding the spatial distribution of channel patterns in their alluvial reaches and in order to find potential causes for the evolution of these channel patterns. Furthermore the results for the individual rivers were compared in order to see if they indicate that there is a characteristic spatial distribution of channel patterns for dryland rivers. The methods used in this study were the analysis of satellite images on Google Earth, including measurements of geometrical features of the rivers, and a literature review. While the analysis of satellite images on Google Earth was a successful method to classify channel patterns of perennial rivers, it proofed to be more difficult for ephermal streams as the images only showed the dry rivers. The literature review produced valuable information on the rivers’ hydrological regimes, sediment characteristics and human impact. The findings were however mostly of qualitative nature or not consistent enough to serve as a basis for calculations. For example, calculating the stream power at different river reaches or flow stages would have allowed more certain conclusions about the causes for the development of a certain channel pattern. Nonetheless, changes of gradient, discharge and other environmental factors served as adequate indicators. The study suggests that there is a distinct difference between exotic, perennial dryland rivers and endogenic, ephermal dryland rivers. The analyzed larger perennial streams are characterized by very low gradients and fine sized sediments which promote the formation of meandering and anastomosing channel patterns. Changes of the channel pattern along the course of the rivers are not frequent and mostly appear to be related to a change of gradient or a reduced discharge. The two analyzed ephermal rivers showed a large variety of channel patters which changed frequently. Braided, meandering and straight reaches were found in both rivers. The causes for changes were often less apparent than in the large rivers. It was suggested that because ephermal streams are mostly smaller their channel planform is more easily affected by local environmental settings such as the occurrence of vegetation, the inflow of tributaries and local rain events that only affect parts of the basin. This study can serve as a starting point for further research regarding channel patterns of dryland rivers. The analysis of additional rivers on the basis of more consistent and comprehensive data might reveal further conclusions on the channel planform development of dryland rivers.

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List of Figures Figure 1: the world’s warm drylands (Stephen Tooth, 2000b) ...... 1 Figure 2: flood hydrographs of emphermal streams in desert regions (Reid & Frostick, 2011) 2 Figure 3: channel patterns, modified from Alabyan and Chalov (1998) ...... 4 Figure 4: channel classification based on pattern and type of sediment load, showing types of channels, their relative stability, and some associated variables (Schumm, 1985)...... 6 Figure 5: Taklamakan Desert (left) (Adams et al., 1979) and Tarim River (right) (Yu et al., 2015) ...... 7 Figure 6: decreasing annual discharge at Alar Station (left) (Thevs, 2011) and at Yingbazha Station (right) (Pang et al., 2010)...... 8 Figure 7: changes of Tarim River course along the middle reaches of the river (Thevs et al., 2008)...... 8 Figure 8: monthly runoff at Yengi Bazar (Thevs et al., 2008)...... 9 Figure 9: decrease of annual runoff and sediment load (Yu et al., 2015)...... 9 Figure 10: area of Tugai forest in the Basin (Thevs, 2007) ...... 10 Figure 11: Euphrates River. Left (Adams et al., 1979), right (Flint et al., 2011) ...... 10 Figure 12: left: average yearly precipitation. Right: potential Evaporation (Flint et al. 2011) .11 Figure 13: discharge at Hit and Haditha cities 1948-2007(Al-Ansari & Knutsson, 2011) ...... 11 Figure 14: drylands of Europe, in most parts of southern Spain the moisture index is 0.2-5 (left) (Estrela et al. 1996), watershed of Guadalquivir River (right) (Wikipedia, 2015a) ...... 13 Figure 15: daily average water flow discharge since October 2009 and peak annual water flow discharge during the last century at Marmolejo dam in the Upper Guadalquivir River (Bohorquez et al., 2014)...... 14 Figure 16: annual average discharge since 1912 (Droogers & Immerzeel, 2008) ...... 14 Figure 17: left: drylands of Australia (Adams et al., 1979); Right: River Murray in Southeast Australia (Wikipedia, 2015b) ...... 15 Figure 18: annual runoff and annual potential evaporation in the Murray-Darling River Basin (Murray-Darling-Basin-Authority, 2010)...... 15 Figure 19: left: effect of regulation on the annual flows at eight stations along the Murray from Albury to the Murray Mouth. Right: variation in average monthly flow at the mouth under natural and present conditions (Maheshwari et al., 1995)...... 16 Figure 20: Plenty River in Northern Territory (Tooth and Nanson, 2004) ...... 17 Figure 21: continuous flood records of the Plenty River 1975-1986 (Tooth & Nanson, 2004) ...... 18 Figure 22: Plenty River. Left: cross-sectional profiles. Right: aerial photographs (Tooth & Nanson, 2004)...... 18 Figure 23: Little Colorado River. Left (Adams et al., 1979), right modified: (Wikipedia, 2015c) ...... 19 Figure 24: mean monthly discharge of the Little Colorado River (1948-1992) at a station near Cameron (Birkeland, 1996)...... 20 Figure 25: changes due to the spread of tamarisk in a cross section on the Green River (Utah) (Graf, 1978)...... 20 Figure 26: morphological changes of the Little Colorado near Winslow and the spread of tamarisk (Block, 2014)...... 21 Figure 27: Little Colorado River: Grand Falls near Flagstaff during a flash flood (http://i.ytimg.com/vi/rDSmxbMWKNg/maxresdefault.jpg)...... 21 43

Figure 28: channel migration of Little Colorado River (Block, 2014) ...... 22 Figure 29: changes in peak discharge, precipitation and temperature in the Little Colorado Basin during the past century (Block, 2014)...... 22 Figure 30: measurement of average channel bed elevation...... 23 Figure 31: braided reach of Tarim River during flood season (left) and dry season (right). ...24 Figure 32: measurement of the active channel width. Two exaples from Tarim River...... 25 Figure 33: the channel count method for measuring the braiding intensity (Egozi & Ashmore, 2008)...... 25 Figure 34: measuring the sinuosity index (here: Euphrates River)...... 26 Figure 35: macromeanders in Murray River...... 26 Figure 36: gradient, elevation profile and distribution of channel patterns along Tarim River...... 27 Figure 37: Tarim River. Left: Active channel width. Rigth: braiding index and sinuosity index ...... 28 Figure 38: left: braided pattern of Tarim River downstream of Alar (Google Earth, 2014). Right: high sinuosity meanders in the middle reaches of Tarim River (Google Earth, 2012). Both photographs are taken in the dry season (frame width: 5.5 km) ...... 28 Figure 39: gradient, elevation profile and distribution of channel patterns along Euphrates River...... 29 Figure 40: Euphrates River. Left: total channel width and number of channels. Right: sinuositiy index and number of channels...... 30 Figure 41: Euphrates River. A: anastomosing reach downstream of Lake Assad. B: straight and meandering reach above the Haditha Dam. C: lower Mesopotamian Plain with widespread river-channel network and the confluence with the Tigris upstream of Basra .....30 Figure 42: Guadalquivir River. Left: active channel width. Rigth: sinuosity index...... 31 Figure 43: gradient, elevation profile and distribution of channel patterns along Guadalquivir River ...... 31 Figure 44: left: Guadalquivir, middle reaches near the foothills of the Sierra Morena and some tributaries with reservoirs. Right: lower reaches, channelized, with still recognizable old meanders...... 32 Figure 45: gradient, elevation profile and distribution of channel patterns along Murray River...... 32 Figure 46: Murray River. Left: channel Width. Right: sinuosity index...... 33 Figure 47: Murray River. A: meander cutoffs.: macromeander. C: dry floodplains. D: confined floodplains...... 33 Figure 48: gradient, elevation profile and distribution of channel patterns along Plenty River ...... 34 Figure 49: Plenty River from upstream to downstream. A: upper reach, highly vegetated straight channel. B: stable vegetated islands. C: smaller channel separation. D: braided appearance of the river. E: anastomosing reach. F: flood-out...... 35 Figure 50: Plenty River. Left: active channel width. Right: sinuosity Index...... 35 Figure 51: gradient, elevation profile and distribution of channel patterns along Little Colorado River...... 36 Figure 52: channel width and sinuosity index Little Colorado River...... 36 Figure 53: Little Colorado River. A: meanders before inflow of the Puerco River. B: wandering channel bed near Winslow. C: transitional channel pattern braided/meandering. 37

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List of Tables Table 1: spatial distribution of planform patters – comparison of the analyzed rivers...... 38 Table 2: natural and human impact factors - comparison...... 41

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