The use of citizen science is well-established in the field of river monitoring (Buytaert et al., 2014), encompassing a wide range of institutions, governmental departments, and community resource groups on a global scale (Conrad and Hilchey, 2011; Njue et al., 2019). Citizen science is characterized by the active involvement of citizen communities in scientific endeavours to support academic research, facilitate information sharing, and encourage public participation in science (Ceccaroni et al., 2017). Citizen science is known to enhance public knowledge, encourage public participation in decision-making, and protect the local natural environment with the information gathered (Walker et al., 2021; van Noordwijk et al., 2021). Numerous studies have demonstrated the reliability of data generated through citizen science, highlighting its effective contribution to diverse facets of river monitoring, including the detection of water quality and monitoring of water levels (Fehri et al., 2020; Njue et al., 2021).

Citizen science is an increasingly important component of river management worldwide. National institutions and NGOs around the globe prioritize river ecology and conservation because rivers play a vital role in being home to flora and fauna, regulating local climates, and providing essential services such as drinking water, irrigation, and recreation for human activities (Milton and Finlayson, 2019). Monitoring is essential for safeguarding natural environments and ecological quality, however governmental and large institutional efforts inevitably fall short in encompassing the entire freshwater system because it is difficult for authorities to cover all tributaries and small water bodies for long-term monitoring (Gurnell et al., 2019). Citizen science projects, organized by communities and local institutions, play a pivotal role in bridging these gaps by providing essential resources for river monitoring data collection, exemplified by large-scale initiatives monitoring for Sustainable Development Goals (Bishop et al., 2020) to regional efforts focusing on microplastic pollution and small waterbodies (Kelly-Quinn et al., 2023).

Professional river monitoring encompasses a variety of methods, including chemical and biological techniques (Pacini et al., 2019; Lamberth and Hughes, 2019), which may be tailored to different monitoring objectives. Many citizen science projects have been developed that either use these methods directly or adapt them to be more user-friendly for volunteers (Simaika et al., 2022). Existing literature on the application of citizen science in rivers primarily focuses on verifying the accuracy and reliance of individual citizen science projects (Krabbenhoft and Kashian, 2020; von Gönner et al., 2023), as well as determining the socioeconomic factors that affect the effectiveness of citizen science projects (Aura et al., 2021; Mgoba and Kabote, 2020). There is surprisingly little information available on how existing citizen science monitoring techniques can be tailored and adapted to meet new monitoring objectives. Without a comprehensive understanding of the common objectives and approaches of citizen science projects, it is difficult to identify opportunities for knowledge sharing, data integration across projects, or opportunities to expand citizen science monitoring as part of national monitoring schemes. It also creates a complex environment for new projects, where a large array of seemingly competing approaches leads to confusion about the best methods to use in any given situation (García et al., 2021).

This study aims to conduct a systematic review of global literature on river monitoring, including academic journals and institutional manuals (grey literature), to identify the principal objectives and monitoring methodologies employed in diverse citizen science projects. The overarching goal is to establish a coherent framework linking objectives with methods for citizen scientists within the realm of river monitoring, outlining cost-effective and operationally feasible methods suitable for citizen scientists. Ultimately, the framework aims to provide foundational guidance for new citizen science initiatives, enabling participants to select appropriate monitoring methods aligned with their conservation objectives effectively. There are three objectives:

a) To identify the primary objectives and monitoring methodologies that are currently being used in existing citizen science projects focused on river monitoring;

To perform a systematic search of the literature, we used the PSALSAR method for environmental science research described by Mengist et al. (2020). Existing literature reviews tend to focus exclusively on peer-reviewed literature, which may exclude a large proportion of active citizen science projects (Ramírez et al., 2023). We therefore used Google as well as Web of Science and Google Scholar as our search databases. The search results from Web of Science and Google Scholar primarily consisted of academic journals, while results from Google provided a substantial number of institutional materials. Given the unique nature of citizen science as a source of data from non-academic fields, including grey literature from non-academic sources is essential to capture a broader spectrum of firsthand information on citizen science projects. However, Google searches can also lack specificity, and our search returned an unmanageable number of hits (>16 million). For this reason, we decided to analyse a representative sample of the full dataset by extracting data from only the first 100 search returns. We applied this to all three search databases to ensure parity.

The search terms and screening methodologies utilized across the three databases are detailed in Figure 1. The types of materials included academic evaluations of single or multiple citizen science projects, comparative studies between citizen science projects and authoritative laboratories/monitoring stations, descriptions of monitoring methods suitable for citizen science projects, and research manuals for various citizen science projects.

The search process involved an initial search using the specified search terms in each database. In Web of Science, further categorization using Web of Science Categories was applied. Due to the large number of search results in Google Scholar, additional filtering based on publication date was applied to obtain more recent academic results. The first 100 entries from each database were then manually screened to identify entries that were directly pertinent to specific river-based citizen science projects. 203 papers were excluded at this stage, including institutional manuals informing basic monitoring methods, general reviews on citizen science, specific perspectives on citizen science projects, and projects that did not relate to both citizen science and river monitoring. ‘Specific perspectives’ refer to studies focusing on aspect of citizen science or a citizen science project that is not related to objectives and methods, including papers with a theoretical or critical emphasis. For example, Blake et al. (2020) discusses the demographic composition of citizen scientists and its implication to project effectiveness, and Horvath et al. (2022) demonstrates how citizen science projects can reveal differences in water quality across economic regions.

It is important to recognize the limitations of limiting screening to the first 100 results per platform. In particular, the way in which search engines like Google and Google Scholar rank and display results is influenced by factors beyond keyword relevance, such as citation counts, user search history, and the search engine’s commercial algorithms. These factors may bias the visibility of certain sources over others. To mitigate this, the screening process focused on identifying a wide range of study types, geographic regions, and methodological approaches within the 100 results. This included ensuring that both high-profile, frequently cited studies and less-cited or local-scale projects were selected for a more balanced representation of citizen science efforts in river monitoring.

Ultimately, a total of 97 entries from the three databases were selected for subsequent classification and analysis, including 16 studies from Web of Science, 30 studies from Google Scholar, and 34 from Google. According to Figure 2, the majority of these entries were published in the last 10 years, with a smaller proportion (24%) representing older projects dated between 1995–2014. No studies published before 1995 were identified in the search, which may reflect the relatively recent emergence of citizen science projects in the field of river monitoring.

The following information was extracted from each paper included in the review:

1. Research aims and objectives.

Initially, the principal objectives identified in each paper were classified into basic types and further subdivided for detailed analysis. Through a comprehensive review of all 97 selected studies, the objectives of citizen science river monitoring projects were extracted and listed. The monitoring methods associated with each individual objective were documented. Many studies report projects with multiple objectives. For these studies, each objective was documented separately along with the associated research methods employed per objective. This resulted in a total database of 9 different objectives and 7 different methods. Both the objectives and methods were then manually coded according to recurring themes.

An exploratory cross-tabulation was conducted to examine the relationship between the publication year and the geographic location of citizen science projects. However, no clear temporal or regional patterns were observed across the dataset. As such, while publication year and project location were recorded, these variables were not included in the main analysis.

We were able to categorise the objectives of river-based citizen science activities into six groups: 1. protecting ecosystem or river health; 2. assessing water quality; 3. assessing water quality for designated uses; 4. monitoring pollution; 5. acquiring hydrological data; and 6. providing education. Figure 3 shows how these objectives were distributed across the reviewed papers. As we detected studies with more than one objective, all objectives of studies were recorded. Among the 97 recorded studies, 27 have two identified objectives, and 2 have three identified objectives. Therefore, the total of percentages in Figure 3 is 130%, which is higher than 100%.

From Figure 4, the continent with the highest number of selected studies is North America (47), followed by Europe (25), Africa (10), Asia (7), Oceania (5) and South America (1), predominantly reflecting the Global North, although the Global South is also represented. Two studies contain citizen science projects that spread across multiple continents, so they are not recorded under specific continents. The countries with the highest number of selected studies are the United States (37), the United Kingdom (14), and Canada (9).

We were able to categorise the methods of river-based citizen science activities into seven groups: 1. kick-net sampling or equivalent; 2. chemical and physical measurements; 3. information on the riverbank; 4. monitoring species activities; 5. nutrients; 6. Rainfall, water level or water flow data; 7. bacterial/compound/diatom/DNA experiments. Figure 5 shows how these methods were distributed across the reviewed papers.

Kick-net sampling or equivalent predominantly involves collecting benthic macroinvertebrates by disturbing the riverbed through kicking and using a hand net to take a sweep sample. The collected specimens, which include insect larvae, mollusks, worms, and other phyla, are then identified either on the field or in a laboratory (Marchant and Yule, 2019). Since different species have varying levels of pollution tolerance and inhabit water bodies with distinct chemical, physical and biological conditions, their presence in a particular river area can serve as indicators of the corresponding environmental quality of the river (Tampo et al., 2021). For example, the Save Our Stream project documented in Gowan et al. (2007) uses four metrics based on macroinvertebrates obtained in field samples, to calculate a score indicative of stream health. This method is relatively cost-effective for volunteers and requires less professionality. However, Storey and Wright-Stow (2017) demonstrates that laboratory-examined samples may be more precise compared to those analyzed in the field.

Chemical and physical measurements is the most common methodological category among the 100 selected studies. This method typically includes testing parameters such as water temperature, pH, electrical conductivity, dissolved oxygen, and turbidity, and can be conducted using on-field test kits, making it accessible for citizen scientists. For example, turbidity can be measured using Secchi tubes, as demonstrated by Miguel-Chinchilla et al. (2019), while parameters such as pH and conductivity can be tested using specific test kits and strips. These chemical parameters provide citizen scientists with direct indicators and underlying determinants of water quality. However, the preparation of these kits and the maintenance of consistent quality assurance and control are labour-intensive and costly (Savan et al., 2003). Ramírez et al. (2023) have produced a very helpful review of citizen science methods for water chemistry which we do not need to repeat here.

Information on the riverbank is used for general methods including documentation and description of various characteristics in and around the river basin, which could facilitate qualitative analysis of the ecological quality. The index used in studies includes characterization of habitats (Scotti et al., 2022), type of surrounding land use (Edmonson, 2004), height, abundance, structure and species of riverbank vegetation (Modular River Survey Team, 2016). It can also be critical for identifying sources of pollution and monitoring urban interference. For instance, detecting discharge points into the river can indicate potential pollution sources (Miguel-Chinchilla et al., 2019), while counting the presence of livestock, farmlands, and settlements near the river can monitor the level of urban interference (Fore et al., 2001). The MoRPh technique is also used to assess the physical habitat condition of river and the extent of human activities impacting the river area (Gurnell et al., 2019).

Monitoring species activities refers to the observation and recording of species, including their behaviours, quantity, and movements within their habitats. Examples include counting bats to indicate riverine forest quality (López-Bosch et al., 2023), monitoring malaria mosquitoes to assess health risks (Murindahabi et al., 2021), and detecting fish landings to ensure sustainable small-scale fisheries (Silvano and Hallwass, 2020). While these methods enable direct monitoring of species, it is essential for citizen scientists to consider the varying traits of different species and to prioritize health and safety factors during field sampling.

Nutrients can be monitored onsite with test kits by citizen scientists, similar to other chemical measurements. However, nutrient measurement is less commonly performed than parameters like conductivity and dissolved oxygen, possibly due to higher costs. Nutrients primarily affect water quality by causing eutrophication, a form of water pollution characterized by harmful algal blooms that deplete oxygen levels, reduce biodiversity, and degrade water quality (Lintern et al., 2018). In selected projects documented in this study, the commonly measured nutrient parameters include nitrogen, nitrate, phosphorus, phosphate, and chloride (Babiso et al., 2023; Herman-Mercer et al., 2018).

Rainfall, water level or water flow data refers to the acquisition of hydrological data and the dynamics of river water flow. Citizen science projects under this objective often involve monitoring parameters such as water levels and rainfall data, equipping volunteers with tools and techniques, like water level sensor and rain gauges, to systematically collect and record data (Ferede et al., 2022). The parameters are collected to gain hydrological insights, typically involving the use of rain gauges and water level sensors, which demands citizen scientists of equipment installation. For example, river water gauges are used to assess the impact of rainfall on water quality, pollution levels, and habitat conditions (Fehri et al., 2020). Additionally, water flow data is valuable for analysing flow patterns, monitor groundwater recharge, and supports effective river management (Ferede et al., 2022; Weeser et al., 2018).

Bacterial/compound/diatom/DNA experiments represent biochemical experiments conducted to examine the microbiological characteristics of river samples, which detects the presence and concentration of chemical and biological contaminations. It should be noted that this method is not the primary approach for all objectives, but it is utilized in studies addressing the majority of the objectives. The most commonly employed methods include E. coli and total coliform tests, exemplified by Burgos et al. (2013) using total coliforms and E. coli as parameters to assess water quality. Most of these experiments require professional laboratory facilities, making them less accessible to citizen scientists. However, the establishment of community laboratories has facilitated the possibility for citizen science projects to conduct more sophisticated experiments (Water Rangers, 2024).

Figure 6 represents the proportion of studies using each method across different research objectives. For each objective, bars represent the proportion of studies that employed a given method, calculated based on the total number of studies associated with that objective. As studies often use multiple methods, proportions can exceed 100% when summed across methods. For example, 20 studies fall under the objective of Protecting habitat and biodiversity, and 6 of them used the method of Chemical and physical measurements, the proportion for this Objective-Method is 30%. The proportion metric reflects how commonly each method is used within the context of a specific objective, allowing for comparisons across methods even when studies are not mutually exclusive.

From our literature search, it is evident that most studies employ multiple methods. Generally, kick-net sampling (or equivalent) and chemical and physical measurements are the most commonly used methods, and they are applied across multiple objectives. Conversely, projects under the objective of acquiring hydrological data reportedly only use methods related to acquiring rainfall, water level, or water flow data. Objectives related to monitoring specific species, providing education, and assessing impact of human activities also tend to utilize more specialized methods. For instance, monitoring species activities are primarily used under the objective of monitoring specific species, and assessing impact of human activities primarily uses methods related to information on the riverbank and chemical/physical measurements.

Furthermore, within the broad category of protecting ecosystems or river health, the sub-objective of assessing water quality tends to use fewer but more dominant methods, such as kick-net sampling and chemical and physical measurements. This contrasts with the general objective of assessing water quality, which employs a broader range of methods.

The flowchart in Figure 7 links the common objectives to the primary methods used for each objective. Primary methods of a specific objective are defined as those utilized by more than 40% of the projects under that objective. Each objective draws an arrow to its main methods, illustrating that most objectives employ multiple methods and that most methods are applied across multiple objectives. Generally, the objectives of protecting ecosystem or river health, monitoring pollution, and assessing water quality for designated uses utilize the most methods. Among all methods, chemical and physical measurements, followed by kick-net sampling or equivalent, are the most widely applied. Objectives directly or indirectly aiming to assess water quality—whether through general water quality measurement, designated use assessment, or pollution monitoring—tend to use a greater variety of methods. Conversely, specific objectives such as protecting ecosystem or river health, acquiring hydrological data, and providing education use fewer methods.

In general, kick-net sampling, chemical and physical measurements and gathering information on the riverbank are reportedly more widely used by citizen scientists, requires less training, and costs less, which could indicate their suitability in further citizen science river monitoring projects. Specific methods like monitoring species activities require a particular understanding of the species, and acquiring rainfall, water level, or water flow data requires the setting of river gauges or sensors, thus they may be limited to specific objectives. The use of bacterial/compound/diatom/DNA experiments requires high cost and professional expertise; it is not a method primarily used in all objectives.

It should be noted that 24% of our documented projects require the use of a laboratory, which may pose obstacles for citizen scientists due to increased requirements for funding, access to resources, and professional assistance. For example, in their evaluation of the Bellingen Riverwatch program, Dickson et al. (2024) note that alongside citizen volunteers, the project operated a complementary professional monitoring stream in which nutrient samples were delivered to an accredited laboratory, with professional scientists providing essential field and laboratory assistance. Similarly, Deutsch and Ruiz-Córdova (2015) reflect on the long-running Alabama Water Watch volunteer network, emphasizing the necessity of rigorous quality assurance protocols, including bacteriological and chemical monitoring, which rely on certified labs and trained staff to uphold standards. Furthermore, the Ilkley Clean River Group’s faecal bacteria testing protocol instructs volunteers to collect samples in sterile bottles, keep them between 2°C and 8°C, and arrange analysis by an accredited laboratory, demonstrating the need for coordinated lab partnerships, logistics, and professional support in otherwise community-led monitoring (Ilkley Clean River Group, 2023).

Moreover, sensors, smartphone applications and Geographic Information Systems (GIS) are also commonly used to assess multiple parameters in the water. In this study, all objectives except providing education include projects that use smartphone applications, sensors, or GIS to monitor water quality or water levels. Under the objective of protecting ecosystem or river health, Luchette and Crawford (2008) describe a public-participation GIS platform in the Pamlico-Tar River basin in North Carolina, the United States, where citizens and researchers collaboratively visualized watershed data. Under the objective of assessing water quality, Cârstea et al. (2022) document smartphone-based monitoring along the Danube in Europe, noting that mobile apps allowed volunteers to record water parameters in situ, improving data collection frequency and facilitating real-time knowledge transfer across regions. Under the objective of assessing water quality for designated uses, Bannatyne et al. (2017) report on the Tsitsa River in South Africa, where a network of citizen technicians used sensors to monitor suspended sediment, standardizing data collection across the catchment and ensuring consistent sediment measurements. Under the objective of monitoring pollution, Thatoe Nwe Win et al. (2019) introduce a low-cost sensor and smartphone app system for the Ayeyarwady River in Myanmar, where volunteers used handheld sensors and the Akvo Caddisfly app to capture electrical conductivity and turbidity, generating data comparable in quality to professional measurements. Finally, under the objective of acquiring hydrological data, Starkey et al. (2017) introduces the value of GIS-linked community observations for catchment modeling, showing how citizen-collected flow and water level data integrated into catchment-scale models, improving hydrological characterization at a high spatial resolution. The use of these technologies enhances data accuracy and reliability and facilitates the acquisition and documentation of real-time data. However, the associated costs, maintenance requirements, and data management challenges pose significant barriers for citizen scientists (Warner et al., 2024).

This framework acts as a guide that links objectives to methods, providing a clear and intuitive visualization of the linkages between primary objectives and the corresponding methods utilized. By understanding the association between objectives and methods, project organizers can better design their initiatives, ensuring the selection of the most suitable and effective methods to achieve their specific objectives.

This systematic literature review offers an assessment of the common objectives and methods employed by citizen scientists in the field of river monitoring. Through a statistical analysis of the global distribution of citizen science projects, we examined their primary objectives and methods used. Our study presents a framework that intuitively links these objectives and methods, providing guidance for new citizen science initiatives in project positioning, objective definition, and efficient method identification. Although this study does not evaluate the effectiveness or efficiency of each method, it offers insight into the frequency and popularity of various approaches, providing a useful reference for selecting methods in future projects. It also categorises, based on all reviewed papers, the main objectives and methods of citizen science projects in the field of river monitoring, with information on their characteristics and examples, which helps identify common practice patterns and supports the strategic design of future initiatives by aligning project goals with suitable methods.

However, the study has certain limitations. To begin with, the literature search is restricted to English language, which likely contributed to the underrepresentation of studies from non-English-speaking regions, such as the very few studies included from South America. Also, the search terms used may not encompass all literature on citizen science projects in river monitoring, and the total of 300 manually screened papers might be insufficient for summarizing general trends in objectives, methods, and their interconnections. Additionally, significant differences may exist between the objectives and methods used in the publications we screened and those in more recent, non-date-limited publications. Furthermore, many citizen science projects may not be documented in the literature, as they may be practiced without being recorded and uploaded to the selected databases. There are also limitations associated with using Google as a source of grey literature, including geographic biases and reproducibility issues related to Google’s filtering processes (Paez, 2017). Also, the analysis did not make reference to the temporal and spatial distribution of the reviewed studies. Moreover, 16% of the recorded projects are studies comparing citizen science with professional measurements from laboratories, governmental departments, or monitoring stations. These studies may select different methods than purely independent citizen science projects.

Future research should investigate the development and variation in objectives and methods used in more contemporary river monitoring citizen science projects, for example, with a focus on geographical distribution. Additionally, since the establishment of citizen science projects requires practical training, which cannot be easily conveyed through written frameworks, should focus on delivering practical support. This includes providing technical guidance and hands-on skills training to better equip citizen scientists. Moreover, we look forward to future research that provides systematic evaluations of the effectiveness and efficiency of the various methods described in this study, as well as studies that incorporate theoretical or critical perspectives on citizen science for a more comprehensive analysis.