Operational Challenges and Potential Environmental Impacts of High-Speed Vessels in the Brazilian Amazon

Abstract

In the Amazon basin, there are few terrestrial communication routes between cities, so river transport is often the only viable alternative for people and cargo. Although high-speed vessels are common in the region, they face operational challenges that endanger crews and passengers. Moreover, their use can impact the environment in various ways. This paper discusses operational difficulties related to the use of high-speed vessels in the Brazilian Amazon, and details possible environmental impacts, based on literature reviews and photographic information from field surveys in the state of Amazonas. The main operational challenges include interacting with complex flows, the possibility of colliding with fixed and moving objects, and the limitation of navigation routes. The main environmental impacts were classified as related to vessel motion, the propulsion system, and waste disposal. There is a need for increased innovation and research into marine applications and sustainability topics. Technical information must be disseminated more widely, including to remote regions. If the region’s sustainability is to be improved, riverine infrastructure must be enhanced and new technologies adopted.

1. Introduction

The Amazon river basin is one of the largest on the planet, and is responsible for the concentration of the majority of the world’s biodiversity [1]. The Brazilian Amazon plays a significant role in this regard, with its intricate network of rivers and tributaries serving as the primary means of transportation for people and cargo due to the lack of terrestrial routes [2,3]. The rivers within the Amazon region exhibit different morphologies, chemical compositions, and hydrodynamic characteristics [4]. On these rivers, it is common to find ports for small- and medium-size regional vessels (Figure 1a,b). In the context of long-distance travel, fast vessels of different sizes (e.g., Figure 1c–f) facilitate the execution of pivotal social activities. These activities encompass the transportation of people between communities, the provision of emergency assistance, the promotion of school attendance, and the acquisition of food resources including fishing [5]. It is worth noting that in many riverine communities, access for large vessels is limited, either due to the reduced depth of the rivers or hydrodynamic conditions. In such cases, smaller vessels play a crucial role in transportation. It is widely acknowledged that a number of operational risks are associated with such vessels. These include a lack of infrastructure to address mechanical problems, human error, and unexpected environmental conditions, which may result in accidents [5,6]. Such accidents have been shown to engender significant negative economic, social, and environmental impacts [7].

The hydrodynamic behavior and morphological characteristics of the rivers in the region vary significantly between their upper and lower courses [8,9], with each area presenting particular challenges for inland vessels. For instance, in upper courses, the rivers and their tributaries generally exhibit rapid, shallow flows, thereby necessitating the restriction of certain navigation routes to smaller vessels. Conversely, in the lower courses, the rivers are usually wider and deeper, thus permitting the utilization of larger vessels. In estuaries, where rivers discharge into the ocean, the complex flows of waves, currents, and tides can make navigation more susceptible to dynamic loads [8]. The extensive area of the Brazilian Amazon renders the monitoring of high-speed vessels operations, the assurance of their safety, and the mitigation of their environmental impacts challenging [5].

Several recent studies have been published to contribute to the United Nations (UN) Sustainable Development Goals (SDGs), which are also of concern of the International Maritime Organization (IMO, [10]). These studies have been mainly focused on preserving health and well-being when using vessels. Some of the topics covered include the risks of river passenger transport vessels [11], the operational safety of river convoys [12], improving the sustainability of river transport operations [3], challenges in initiatives to prevent accidents with passenger vessels [6], and possible environmental impacts resulting from vessel accidents [7]. Maia and Said [13] carried out studies on the efficiency of vessels in the Amazon. More recently, Fontes et al. [5] discussed the main challenges in preventing accidents involving high-speed vessels and described the main risk conditions. The authors presented the main types of regional high-speed vessels, including a description of accident risks and some mitigation strategies. While the research highlighted the main risks in navigation, and was supported by information obtained in some cities in the state of Amazonas, it did not detail the specific challenges that occur in the upper courses of the rivers. Addressing this gap, the present study builds on the work of Fontes et al. [5] by discussing the primary operational challenges of high-speed vessels in the Brazilian Amazon, with a particular emphasis on the upper river courses, where many communities rely almost exclusively on small motorized boats for transportation. Moreover, the present study extends the research by Fontes et al. [7], by discussing potential environmental impacts deriving from the use of high-speed vessels in the Brazilian Amazon. This study also extends previous work, to provide an integrated approach to identifying challenges in preventing marine accidents and increasing the sustainability of vessel operations in the Amazon region.

The discussions presented were supported by bibliographic reviews primarily from articles published in indexed journals, as well as graphical information collected through fieldwork. This provided insights into needs and possible solutions. To illustrate this, the case study of the Negro River, a major river in the state of Amazonas, is considered. It is expected that this study will contribute to the third and thirteenth sustainable development goals from UN, which aim to improve health and well-being, by identifying the need to increase the safety of regional vessels and addressing climate change by identifying possible environmental impacts due to the use of high-speed vessels.

2. Methodology

2.1. Region of Study

This study considered the vessels operating on the rivers of the central Amazon, particularly the River Negro. This river flows from the most northerly city in the state of Amazonas, São Gabriel da Cachoeira, to an area near its capital, Manaus (Figure 2a). Figure 2a shows a map of the state of Amazonas, displaying the main rivers and tributaries, principal cities and their populations, and elevations. The River Negro begins close to the borders with Colombia and Venezuela, at an altitude of over 100 m, where it tends to be steep and narrow (Figure 2b). Brazil’s highest mountain, Pico da Neblina (2994 m.a.s.l), is located here in a protected area known as Pico da Neblina National Park [14].

The lower course of the River Negro is home to important cities, such as Manaus and Novo Airão. The latter is the entrance to the Anavilhanas National Park [15]. The region has an extensive network of rivers, with exuberant vegetation (Figure 2c). Near Manaus, the River Negro joins the River Solimões [4] (Figure 2d), to form the regionally known River Amazonas that flows into the Atlantic Ocean. Manaus is the main hub in the basin, connecting inland navigation routes in the central Amazon, with some terrestrial highways linking the city to other states. In other cities in the region, communication is limited to marine vessels or aircraft only because there are no highways.

Figure 2. (a) Physical–political map of the state of Amazonas, Brazil. Population 3,941,613 people, 2020 census. (b) A beach in River Negro at São Gabriel da Cachoeira. (c) River Negro and Anavilhanas archipelago in Novo Airão. (d) Confluence of the rivers Negro and Solimões, near Manaus. Source: (a) Instituto Brasileiro de Geografia e Estatística (IBGE, [16]). The figure was used with the authorization of IBGE (protocol number 20240905003). A higher resolution version of this map is available in IBGE [16]. Credits: (b) João Carlos Fontes de Araújo (in 2025); (c,d) Jassiel V. H. Fontes (in 2025).

Figure 2. (a) Physical–political map of the state of Amazonas, Brazil. Population 3,941,613 people, 2020 census. (b) A beach in River Negro at São Gabriel da Cachoeira. (c) River Negro and Anavilhanas archipelago in Novo Airão. (d) Confluence of the rivers Negro and Solimões, near Manaus. Source: (a) Instituto Brasileiro de Geografia e Estatística (IBGE, [16]). The figure was used with the authorization of IBGE (protocol number 20240905003). A higher resolution version of this map is available in IBGE [16]. Credits: (b) João Carlos Fontes de Araújo (in 2025); (c,d) Jassiel V. H. Fontes (in 2025).

2.2. Procedure of Analysis

The descriptions of operational challenges and environmental impacts were supported by bibliographic reviews and pictures taken during field surveys. The photographic evidence was collated during field surveys around São Gabriel da Cachoeira, in the upper course of the river, and Manaus and Novo Airão, in the lower course. The literature review included regional media, and recent academic works, prioritizing scientific articles selected from journals indexed in Scopus or Web of Science databases. Keywords used to search for papers included “marine accidents”, “marine environment risks”, “ship environmental impact”, “operational risks of ships”, “marine pollution”, “ship effects in the environment”, and correlated phrases. The information obtained was used to support discussions on the topics.

The regional vessels referred to in this paper are those used for various activities in the Brazilian Amazon, as described by Fontes et al. [5] and illustrated in Figure 1. As detailed by these authors, the high speed of a vessel can be defined when its Froude number (F_n) exceeds, approximately, the value 0.5 [17]. This number is defined as the ratio between the vessel velocity and the root square of the product between the acceleration of gravity and vessel’s length (F_n = U/(gL)^1/2). Fontes et al. [5,6] presented a preliminary classification of regional Amazon high-speed vessels commonly used in the central Amazon, including the description of some safety challenges. However, the reader is referred to [17,18] for more technical information regarding the classification, particularities, behavior, and safety of high-speed vessels.

The results in Section 3 and Section 4 present the operational challenges analysis and the proposal of possible environmental impacts, respectively.

3. Operational Challenges

3.1. General Considerations for High-Speed Vessels

3.1.1. Types of Vessels

For the local population, inland navigation is a vital means of transport, allowing them to travel between the riverside communities and urban areas, and access essential services, such as food, healthcare, and education. The Negro River has many tributaries that connect riverine communities, including those belonging to indigenous ethnic groups.

In urban areas, there are some main ports, as discussed by Fontes et al. [5], where large vessels arrive, transporting people and cargo from surrounding areas and from main cities like Manaus. These large vessels are known regionally as “Recreios”, “ferryboats”, and “Expressos” [6,19], and depending on the river conditions and operation mode, these can take several hours (as “Expressos”) or even days (as “Recreios” and ferryboats) to transport passengers and cargo from Manaus, the main transportation hub. Port installations present accessibility challenges for vessels of different sizes. Most vessels usually dock at improvised ramps or naturally lower points on the riverbanks [5,20]. Accessibility to ships in different Amazon environments is a need that has previously been reported [5,21].

Small regional boats are commonly employed to access shallow or breadth-restricted regions. Although modern, well-equipped boats are available in the region, the most common are regional boats [5,22]. These vessels need to be able to operate with a restricted draught, be lightweight, and be able to navigate in shallow waters with complex flow behavior, including rapids. They also require propulsion systems that can operate close to the free surface. Examples of such boats are the “Voadeiras” (Figure 1e) and the “Rabetas” (Figure 1f); see [5,22] for details. In remote regions, it is also common to use artisanal boats built from wood, sometimes from a tree trunk, which are usually propelled by stern outboard systems, as seen in “Rabetas”. Small, motorized boats are commonly used to sell local products, such as handicrafts and agricultural products.

3.1.2. Seasonal Variations

Depending on the water depth available in the different seasons (wet and dry), vessels of different sizes navigate the various rivers of the Brazilian Amazon. In the study area, the River Negro and its tributaries have very little signalization to assist crews, and remote regions have scarce infrastructure for specialized shipbuilding and repair. These are common challenges in many Amazon rivers, as also described by Fontes et al. [6].

In the Central Amazon, particularly around Manaus, the minimum water levels recorded in recent years have been exceptionally low. During these periods, severe river erosion, including landslides, has impacted local communities, destroying infrastructure such as ports and schools. Moreover, extreme droughts have caused entire communities to become isolated from the main rivers, and have contributed to shortages of food and water [23].

3.2. Operational Challenges Identified in the Upper-Course River Region

Previous studies have documented the safety risks associated with marine accidents involving large passenger vessels [6] and high-speed craft [5] in the Brazilian Amazon. However, the regions discussed in those works did not detail the operational challenges in the upper courses of the rivers. For example, Figure 3a,b, taken around Novo Airão, illustrate some of the challenges associated with the harsh environmental conditions, such as rain and waves produced by other vessels, respectively, as reported by Fontes et al. [5].

Figure 3c–l, taken around São Gabriel da Cachoeira, illustrate some of the challenges related to smaller, high-speed vessels that operate in the upper courses of the rivers, where depths tend to be lower and currents more rapid [8]. Here, the rivers are often obstructed by tree branches and trunks (Figure 3c), and turbulent waters (Figure 3d,e), rocks (Figure 3f), or sand banks (Figure 3g). These obstacles may slow the journey as the crew prefer to stop and plan the most convenient route. Collisions often damage the hull and propulsion system of the vessels. As most boats used in such waters have light-material hulls, they are easily damaged, increasing the possibility of flooding or capsizing. Experienced boat operators, with practical, local knowledge, are required. They may even have to lift the boat out of the water and transport it over land for long distances. Trails in the forest can be found close to the water, with tree trunks distributed transversely on the ground to help with such displacements (Figure 3h). It may take several hours, or even days, to reach remote communities in the region, which may compromise health and food security, especially in dry seasons.

As seen in Figure 3i, the geomorphological characteristics of the upper course of rivers, which present with areas of variable altitude, can produce ramps of water or even small waterfalls in river tributaries. Navigating these environments can compromise the stability of boats, causing them to collide with rocks or allowing water to enter.

In some circumstances, interactions between river flow and underwater obstacles can produce localized flows that resemble stationary waves (Figure 3j) or swirling flows (Figure 3k). These flows could destabilize boats traveling at a high speed, contributing to dynamic instabilities or impacts with submerged obstacles.

The location of São Gabriel da Cachoeira is in the Tropics, where tropical storms produce complex environmental conditions. These storms can cause strong winds and heavy rain, which in turn interact with the river’s surface, forming small waves. Some works have documented the potential effects of the strong winds that occur in the Amazon environment [24,25].

Considering the information presented above,

Table 1. Operational challenges for high-speed vessels in the upper courses of rivers in the Brazilian Amazon and their main related issues.

Operational Challenge	Issues
Navigation in shallow and/or rapid waters is dangerous	Lightweight hulls with flat bottoms are needed. The propellers tend to operate close to the water surface. Acceleration effects can cause displacement of cargo and passengers. Vessels must have good maneuverability and experienced pilots. Sudden motions can exceed recommended acceleration thresholds, affecting people’s safety.
Impacts with fixed or moving rocks are an ever-present threat	Hull damage (deformation, fractures). Shaft–propeller system damage. If the hull is perforated, the vessel would let in water. Capsizing. Injuries due to impacts, particularly at high speeds.
Interaction with small waterfalls is perilous	A loss of stability is increased if the hull comes out of the water. The bow may become submersed and then water could enter the boat. Sudden motions may propel people into the water.
Interaction of currents with underwater obstacles can cause waves or swirling flows	Dynamic instabilities are possible, particularly if the vessel is travelling at a high speed. Capsizing is a possibility if the formed flows are encountered at a high speed. The water may lie over rocks and resemble waves, which may confuse operators, and thus contribute to collisions.
Sand banks are often difficult to see	Grounding and collision damage to the hull or propulsion system. The vessel may need to be lifted off the sand bank and perhaps carried for a long distance. If the sand bank is extensive, it could cause water bodies to become isolated, and lead to subsequent difficulty in accessing fuel and essential services.
Severe environmental conditions make navigation more difficult	Perturbation of the river surface can cause waves that may affect small vessels. Strong winds could induce additional loads on the vessels, contributing to instability or capsizing. Heavy rain can flood open-hulled vessels.

summarizes the operational challenges identified in fieldwork in São Gabriel da Cachoeira, which was considered as representative of the upper-course regions of other rivers.

3.3. Suggested Strategies to Overcome the Operational Challenges

This section describes possible strategies that could be implemented by the government, industry, and academia to overcome the above operational challenges. Research and innovation alternatives should be sought to improve the design, construction, and operation of the river crafts working in these waters.

3.3.1. Research and Innovation

• Application of knowledge in naval architecture and marine engineering: More advanced specific technical and scientific knowledge is required in analyzing every part of vessels’ project and operation, particularly with regard to structural integrity, materials, hydrodynamics, and propulsion. In the study region many vessels are built using age-old empirical techniques [26,27]. Digitalization and the three-dimensional design of hulls could allow for better analysis of stability, hydrodynamics, and structural optimization employing numerical simulation methods.
• Analysis of structural behavior: The structural analysis of the vessels used in the area would improve our understanding of their resistance during critical situations, such as collisions with rocks or sand banks. Investigating the stresses and deformations on hulls could be done, as proposed for ship grounding analysis by Zhou et al. [28]. Approaches based on Finite Element Methods (FEMs) to investigate structural behavior [29] are commonly applied elsewhere. Composite or plastic-based materials could be considered to make more flexible hulls. Moreover, composite patch techniques to repair damaged hulls, as proposed by Bianchi [30] for aluminum hulls and by Bianchi et al. [31] for underwater structures, could be considered, as they increase the useful life of vessels.
• Investigation of the hydrodynamics of vessels: High-speed vessels are used around the world for transportation, racing, and military purposes [32]. A more thorough analysis of the hull behavior of moving vessels improves their reliability in different conditions. Validated computational fluid dynamics (CFD) methods can be used to examine the hydrodynamic behavior of high-speed boats, exploring parameters such as drag and lift forces, and flow kinematics, both in steady-state or transient analyses [33]. Research into the dynamic behavior of the vessels in complex flow environments, such as those in Figure 3e, would require the use of time-domain simulations because of the transient flow conditions. In harsh environments, the effects of wind and waves on vessels could also be investigated. Again, CFD approaches can be used in investigating complex boat–flow interactions, either with mesh [34] or meshless [35,36] methods. Coupled FEM and CFD approaches [37] would allow for a more detailed fluid–structure interaction analysis. To improve the vessels’ dynamic stability, control systems such as interceptors or flaps [17] are often suggested. However, these could easily be damaged in the interaction of the vessels and obstacles.
• Investigation into the propulsion characteristics of the vessels: Navigating in rapids, see Figure 3d–f, using outboard motors, requires experienced operators who can avoid obstacles. In the study region, journeys can sometimes be undertaken in complex hydrodynamic conditions thanks to the operator’s knowledge of the region. However, there are always risks as conditions change throughout the year. Fixed and moving obstacles, such as tree trunks and rocks, are also an ever-present risk. It may be advisable to provide some degree of structural protection for the propellers using shock-resistant materials. Research could be encouraged on integrating shock-absorption technologies into motor-shaft-propeller systems to minimize the effects of impacts. Finally, the use of composites as potential replacements for propellers should be evaluated, as proposed by Islam et al. [38]. Although these authors described the pending tasks of this type of approach, they also presented potential benefits of marine composite propellers. Among them, it is possible to mention the high strength-to-weight ratio, damping, design flexibility, and susceptibility to low-velocity impacts with floating and submerged debris and aquatic animals.
• Applying national and international regulations to improve safety: Currently, marine activities in the Brazilian Amazon are mainly regulated by the Brazilian Navy through Maritime Authority Norms (NORMAM, [39]). However, it is possible that navigation with small high-speed crafts in remote regions presents particularities that need to be reviewed for implementation purposes. New recommendations related to the small vessels under study could be found in updates provided by International Maritime Organization (IMO) and in scientific literature on new technologies and mechanisms that would decrease damage to vessels operating in risk conditions [18,40].
• Possible new technologies: aerial or amphibious transport: In the future, perhaps marine vehicles will be designed to avoid obstacles, such as rocks and sand banks. A possibility for transportation in the dry season could be amphibious vessels that can operate in water and on land, as shown in Pan et al. [41]. Another possibility could be Wing-In-Ground (WIG) crafts, which operate in water and in the air, flying just above the water surface [42]. Yang et al. [43] gives examples of these devices, and the Aeroriver Barco Voador company has recently sought to implement this type of device as a means of transportation in the Brazilian Amazon [44].
• Research alternatives for signalization in remote regions: To improve signage and thus reduce accidents, devices powered by renewable energies, such as buoys [45], powered by solar, wind, and hydrokinetic energy, could be developed and installed in strategic positions to provide information, such as the direction of the main routes or warnings of dangers. These devices could also be equipped with sensors to measure and communicate environmental parameters to control stations (e.g., [46]); however, connectivity between the stations and the devices could be an additional challenge to be evaluated for remote regions.

3.3.2. Promote Education

• Disseminate technical knowledge: In remote regions of the Amazon, academic training in naval architecture and marine engineering is scarce. Although marine activities are common in the Brazilian Amazon, including frequent shipbuilding and repair activities [47], the only undergraduate courses in this region are in Manaus (at Amazonas State University) and Belem (at Federal University of Para). It is therefore suggested that lectures and courses are organized in Amazonian cities, including remote communities. Relevant technical advice could be presented to the owners and users of the small vessels regarding safety requirements, emergency procedures, and alternatives for safer navigation. Although this kind of information is available from the Maritime Authority in Brazilian Norms (e.g., NORMAM, [39]), perhaps there is a need to transmit such information in a more didactic way, through educational campaigns. Operational information should also be disseminated, such as recommendations for load distribution and stability on vessels, the proper use of propulsion systems, and emergency procedures.
• Disseminate information on safety to the local populations: The small vessels operating in the Brazilian Amazon should be equipped with basic safety kits, and users should know how to use the equipment properly. In the study region, it can be very difficult to supervise the use of safety equipment in remote regions, so users should constantly be kept up to date on safety procedures.

3.3.3. Improve Infrastructure

• Increase signage in dangerous locations: Due to the vast size of the study area, navigation signage is a complex issue; it cannot be installed in a short period. However, signage should be installed on the main navigation routes in especially dangerous and/or protected areas.
• Improving communication and maintenance services: It is suggested that communication and maintenance services for vessels be improved, including the implementation of boat repair services and emergency assistance, along the main routes connecting remote communities. Although financial resources would be needed for this, emergency assistance for navigation in complex water environments, such as in areas with rapids, could save lives and cause positive impacts to the transportation of essential goods.
• Alternative ways to get into the forest to complement the transport of boats: For the more remote communities, where some stretches of the river are not navigable and boats must be lifted out of the water and carried into the forest (Figure 3h), monitoring of these inaccessible routes would provide valuable data. Identifying where infrastructure could be improved, to reduce the energy and time required to access these communities, would greatly improve the well-being of their inhabitants.

4. Potential Environmental Impacts

The research by Fontes et al. [7] listed possible environmental impacts that could result from accidents involving vessels used in riverine transport in the Amazon environment. The potential effects of collision, grounding, sinking, fire, and explosion were discussed. In this work, a preliminary list of potential environmental impacts related to the use of high-speed vessels in the Amazon region identified from the research is explained.

Figure 4 summarizes the possible environmental impacts that are outlined in this work. These preliminary results are based on findings from published articles, such as Carreño and Lloret [48], and others referenced in our analysis. Three main areas of concern are discussed: vessel operation (motion), propulsion systems (integration of engines, shafts, and propellers), and waste.

4.1. Impacts Related to Vessel Motion

4.1.1. Interaction with Flora and Fauna

Further research is needed into the effects of high-speed vessels in the Brazilian Amazon on flora and fauna. According to [49], aquatic animals can alter their behavior due to vessels’ traffic and the size of the operating vessels. In addition, the animals may be struck by the hull or by the propulsion system. As reported by Carreño and Lloret [48] and Panigada et al. [50], for recreational vessels in Mediterranean waters, collisions tend to be more frequent at certain times of the year. In this context, the situation in the Brazilian Amazon is more complex, due to the tremendous diversity of aquatic species [51,52]. Regarding the impacts on flora, negative consequences often may occur when incursions are made into the flooded forests (Figure 3a), since the vessels may interact with plants.

Anchoring vessels in marine protected areas can also have an impact on flora and fauna [48,53]. There is a lack of research regarding the impacts of anchoring in the Amazon basin; however, research reported for sea environments can help to contextualize the impacts in the region. For example, Boudouresque et al. [54] described how sensitive marine habitats can be affected according to the type, size, and format of the anchors, as well as the characteristics of the sites chosen for anchoring and docking.

In some parts of the Amazon, spotting aquatic animals is a common tourist attraction. Floating structures and high-speed vessels are used to facilitate the observation of the local fauna. Figure 5 shows a typical scene on the River Negro where tourists hope to interact with the animals. Other means of spotting are performed onboard high-speed vessels to reach more remote locations. High-speed vessels are also employed to perform leisure and sportive activities, such as fishing, which is common in some Amazon regions [55,56]. Recreational fishing may affect the behavior of different aquatic animals, as discussed by Powell and Wells [57] for sea conditions. Moreover, poor practices such as feeding the animals can alter their eating habits and behavior, as explained by Clua [58].

4.1.2. Transfer of Non-Native Species

It has been reported in scientific literature that maritime transport contributes to the dispersal of aquatic animals of different sizes in non-native environments [59,60]. In considerably large vessels, aquatic species can be transported in anchors, as hull incrustations, or as solid material in ballast water, as reported by Ulman et al. [61] regarding marine activities at sea. In the case of the Amazon region, it may also be possible to introduce exotic species from vessels that interact with sea–river environments, not necessarily high-speed ones. Some researchers have documented this problem regarding the invasion of exotic species from ships into the Amazon region [62,63]. However, more initiatives to research this topic considering the risks for different types of vessels, including high-speed vessels, are still required.

4.1.3. Impact of Artificial Light Emissions

Longcore and Rich [64] described how light pollution can affect many aquatic organisms in various ways, since there are many animals that are guided by daylight, as pointed out by Cruz et al. [65] for marine turtles. Brüning et al. [66] suggest that nocturnal illumination can alter the heart rates of some fish species, whereas Newport et al. [67] suggest that it can affect those that use visual signals for communication. Moreover, Rodríguez et al. [68] explain how birds could feel attracted or disoriented by bright lights. It is important to mention that most of these observations have been reported in maritime environments. Studies should be performed to identify the main impacts of artificial lights on creatures in the surroundings of vessels in the Amazon waterways, where light pollution may be a concern, especially at night.

4.1.4. Release of Toxic Compounds

One of the main means of toxic compound release from vessels is antifouling paints. These are commonly used in the maritime industry to coat the underwater surfaces of ships to prevent attached sea life such as algae and mollusks [69]. However, for marine applications at sea, it has been reported that there are some paint compounds that can persist in the water, killing species, harming the aquatic environment, and entering to the food chain, such as trybutylin (TBT), [48,69]. Due to its toxicological effects, the risks associated with TBT have been of concern for decades [70,71]. Although international restrictions have been imposed on the use of TBT-based antifouling paints, Castro et al. [72] reported that TBT contamination is still an important issue along some Latin American coastal areas, reporting that marinas are still acting as possible sources of TBT. Nevertheless, it may be possible that there are other paint compounds in hull paintings that could be potential risks to the aquatic environment, since they can contain heavy metals, which can deposit in aquatic animals and reach the human food chain, as described by Ytreberg et al. [73]. In the Amazon, including remote regions, it is suggested to increase inspections to evaluate the painting procedures and verify whether the paints used are free of toxic compounds that could threaten the aquatic environment.

4.1.5. Wave Wash

This is also known as wake wash, and it is a problem related to the generation of waves when a vessel moves forward [74]. Figure 6a–c show examples of wave wash generated by high-speed vessels of different sizes in the rivers of the Amazon. These waves tend to propagate away from the vessels and may interact with other vessels, plants, swimmers, or riverbanks. Vessels moving at high speeds generally produce larger and more energetic waves than vessels of the same size moving more slowly [75]. As stated by Zajicek and Wolter [76], all types of motorized vessels create waves and can contribute to the erosion of shallow shore areas. It has also been reported that this phenomenon can disturb seagrass [77] and algal communities [78] in coastal environments. Although several works have reported that this can contribute to coastal erosion [79,80], there has been little research into its potential effects in the Amazon waterways [5,81].

High-speed vessels in the Amazon navigate in rivers and tributaries with different depths and widths. The potential effects of wave wash could be more considerable when there are reductions in these dimensions, or when a vessel navigates close to the river banks, as pointed out by Li et al. [82].

Research is needed for the Amazon region, and expert recommendations, such as in [83], should be followed to manage the problem. For example, to limit wave wash damage, wave heights or energy thresholds should be considered in order to set appropriate operational speeds for vessels [84,85].

4.2. Impacts Related to the Propulsion System

4.2.1. Fuel and Oil Leaks Including Bilge Waters

In the Brazilian Amazon, leakages might occur during the process of re-fueling the high-speed vessels on the rivers. In the study region, it is common to find floating structures, which offer fuel to the vessels, as in Figure 7. During this process there is a high risk of leakage, either because of the waves caused by other vessels, or because of wind or heavy rain. This is mainly because it is usually necessary to move the hose delivering the fuel outside of the floating station to reach the vessel’s fuel tank. Elentably et al. [86] discussed that the use of floating fuel stations can present some benefits for navigation and can increase regional productivity. These authors also discussed that these devices are subjected to environmental concerns, including safe operation in terms of fire safety, pollution prevention, and comprehensive emergency response plans. For existing floating fuel stations on the Amazon, it is suggested that they are periodically inspected, as well as the procedures used for supplying them with fuel.

On the other hand, the inadequate release of bilge waters, which are those residual waters that are generated in the engine room in large vessels and can contain toxic substances, including fuel and oil, can be an environmental concern [48]. Some alternatives to reduce oily water discharge and make this practice less potentially harmful include proper maintenance of the equipment, raising awareness of environmental and legal consequences of this practice, and providing suitable facilities for the disposal of water–oil mixtures at ports or other structures [87].

4.2.2. Air Pollution Emissions

Any motorized vessel that uses a combustion-based motor is a potential contributor to emissions that pollute the air. The release of hydrocarbons into the environment can affect the air and water, contributing to climate change because of warming effects [48]. For example, NOx and SOx hydrocarbon components that are emitted can react in the atmosphere, generating nitric and sulfuric acids, respectively, which contribute to the acidification of the oceans [88] and forests [89]. Newman et al. [90] stated that air pollution can affect aquatic and terrestrial animals, highlighting the concerns of this problem in protected areas, such as the Amazon Basin. Small vessels do not emit as much pollution as large ships, although they can be a significant problem locally, as stated by Moreau et al. [91], especially when the number of the vessels is considerable, and if the propulsion systems do not have adequate maintenance. In the remoter parts of the Brazilian Amazon, the high-speed vessels used are usually small, and adequate maintenance of the propulsion system may be difficult. There is no documented research on the effects of the emissions of high-speed vessels on the regional environment, so increased monitoring of this is suggested to measure and quantify the negative effects.

4.2.3. Sediment Resuspension

The sandy or muddy beds of shallow waters in the Amazon rivers can be disturbed by motorized vessels, contributing to the resuspension of the sediment (e.g., Figure 8). From research performed in marine environments, it has been reported that sediment resuspension can increase the turbidity of the water, which decreases light penetration. This may have negative effects on marine algae [92] or increase the risk of eutrophication [48]. Alexander and Wigart [93] explained that in the shallow, nearshore waters of lakes, wave action and turbulence from boats produces suspended sediments and this releases nutrients. Moreover, it also can affect the gills of fish [94].

In the Brazilian Amazon, the use of motorized vessels, including water jet vehicles, is common in shallow regions, and can contribute significantly to resuspension of sediments [48]. Technical studies, similar to that performed by Chakraborty et al. [95] in the Hoogly river in India to measure the effects of sediment resuspension on aquatic ecosystems, would be useful. Through experimental work, these authors concluded that ship speed, river geometry, and irregular bathymetry were determining factors in sediment resuspension in this restricted waterway. Furthermore, through numerical simulations in a shipping channel, Ji et al. [96] found that ship-generated waves and accelerated currents significantly affect sediment transport. Similar experimental and numerical approaches are highly suggested for the Amazon region.

4.2.4. Noise Disturbance

From research performed in ocean environments, Cominelli et al. [97] suggested that the continuous passage of vessels in specific areas elevates noise levels, which may affect the behavior of marine fauna (fish, mammals, and birds). In the review from Carreño and Lloret [48], most evidence on aquatic animals and noise levels was obtained through laboratory work. Details on these effects in natural habitats require further study. Wale et al. [98] investigated the effects of ship noise on the behavior of shore crabs in a controlled experimental study, determining that noise can alter foraging and anti-predator behavior. They suggested that environmental management strategies should be implemented, stating that even invertebrates are sensitive to noise. Possible impacts of noise on marine mammals in Arctic regions were researched by Halliday et al. [99]. They suggested that the noise of ships can be heard over 100 km away, and that the noise can affect the behavior of mammals at a distance of up to 52 km. In another study, ship noise was deemed to affect the hearing of fish, hindering acoustic communication [100].

Jalkanen et al. [101] investigated underwater noise from ships in the North Sea, 2014-2020, suggesting the establishment of regulations and procedures to mitigate the effect of submarine noise in marine ecosystems. Picciulin et al. [102] reported that even small fishing vessels with low-powered engines can emit intense noise, especially at frequencies below 1 kHz, which are critical for communication among some marine species, such as certain types of dolphins. They also concluded that the frequencies emitted by the motors are the main contributors of marine noise and that increasing vessel speed does not necessarily increase the noise in all the frequency bands.

In common with the conclusions of several authors working in marine environments [103,104], in the Amazon region, strategies related to high-speed vessels and noise emissions should be based on regulations and public policies that take into consideration environmental management to mitigate the effects of noise.

As suggested by Halliday et al. [99], other strategies would include reducing speed limits of vessels, redefining routes to avoid concentrations of animals, and developing specific regulations for protected areas. Developing standardized methods for measuring noise levels would also contribute to understanding the issue.

4.3. Impacts Related to Waste

4.3.1. Black and Grey Waters

According to their size and use, some vessels can discharge waste known as black waters. This is toilet waste, which can contain bacteria and viruses that are problematic in confined waters, such as creeks and marinas [48,91]. Sewage holding tanks could solve this problem, but there are no established, international procedures for this on small vessels [48]. More difficult to treat than this is the discharge of so-called grey waters, which are discharged from accommodation facilities: bath/shower, laundry, kitchen, and dishwater. These waters may contain chemicals and fats, residues from detergents and soap, metals, bacteria, and organic matter. In the Amazon region, large- and medium-size high-speed vessels may be more likely to discharge black and grey waters. Ytreberg [105] discussed the problem of grey water discharges in the Baltic sea, explaining that this is mostly unregulated, and identifying the main contaminants: organic compounds and metals, particularly zinc and copper. Inspections to check the treatment and disposal of these residues are needed, as is the implementation of existing legislation and protocols.

4.3.2. Other Residues

Liquid and solid residues, derived from food packaging and other refuse, could be a problem in the Amazon rivers (e.g., Figure 9). Although this can be accidental, in some situations it could be more likely to have been dumped by the crew or passengers on the vessels. This practice, occurring in small, recreational vessels, is becoming a critical threat in aquatic protected areas [106]. Plastics derived from different food packaging, in particular, are becoming a concern in aquatic environments [107], inhibiting the growth and behavior of flora and fauna [108]. After time, plastics may become degraded into micro- and nano-plastics, which are of serious concern since they can be ingested by animals and eventually affect the food chain of human beings [107]. While some experimental campaigns have been initiated to verify water quality in the Brazilian Amazon (e.g., [109]), to the authors’ knowledge, no research regarding the amount of rubbish dumped from high-speed vessels in the region has been documented.

5. Further Research Developments

From the results presented in the previous sections, additional research is needed regarding the sustainable use of high-speed vessels in the Amazon region. Figure 10 offers a preliminary roadmap with four main branches to guide future research aimed at improving navigation.

• Data acquisition: Firstly, information on vessel parameters and environmental conditions, acquired through measurement campaigns, is needed. In terms of operational challenges, it is necessary to gather more data on the motion of vessels in different operating conditions, which is relevant for research into control systems and propulsion [74,110]. Data on the stability of vessels is needed to determine safe loading conditions and evaluate potential dynamic instabilities [17]. Furthermore, data on the structural behavior of vessels is also required, particularly to quantify the typical damage on hulls resulting from collisions and groundings, and to develop alternative materials for repair and construction procedures [111]. Measurements of environmental conditions, such as wave height, currents, and wind speed, in regions frequented by high-speed vessels are crucial for analyzing possible real-life scenarios through numerical simulation, as done in sea-related applications [112]. Regarding environmental aspects, data on noise emissions, kinematics, and the energy of wake wash, and possible interactions with aquatic species, should be gathered, as suggested by Suprayogi et al. [85] for the wave wash phenomenon. To achieve this, research protocols may need to be developed to maintain ethical standards in research in protected environments.
• Technological innovation: The above information could be used in research and development in technological innovation. For example, more ecological and efficient propulsion systems could be developed, as suggested by [113,114]. This could be attained by optimizing vessel hull forms to minimize drag, wakes, and energy consumption, and by researching sustainable energy sources (e.g., biofuel, hydrogen fuel cells, etc.). The use of alternative materials, including composites, should be investigated for the construction and repair of lightweight, sustainable vessels. New technologies for safer and more efficient navigation, including smart technologies and monitoring sensors, should be tested [115]. Research into the relationship between analytical, numerical, and experimental approaches is crucial for the technological innovation of high-speed vessels, as demonstrated by [116].
• Social engagement: Research that promotes social action and seeks to engage the community and stakeholders should be encouraged [6,117]. Relevant topics include increasing the training of local builders and the pilots of high-speed vessels. Participatory research in communities could be carried out by engaging riverine communities to identify potential issues and co-develop solutions based on local needs and knowledge. Furthermore, public awareness campaigns and disseminating information through schools and the media are also advisable. The planning of participatory frameworks that involve local communities in decision-making and impact mitigation strategies are also important.
• Policy and regulation: Ensuring compliance with safety and sustainability objectives, in line with available policies and regulations, requires substantial work due to the particular operational challenges posed by high-speed vessels in some Amazon regions [5,7,11]. For example, research should be conducted into the speed limits of vessels, monitoring of loading procedures, optimized routing, scheduling, safety of operations, and so on. Moreover, policies and regulations applicable elsewhere should be collated to determine the feasibility of adopting green vessel technologies, facing decarbonization challenges, as discussed by [118]. Approaches to increase the monitoring and fiscalization of unsafe practices, including the operation of high-speed vessels in remote areas, are also needed. Proposals for integrated river traffic management mechanisms are also required, including the application of monitoring and control systems, to make vessel operations sustainable.

While the proposed research branches cannot be easily or quickly implemented, they ought to be considered when planning the future of transportation and the achievement of the sustainable development goals in the region. In addition, there are several protected areas in the region [119], which could restrict the implementation of some short-term risk mitigation initiatives. We therefore recommend that these strategies be accompanied by further technical, social, and environmental assessments, to preserve the area’s environmental and cultural status.

6. Conclusions

In this paper, the main operational challenges of high-speed vessels in the Brazilian Amazon were discussed, with a particular emphasis on the regions in the upper courses of the rivers. Moreover, a preliminary list of potential environmental impacts related to the operation of such vessels in the region was presented.

It was found that the main causes of operational challenges for high-speed vessels are the shallow and rapid waters, collisions with fixed and moving obstacles, complex river flows, and climate conditions. Suggested mitigation strategies to overcome these challenges are focused mainly on extending research, developing innovation, promoting education, and improving regional infrastructure.

With respect to potential environmental impacts from the use of high-speed vessels in the region, a list of proposed actions was presented. This preliminary list was based on findings and suggestions from published literature and is organized according to vessel operation (motion), the propulsion system (integration of engine, shaft, and propeller), and waste.

The results of this work could form the basis for further research into the sustainability of high-speed vessels in the Amazon region, helping academia, the government, and industry to plan risk mitigation strategies. Further research in this area should focus on measuring and quantifying vessel parameters and environmental data, increasing technological innovation in vessel design and operation, implementing policy and regulatory procedures, and promoting community and stakeholder engagement.