Review: Water quality in non-perennial rivers (with erratum)

Abstract

More than half the river-lengths of rivers in southern Africa dry up occasionally or – more commonly – seasonally. Here we review the literature on water quality (WQ) in non-perennial rivers (N-PRs), with emphasis on river management and southern African systems. Hydrological regimes cover a spectrum from relatively predictable and unvarying in perennial rivers, to unpredictable and highly variable in non-perennial rivers, which are complex, continually shifting mosaics of flowing water, standing-water pools and terrestrial habitats. N-PRs are uncommonly difficult to manage because they represent a limited source of water that is renewed unpredictably and is competed for by local people as well as being required by wildlife. Groundwater, and therefore its chemical and physical features, contributes significantly to base flow and to the maintenance of pools remaining in the bed when the river is not flowing. Water chemistry reflects catchment geology except in polluted systems. Salinity varies temporally, and spatially over three dimensions, and is the variable controlling the composition of the biotas of many N-PRs. Hydrological regimes are seldom predictable with any certainty; WQ varies naturally over time and space; groundwater often determines the WQ of surface water, especially in pools; and WQ in non-perennial rivers and pools may be affected by activities far upstream in the catchment. As yet we have no more than a sketchy understanding of the extent to which data on any one system can be applied to any other. Until we have a better understanding of these systems, the following basic principles should guide the management of WQ in N-PRs: (i) Rivers need to be assessed on a case-by-case basis. (ii) Understanding of the groundwater regime, including its chemistry, is crucial. (iii) Effluents need to be controlled by conservative effluent standards set for both ground and surface waters. (iv) Flows may need to be augmented at certain times of the year.

The effect of increasing fine sediment load and drying duration on the re‐emergence of Gammarus pulex (Amphipoda: Gammaridae) from the subsurface following flow resumption

“…Temperature, pH, and conductivity varied as the duration dry period increased, and dissolved oxygen showed a decreasing trend with increasing dry period (Table 2). However, the nature and direction of the changes recorded in pH, conductivity, and dissolved oxygen are similar to those reported for naturally drying lotic systems (Day et al, 2019).…”

Conservation and Management of Isolated Pools in Temporary Rivers

“…In IPs, a lack of water renewal, low gas exchange, high water temperature, organic detritus accumulation, and high respiration rates commonly create a low oxygen and slightly acidic environment, where micro-aerophilic or anaerobic processes dominate [65,66,79]. Figure 4 illustrates such changes with data from an IP of a well-studied Mediterranean forest river (von Schiller et al [66], unpublished data).…”

“…Some studies have investigated biogeochemical shifts in the surface water of IPs, but much less is known about changes in subsurface compartments [72,82,83]. Likewise, more information is available for the dynamics of organic matter and nutrients, especially nitrogen and phosphorus, than for other compounds such as metals [65,66,79].…”

“…Low oxygen conditions in IPs causes significant changes in the composition and activity of microbial communities [84][85][86] and shifts biogeochemical processes toward reductive pathways, increasing the concentration of reduced solute forms in surface water [66,79]. For instance, denitrification is enhanced, and, thereby, reduces the concentration of nitrate in the water column [78].…”

A conceptual framework for gender and climate mainstreaming to mitigate water inaccessibility in ruralsub‐SaharanAfrica

“…This is dependent on the terrain and distance between households and water points and can vary between wet and dry seasons (Jeil et al, 2020; Van Houweling, 2016). For example, muddy paths can be more difficult to traverse than dry ones (Drafor & Jones, 2008; Wilbur & Danquah, 2015; Wrisdale et al, 2017), and ephemeral rivers and lakes can impede travel or provide a closer water source that can also be contaminated (Day et al, 2019; Wekesa et al, 2020). Steep slopes, varying relief of the physical terrain, and long distances particularly impact people who are young, elderly, pregnant, sick, or with a disability (Jeil et al, 2020; Mushavi et al, 2020; Pommells et al, 2018; Stevenson et al, 2012; Van Houweling, 2016; Wrisdale et al, 2017).…”