Introduction

Further deterioration of freshwater quality due to pollution is a major challenge humanity will face in the 21st century (UNEP, 2016). Consequently, we must strike a delicate balance between meeting increasing demand for water, food, and energy without causing irreparable damage to freshwater ecosystems. According to the United Nations Environment Programme (UNEP (2016)), deterioration of freshwater ecosystems has primarily been driven by human population growth, economic activity, land use, and climate change. These driving factors continue to exert pressure on the quality and quantity of freshwater resources. As a result, deteriorating freshwater quality is a global concern as it threatens to destabilize water use, freshwater ecosystem integrity, and the biodiversity of these ecosystems.
 
Freshwater is a critical natural resource as it is vital for the delivery of ecological goods and services, such as domestic water, food production, energy, industrial processing, transportation, waste disposal, and human health (Gleick, 1993; Malmqvist & Rundle, 2002). According to Shaklomanov (1993), the total global water resources are an estimated 1386 million km3, over 96% of which is saline, and just 4% is freshwater. Of these freshwater resources, 68% is captured in ice and glaciers, and the remaining 30% is restricted to the ground. Freshwater sources, such as rivers and lakes, only account for 93100 km3, which is approximately 0.0067% of the world’s total water. Nevertheless, rivers and lakes are major sources of water for humans as well as wildlife. Of all the water in the hydrosphere, surface water and other freshwater resources are further categorised as follows: atmospheric water 0.22%, biological water 0.22%, rivers 0.46%, swamps and marshes 2.53%, and soil moisture 3.52% (Shaklomanov, 1993). The focus of this study is freshwater river resources at watershed scale.
 
Degradation of river ecosystems has been extensive in developed countries such that it is now necessary to either protect what remains or restore degraded systems. In contrast, in developing countries, destruction of river ecosystems is currently peaking, but it may present an immediate threat (Dudgeon, 1999). According to Dudgeon (1999), information on the impacts of human activities on river ecosystems in developing countries is scarce. There are critical knowledge gaps concerning the impacts of many pollutants, and there is insufficient data for this in developing countries, most of which are likely to become vulnerable in the near future. Additionally, fewer studies have been undertaken to predict changes in the status and ecology of river water systems.
 
With the global population projected to increase by 2–8 billion by 2023, pressure on river systems is expected to increase dramatically (Malmqvist & Rundle, 2002; Rijsberman et al., 2006). Unsustainable land use and management practices within a watershed may be the underlying cause of resource degradation within the watershed. This could have primary implications for the socio-economic well being of resource users and the environment (Fock & Cao, 2016). The terms ‘basin’, ‘watershed’, and ‘catchment’ are often used interchangeably (World Bank, 2001). However, for the purpose of this study, the term ‘catchment’ is used to refer to the segmentation of the NPW into ‘subwatersheds’. River ecosystems are characterized by temporal variations in flow regimes due to spatial differences, the occurrence of maximum precipitation, evapotranspiration, and ice melting (Dettinger & Diaz, 2000). Therefore, the focus of this study is to assess water quality parameters that could be indicative of an optimal freshwater ecosystem in a peri-urban watershed.
 
In the context of Sub-Saharan Africa (SSA), the outlook on freshwater resources is no different, if not worse. For instance, findings from a study by Conway et al. (2009) highlighted significant temporal variation of up to 14% and 51% for rainfall and river flows, respectively, in SSA. A prospective study of Africa indicated that rapid urbanization is the paramount threat to the ecosystem (Clancy, 2008). Furthermore, according to the United Nations Population Fund (UNFPA) (2007), Africa’s annual urban population growth has been the highest in the world recently, at a rate exceeding 4% per annum. Africa’s urban population is expected to grow from 294 to 724 million by 2030. Several authors, including Mokwunye et al. (1996), Bridge (2001), Marcotullio (2003), Rijsberman et al. (2006), Cities Alliance/ICLEI/UNEP (2007), Simon (2008), Bhatta and Doppler (2010), Kasa et al. (2017), and Schwärzel et al. (2014), have elucidated the connection between the population, urbanization, and environmental degradation. According to Clancy (2008), in the current decade and the future, high annual urban growth will continue in Africa as well as across the world. This suggests that there will be a major strain on water resources and disruption of river ecosystems will accelerate. For example, unsustainable groundwater exploitation has decreased water table levels and increased degradation such that, in Africa, there has been a shift in the water supply for urban communities from groundwater to surface water (Showers, 2002).
 
Furthermore, according to Kusangaya et al. (2014), in Southern Africa, climate change will negatively affect the availability and demand for freshwater resources. Typically, Southern Africa has highly spatiotemporally variable rainfall. This phenomenon subsequently manifests as water scarcity. In addition, there are indications that, in Southern Africa, no facet of anthropological welfare will be spared from the negative impacts of climate change on freshwater resources. The susceptibility of the region to freshwater insecurity is worsened by its low capacity to adapt, poverty, and poor access to appropriate technology. Thus, it is envisaged that deterioration of freshwater resources in Southern Africa will impact agricultural, energy, domestic, and industrial water needs, as well as environmental flows (Kusangaya et al., 2014). In Zambia, a study by de Waele and Follesa (2003), which assessed human impacts on a vulnerable karst environment, revealed that demographic changes have led to unrestrained urbanization in Lusaka. This resulted in poor water resource management over the past three decades, putting the future social and economic development agenda at risk.
 
2 Materials and Methods

2.1 Study area

For data analysis, the NPW was divided into three segments, referred to as watershed positions (WPs). The three WPs are upstream catchment (USC), midstream catchment (MSC), and downstream catchment (DSC). There were three sampling locations (SLs) in each WP. Given the wide variation in the types and sources of pollution at a watershed level and potential uses for water, different types of parameters, including physical, chemical, and biological, can be applied to assess WQ. In this study, chemical WQ parameters were assessed for surface water following a more generalised approach for the final uses of this water. Water samples were collected from nine sampling points within the watershed (Fig. 1).