Alluvial plain dynamics in the southern Amazonian foreland basin

Alluvial plains are formed with sediments that rivers deposit on the adjacent flood-basin, mainly through crevasse splays and avulsions. These result from a combination of processes, some of which push the river towards the crevasse threshold, while others act as triggers. Based on the floodplain sedimentation patterns of large rivers in the southern Amazonian foreland basin, it has been suggested that alluvial plain sediment accumulation is primarily the result of river crevasse splays and sheet sands triggered by above-normal precipitation events due to La Niña. However, more than 90 % of the Amazonian river network is made of small rivers and it is unknown whether small river floodplain sedimentation is influenced by the ENSO cycle as well. Using Landsat images from 1984 to 2014, here I analyse the behaviour of all 12 tributaries of the Río Mamoré with a catchment in the Andes. I show that these are very active rivers and that the frequency of crevasses is not linked to ENSO activity. The data suggest that most of the sediments eroded from the Andes by the tributaries of the Mamoré are deposited in the alluvial plains, before reaching the parent river. The mid-to-late Holocene paleo-channels of these rivers are located tens of kilometres further away from the Andes than the modern crevasses. I conclude that the frequency of crevasses is controlled by intrabasinal processes that act on a yearly to decadal timescale, while the average location of the crevasses is controlled by climatic or neo-tectonic events that act on a millennial scale. Finally, I discuss the implications of river dynamics on rural livelihoods and biodiversity in the Llanos de Moxos, a seasonally flooded savannah covering most of the southern Amazonian foreland basin and the world’s largest RAMSAR site.

. In the SAFB, the patterns of paleo channels show that it is not the large Río Mamoré but rather its tributaries that have deposited most of the sediments that form the modern alluvial plains (Lombardo et al., 2012;Lombardo, 2014;Hanagarth, 1993). Hence, it is important to further our understanding of the behaviour of these tributaries and the mechanisms controlling alluvial plain sediment 10 accumulation.
Thanks to the availability of Landsat imagery with sub annual temporal resolution covering the last three decades, it is now possible to document river spatial and temporal changes (Buehler et al., 2011;Peixoto et al., 2009;Constantine et al., 2014). Here, I use several time series of LANDSAT images from 1984 to 2014 to analyse the 15 behaviour of the twelve tributaries of the Río Mamoré which have their headwaters in the Andes: the Maniqui, Sécure, Moleto, Isiboro, Chipiriri, Chapare, Chimoré, Sacta, Ichilo, Yapacaní, Piraí and Grande (Fig. 1). The geomorphology of these rivers has never been studied before and hydrological and geochemical data only exists for four of them: the Grande, the Piraí, the Yapacaní and the Ichilo rivers (Guyot et al., 1994, 20 2007). In this paper I analyse the occurrence of crevasses, a breach in the river levee, and river avulsions, the abrupt abandonment of a channel for a new course at a lower elevation Smith, 1998, 2004) and the link between these processes and strong to extreme ENSO events. I investigate how these rivers contribute to the formation of the alluvial plain and affect the local forest-savannah ecotone and forest 25 disturbance. Moreover, the impact of river dynamics on indigenous communities and the rural economy is also explored, with particular emphasis on how these highly active rivers may affect the viability of the planned highway across the National Park Territorio Indígena y Parque Nacional Isiboro Secure (TIPNIS) in Bolivia.

Study area
The SAFB is a largely pristine environment, where rivers move freely across the alluvial plains. The SAFB is drained by three large rivers: the Beni, the Mamoré and the Iténez (o Guaporé). It comprises two regions, the seasonally flooded savannah of the Llanos de Moxos (LM), where nine out of the twelve tributaries of the Mamoré are located, and the northern part of the Department of Santa Cruz, where the remaining three rivers are located (Fig. 1). These rivers drain the Andean catchment of the Mamoré, which includes the second most important rainfall hotspot of the southern tropical Andes (Espinoza et al., 2015). Several paleocourses of the Río Beni have been identified, these seem to be the result of avulsions caused by a fault located a few kilometers 10 from the Andes (Dumont and Fournier, 1994). The Mamoré avulsed during the midto late Holocene (Plotzki et al., 2013) and occupied one of the Río Beni paleocurses (Lombardo, 2014). Stratigraphic cores performed across the alluvial plain have shown that, since the mid Holocene, distributary fluvial systems formed by the Mamoré's tributaries ( Fig. 1) have deposited thick layers of sediments over the southern and 15 central part of the LM (Lombardo, 2014;Plotzki et al., 2015). This region hosts one of the most important collections of pre-Columbian earthworks in Amazonia, including monumental mounds, raised fields, ring ditches, fish weirs, canals and causeways (Lombardo et al., 2011;Prümers and Jaimes Betancourt, 2014). Throughout the Holocene, river avulsions have played a central role in both causing the abandonment 20 and burial of early Holocene archaeological sites (Lombardo et al., 2013) and later favouring the development of pre-Columbian complex societies through the deposition of fertile and relatively well drained sediments (Lombardo et al., , 2012 made it the world's largest Ramsar site (http://www.worldwildlife.org/press-releases/ bolivia-designates-world-s-largest-protected-wetland, last accessed 27 July 2015). The LM constitutes the southern border of the Amazonian rainforest, hence a preferential area to study forest-savannah dynamics (Carson et al., 2014;Mayle et al., 2000;Whitney et al., 2011).

Methods
All the tributaries of the Río Mamoré with a catchment in the Andes have been included in this study. Crevasse splays and avulsions since 1984 have been identified using the Landsat Annual timelapse in Google Earth Engine (https://earthengine.google.org/#intro/Amazon). A total of 315 Landsat subsets have 10 been downloaded from the USGS service LandsatLook (http://landsatlook.usgs.gov/ viewer.html), these include all the river reaches identified for all the years where high quality coverage is available. Images have been transformed into 2 bit (black and white) datasets and channel centrelines have been digitalized using the ArcScan extension of ArcGis software. Meander migration rates have been calculated as in 15 Micheli et al. (2004) and Constantine et al. (2014). Values of the Multivariate ENSO Index (MEI) (Wolter and Timlin, 2011) have been downloaded from http://www.esrl. noaa.gov/psd/enso/mei/rank.html. As in Aalto et al. (2003), only the ranks of the early rainy season months for Bolivia have been included in the analysis. 20 During the 30 year period for which images are available, the Mamoré's tributaries show extremely high activity: 41 crevasses opened up along seven of the twelve tributaries, 29 of which initiated an avulsion process (Tables 1 and 2 Niño years 1987, 1992, 1998and 2003or during La Niña in 1989and 1999. Except for the Río Grande, crevasses are found at an average distance of 68 km (σ 23) from the point in which the rivers enter the alluvial plains. In the case of the Río Grande this distance is 276 km (σ 63.4). All the modern crevasses are closer to the Andes 5 than the mid-to late Holocene terminal splays of the distributary systems formed by these rivers (Fig. 1), particularly in the case of the Río Grande, which during the midto late Holocene deposited a sedimentary lobe 280 km further away from the Andes (Lombardo et al., 2012).

Results and interpretation
Based on their behaviour (

Rivers with low avulsion rates
These rivers have high sinuosity and high meandering migration rates and show very little or no evidence of crevasse splays in the last 30 years (Table 2). They bring most of their total suspended sediments (TSS) to the Mamoré. In the case of the Chimoré, Ichilo and Sacta rivers, there seems to have been no change in the amount 20 of sediments brought to the Mamoré since the 1980s. However, important changes in the sediment load of the Chapare and Chipiriri can be detected. The Chapare and the Chipiriri, a tributary of the Isiboro, fan out from a common catchment. This catchment, although relatively small, includes the second most important rainfall hotspot of the southern tropical Andes (Espinoza et al., 2015), where precipitation easily reaches 25 5000 mm yr −1 . Since 1984, the Chipiriri has been gradually taking over a larger share of the total basin discharge at Villa Tunari, the fan apex, at the expense of the Chapare (Fig. 3) 1998, as it can be observed by the increase in the meandering of the Chipiriri (Fig. 3f). A similar process was described in the piedmont of the Chaco basin, where stream captures can change the size of a given river's drainage basin (Baker, 1977). The width of the Chipiriri is about one third of the Chapare in 1986 (Fig. 3a), but by 2014 the Chipiriri is far wider than the Chapare (Fig. 3e). Between 1984 and 2014 two crevasses 5 opened up in the Chipiriri, at 61 and 65 km downstream from Villa Tunari; none of these crevasses led to avulsions.

Rivers avulsing on a multi-decadal time scale
The second group comprises rivers with one or two full avulsions since 1984. The Yapacaní, a tributary of the Río Grande, started an avulsion before 1984 in its distal part, about 40 km before reaching the Grande; it was completed in 1994. The DEM in Fig. 1 shows that the Yapacaní formed a 10 000 km 2 fan at its exit from the Andes, which, in its middle part, is about 15 m higher than its surroundings. Other than the Río Grande, the Yapacaní is the only river, of the twelve studied, that created such a large convex up topography. 15 The Isiboro shows evidence of five distinct crevasses, located between 70 and 85 km downstream from Villa Tunari. In 1984, when the record begins, a crevasse splay was already triggering an avulsion. By 2014, when the record ends, the avulsion had not yet been completed, as part of the water still flows through the original channel. The Isiboro is currently depositing its sediments on the invaded flood basin through 20 a sequence of crevasses and avulsions that expand downstream (Fig. 4). More than 200 km 2 have been covered with alluvia, causing important changes in the landscape. Figure 4a and b show how, between 1996 and 2013, a lake was completely infilled and erased from the landscape. As the Isiboro receives water from the Chipiriri, which in turn is receiving an increasingly larger share of the water flow of the Chapare, an 25 important part of the sediments that the Chapare used to bring to the Mamoré are instead being deposited on the avulsion belt of the Isiboro. The Sécure and Moleto rivers' avulsions began by channel annexation, but then changed to a progradational style because the annexed channel was too small to convey the whole diverted flow. Currently, overspills and new crevasse splays are depositing most of the Sécure and Moleto rivers' sedimentary load in the floodplain (Figs. 5 and 6). 5 In the case of the Sécure, a full avulsion, initiated in 1986, was completed in 2006. With this avulsion, the river, which was a tributary of the Mamoré, occupied a preexisting channel and became a tributary of the Río Tijamuchí. This channel is not large enough to accommodate the total flow of the Sécure, causing repeated large floods (Fig. 5) and new crevasse splays and avulsions as part of the process of building a new course (Fig. 5a). The diversion sites are located between 40 and 50 km downstream from the point where the river enters the alluvial plain, with recent diversion sites forming downstream from the older ones. The planned road from Villa Tunari to San Ignacio de Moxos cuts through this very region in which the Sécure is building its new course. 15 The process of avulsion of the Sécure that was completed in 2006 had devastating effects on local inhabitants, as twenty indigenous communities were settled along the section of the river channel that was cut off. The abandoned channel now holds standing water, triggering a sharp increase in waterborne diseases and limiting peoples' access to fish resources and navigation courses (Sécure, el río se está 20 muriendo, Escape -Diario La Razón, last accessed 12 May 2015 http://la-razon.com/ suplementos/escape/Secure-rio-muriendo_0_1729627098.html).
The Moleto is a tributary of the Río Isiboro. The analysis of satellite images shows that since 1988 six crevasses have opened up, three of which initiated a process of avulsion. These are located between 6 and 60 km from the point at which the river exits 25 the Andes. In 1988 a crevasse started an avulsion process that went on for 12 years (Fig. 5). This process was interrupted in the year 2000, when the Moleto annexed a pre-existing channel and avulsed about 60 km upstream from the 1988 diversion site (Fig. 5) the avulsion was completed in 2001, a series of processes started to transform the original channel. In the upper part, the new flow has been accommodated by the preexisting channel thanks to the formation of larger meanders (Fig. 6c). At about 30 km downstream from the diversion site, the channel collapses into a series of crevasses ( Fig. 6d and e). Two scenarios are possible for the evolution of the Río Moleto. The 5 partial avulsion that began in 2002 (Fig. 6a) could be completed and the river could establish a new course further north, or the 2002 crevasse could heal and the totality of the river's water and sedimentary load would then go to the channel annexed in the year 2000. If the latter case takes place, it is likely that there will be another avulsion about the middle of the annexed channel, where crevasse sites are currently moving 10 backwards ( Fig. 6d and e). As most of the sediments are deposited through crevasses along the annexed channel, and most of the water is diverted into the floodplain, the second half of the pre-existing channel does not show any change in its meandering rate (Fig. 6b). The planned road from Villa Tunari to San Ignacio will cut through the Moleto's avulsion belt, as well as the Secure's avulsion belt (Fig. 6).

Rivers avulsing on a sub-decadal time scale
Rivers belonging to the third group, the Maniqui, the Piraí and the Grande, show a decreasing discharge down-flow, forming distributive fluvial systems (Nichols and Fisher, 2007), with avulsions completed immediately after the formation of the crevasses. 20 The Río Maniqui is the first river south of Río Beni. The Maniqui's paleo-channels cover a large extent of the western part of the LM (Fig. 1). Up to date, the only study describing the Maniqui is a report by Hanagart and Sarmiento (1990) where they notice that, during the rainy season, the Maniqui's overflow forms sheet-floods of turbid water that reach the Río Rápulo as black waters after being filtered by the vegetation. In the 25 last 30 years this river has been highly active. A total of ten crevasse splays have been identified, seven of which led to an avulsion (Fig. 7). These are located between 60 and 90 km downstream from the point at which the Maniqui enters the alluvial plain. Two crevasses opened up before 1984. Until 1994, the Maniqui was connected with the Rápulo, which is a tributary of the Mamoré. The connection with the Rápulo was lost when the avulsion that started with the pre-1984 crevasse was completed and the former channel was abandoned. Since then, the location of the crevasse splays has gradually moved upriver. The location of the 2014 crevasse is approximately 30 km 5 south-east of the 1997 one (Fig. 7). Following this upward movement of the levee breakage, new areas of the alluvial plain have been flooded by the Maniqui every year. Those areas that have been flooded for several consecutive years, for example, the region in the upper part of Fig. 7 between 1984 and 1997, show a change in the land cover from savannah to forest (Fig. 8) due to the deposition of alluvia and a change in  Figure 7 shows that crevasses immediately followed by avulsions can happen on a yearly basis. The sharp drop in river discharge and the frequency of the avulsions suggest that the river bed becomes seasonally perched during the dry season. While the infilling of the channel progresses, the point of the next siltation/logjam formation moves upwards and so does 20 the location of the next crevasse. This sequence of events probably continues until the crevasse opens up at a point where the discharge is large enough to force a full avulsion, limiting the formation of other crevasses upstream. Given the speed at which crevasses are moving upstream, it will probably be less than a couple of decades before the river takes a completely new course. Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | between 50 and 110 km from the point in which the Piraí enters the alluvial plain. Of these 11 crevasses 9 initiated an avulsion. The Crevasse splays are concentrated in two regions: one in the proximity of the city of Montero and another one about 30 km further north (Fig. 9). During the period between 1977 and 1981, measurements at gauging stations located before and after Montero showed an increase in the annual 5 discharge from 13 to 20 m 3 s −1 . The total suspended sediments (TSS), on the other hand, showed a decrease of more than 50 % (Guyot et al., 1994). The reduction in the TSS of the Piraí is probably larger than 50 %, as several rivers join the Piraí between the two stations. Between 1984 and 1988, the southern part of the river ended in a terminal crevasse just a couple of km west of Montero (Fig. 9a). An avulsion in 1988, which was completed in 1990, now connects the two reaches of the river. This new setting is maintained until 2014, with the exception of 1993 when the river briefly switched back to the pre-1988 channel for one year (Fig. 9b and c). The aggradational area occupied by the crevasse splay deposits close to Montero is now under cultivation and the city of Montero has greatly expanded, occupying the very same areas that were 15 under severe flooding and high rates of sediment deposition until 1993. In the northern reach, a crevasse in 1986 caused the flooding of a large area and the death of the vegetation cover (reddish area in Fig. 9). After a second crevasse in 1990, the river underwent two consecutive avulsions in 2008 and 2010. The river channel has been artificially straightened, first in 2010 and again in 2013 (see upper part of Fig. 9). If it 20 persists, this artificial straightening of the river channel will probably push the zone of future crevasses and avulsions further downstream. Río Grande, the most south-eastern of the 12 tributaries, has by far the largest Andean catchment of all the Mamoré tributaries and, when it leaves the Andes, it carries 138 Mt yr −1 of TSS (Guyot et al., 1996). The Río Grande exits the Andes forming 25 braided channels and becomes a single-thread channel at about the same point where avulsions begin (Fig. 10). At the town of Abapo, where the river enters the alluvial plains, the Río Grande carries 138 Mt yr −1 of suspended sediments (Guyot et al., 1996). Except for the first one in 1986, all the crevasses led to avulsions. The first avulsion took place in 1993. In 2008, after the last recorded avulsion, the Río Grande established its modern course. It has been estimated that about half of the Río Grande's TSS is deposited in the floodplain after its exit from the Andes (Guyot et al., 1996). However, this could be a major underestimation. The 5 estimate is based on the comparison of measurements between a gauging station located at Abapo, on the Río Grande (AP, TSS 138 Mt yr −1 ; discharge 330 m 3 s −1 ), and a station on the Mamoré, in the proximity of Trinidad (PG -Río Mamoré at Puerto Varador, TSS 63 Mt yr −1 ; discharge 2970 m 3 s −1 ) (Guyot et al., 1996). The estimate implicitly assumes that other tributaries of the Mamoré do not represent an important 10 contribution to its TSS. However, several other rivers join the Río Grande and the Mamoré between Abapo and Trinidad: the Ichilo, the Piraí, the Chimoré, the Chapare, the Sacta, the Isiboro and the Yapacaní. Data on the TSS of these rivers is very limited, but they cause an almost tenfold increase in river discharge from AP (330 m 3 s −1 ) to PG (2970 m 3 s −1 ). Because of the important contribution of these tributaries to the 15 Mamoré's discharge, and in light of the high meandering rate of some of them, it can be safely assumed that an important part of its TSS at the station PG comes from these other tributaries, and not the Río Grande. Also the analysis of the meander migration rate of the Río Grande just before joining the Mamoré (Fig. 11)  The behaviour of these three rivers seems to be controlled by the seasonal lowering of the water table that takes place at the end of the rainy season. This causes a sharp reduction in the rivers' sediment transport capacity, increased channel infilling and likelihood of logjam formations. However, as described in the similar case of Río Pilcomayo in the Chaco plains (Martín-Vide et al., 2014), it could also be the result 5 of an increased sediment discharge due to modern landuse change in the Andes.

Discussion
Crevasse splays and river avulsions are the most important depositional processes in alluvial plains (Slingerland and Smith, 2004;Smith et al., 1989). Despite a large body of studies, the exact mechanisms controlling crevasse splays and river avulsions are not 10 entirely understood (Hajek and Edmonds, 2014;Stouthamer and Berendsen, 2007;Ashworth et al., 2004). When the various processes that push the river towards the avulsion threshold proceed at a faster pace than those that act as triggers, the latter control the frequency of crevasses and, eventually, avulsions (Jones and Schumm, 1999). It is generally accepted that in southern Amazonia the trigger behind the 15 formation of crevasses in large rivers is the sudden increase in river discharge that follows extreme precipitation events linked to La Niña (Aalto et al., 2003). In the SAFB, research suggests that the frequency of river crevasse formation increases during La Niña events (Aalto et al., 2003), because higher precipitation in the eastern flanks of the Andes is accompanied by reduced precipitation in the lowlands. This increased 20 precipitation towards the Andes causes an important rise in the rivers' discharge, whilst the floodplain water table remains relatively low. Under these conditions, the formation of crevasses becomes more likely because of the higher hydraulic head (Slingerland and Smith, 1998). The thick deposits of sediment in the Mamoré and Beni floodplains are believed to be the result of crevasse splays, that formed in this way (Aalto et  conclude that these deposits, triggered by La Niña events, cause most of the flood-plain sediment accumulation across the lowland plains. The new data here presented challenges both the importance of large rivers in controlling alluvial plain dynamics in the lowland plains of the SAFB and the role of La Niña in controlling the timing of crevasse splays. 5 The study shows that small rivers are highly active and play a dominant role in shaping the SAFB alluvial plains. These rivers, and in particular the Sécure, Isiboro, Moleto, Maniqui, Piraí and Grande rivers, show extremely reduced meander migration rates downstream from where the crevasses opened up (for example Fig. 6), probably as a consequence of a decrease in their sedimentary load. Most of the sediments, along with associated nutrients and carbon, eroded from the Andean catchment of the Mamoré are therefore sequestered in the flood plains of its tributaries through the formation of crevasse splays and avulsions. This can explain why about half of the total sediment flux discharged from the Bolivian Andes is deposited in the SAFB (Guyot et al., 1996), including most of the sand fraction (do Nascimento Jr. et al., 2015). It 15 also explains why the Río Beni, which only has one tributary with a catchment in the Andes (the Río Madidi), brings to the Madeira three times more sediments than the Mamoré (Guyot et al., 2007;Aalto et al., 2002), despite the fact that the Beni has a smaller catchment and a water discharge of 3070 m 3 s −1 , vs. the 5080 m 3 s −1 of the Mamoré (Guyot et al., 1996). This reinforces the observation that, in the mid Holocene, 20 the tributaries of the Mamoré deposited thick layers of sediments over the southern and central part of the LM (Lombardo, 2014). This research adds new evidence to the idea that most of the modern continental sedimentary basins are filled primarily by distributive fluvial systems (Weissmann et al., 2013;Hartley et al., 2010) and shows that the SAFB is an excellent natural laboratory for the study of river processes in 25 sedimentary basins. The study of the tributaries of the Mamoré over a period of thirty years shows no link between the timing of the crevasses and La Niña events (Table 1). The behaviour of the rivers studied, and in particular the Maniqui, Piraí and Grande, suggests that, on a year ESDD 6,2015 Alluvial plain dynamics in the southern Amazonian foreland basin to decade time scale, the activity of southern Amazonian small rivers is controlled by channel siltation and logjams. These are caused by the rivers' high sedimentary load combined with a perched river bed during the dry season and an extremely low alongvalley slope, which not only bring the rivers to the threshold conditions for the formation of crevasse splays, but also trigger the crevasses. In this setting, the decrease in 5 average precipitation over the SAFB experienced in recent years (Espinoza Villar et al., 2009) and the lengthening of the dry season (Fu et al., 2013) increase the frequency of river crevasses and their formation closer to the Andes. The fact that all the modern crevasses are closer to the Andes than the mid-to late Holocene distributary systems formed by the rivers in groups 2 and 3 suggests, on a millennial scale, a common 10 climatic (Mayle et al., 2000;Baker, 1977) and/or neo-tectonic (Lombardo, 2014;Dunne et al., 1998) control over the shifting of these rivers' depozone. A lack of discrete deposition events has been reported along the Mamoré floodplain after 1971, which could have been caused by a change in regional climate that took place around this time (Aalto et al., 2003). Thus, further research is needed in order to assess whether 15 and how this change could have affected the dynamics of the Mamoré tributaries. Further research is also needed in order to better understand the exact mechanisms behind the formation of crevasses; the contribution of La Niña driven sheet sand deposits to the total floodplain sediment deposition of the Mamoré's tributaries; and the shift of the tributaries' sedimentary depozones. Introduction

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Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | example the reddish area in the upper part of Fig. 9). Aquatic animals, especially migratory fish, must continuously adapt to the frequent changes in the fluvial network. The regime of continuous river fragmentation and forest disturbance could explain the unusually high fish and plant biodiversity found in this area (Pouilly et al., 2004;Thomas, 2009). 5 It is important that Bolivian policy makers take into account the dynamics of the SAFB's river network in order to mitigate future risks to the local population and to better assess the feasibility of new development plans. In particular, river avulsions can have catastrophic consequences on indigenous communities, as shown in the case of the Sécure. If the trend in the formation of new crevasses continues in the future, it is likely that the rivers Maniqui, Moleto and Isiboro change their course in the next decade or two. When this happens, the indigenous communities settled along these rivers will have to be relocated. The frequent avulsions of the Piraí following the 1988, 1993 and 1994 crevasses suggest that it could avulse again in the near future in this same diversion site, flooding the 15 city of Montero, with important economic and human costs. The planned highway cutting through the Territory and National Park Territorio Indígena y Parque Nacional Isiboro-Secure (TIPNIS) has received strong protests, for and against it (see for example http://www.bbc.com/news/world-latin-america-15138784 and http://www.bbc. com/news/world-latin-america-16804399). Nevertheless, the technical feasibility of the 20 project has received little attention. The road, linking Villa Tunari to San Igancio de Moxos, will go through areas that are under constant threat of major floods resulting from the frequent formation of crevasse splays and the avulsion of the rivers Moleto and Sécure (Figs. 5 and 6). It is likely that, if built, the road will require costly and continuous maintenance works. Moreover, the road will dam these rivers, with unpredictable effects 25 on the aquatic ecosystems of this still largely pristine environment. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | do Nascimento Jr., D. R., Sawakuchi, A. O., Guedes, C. C. F., Giannini, P. C. F., Grohmann, C. H., and Ferreira, M. P.: Provenance of sands from the confluence of the Amazon and Madeira rivers based on detrital heavy minerals and luminescence of quartz and feldspar, Sediment. Geol., 316, 1-12, doi:10.1016/j.sedgeo.2014.11.002, 2015. Dunne, T., Mertes, L. A. K., Meade, R. H., Richey, J. E., and Forsberg, B. R.: Exchanges of 5 sediment between the flood plain and channel of the Amazon River in Brazil, Geol. Soc. Am. Bull., 110, 450-467, doi:10.1130/0016-7606(19982, 1998. Espinoza, J. C., Chavez, S., Ronchail, J., Junquas, C., Takahashi, K., and Lavado, W.: Rainfall hotspots over the southern tropical Andes: spatial distribution, rainfall intensity, and relations with large-scale atmospheric circulation, Water Resour. Res., 51, 3459-3475, Introduction

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