## Materials and Methods  
### Study Setting  
#### 2.1.1 Hydrological Setting  
The Lake Chilwa wetland comprises the open lake surrounded by Typha swamps, marshes and seasonally inundated grassland floodplain (Figure 1). It is set within a watershed area of 8350 km\({}^{2}\), 68% falling in Malawi and 32% in Mozambique [10]. Our focus is on Water Resource Area 2 (WRA 2) Lake Chilwa Catchment Area of the Basin within Malawi that comprises four water resource units (WRUs) 2A, 2B, 2C and 2D (Supplementary Materials Figure S1a). A groundwater quality survey assessment of the Phalombe District (Figure S1b) that includes the area of former lake approach to the Mount Mulanje foothills noted by [PERSON] is specifically undertaken. This jurisdictional area covers all of WRU 2A that includes the Sombani River and Mpoto Lagoon and part of WRU 2B Phalombe River catchment (Figure S1c). These perennial rivers draining the Mulanje Mountains to the south together with the Domasi, Likangala, Thondwe, and Namadzi, draining the Zomba Highlands to the west (Figure 1), account for approximately 70% of lake inflow [5; 10]. Distinctive features, besides its shallowness, are that Chilwa is a large East African lake that lies close to, but not within, the Rift Valley. Further, it is one of just two East African lakes believed to have formerly drained into the Indian Ocean [5].  
Basin hydrology is driven by 80% rainfall occurrence between November and April due to north easterly monsoon and south easterly trade wind convergence. Annual rainfall averages from 1100 to 1600 mm, but may vary from 800 mm in the lowland plains to over 2000 mm in the mountains [7; 10]. Sporadic winter rainfall (May to October) only occurs in the highlands [29]. The mean annual temperature is 21-24 \({}^{\circ}\)C, with a climate described as tropical wet and dry [8; 30]. This area is susceptible to climate change impact [10].  
Rivers incise up to 10 m in soft lacustrine-alluvial sediments and become influent around their mid-to-low reaches, where underlying sediments are permeable, allowing groundwater recharge [5]. Together with high evaporation and interception of flows for irrigation, perennial-source rivers may fail to reach the Chilwa shoreline in the dry season, including the Phalombe and Sombani historically [5]. Wet-season high suspended-sediment loads causing downstream siltation problems are attributed to drainage of intensive agriculture [5], exacerbated in modern times by increased near-river agriculture (e.g., rice fields), upland deforestation and drought/flood events [8; 10]. The Phalombe River has developed an extensive delta, with lake entry varying over a 20 km shoreline.  
The dynamic nature of Lake Chilwa is not restricted to modern times. Additional to its modern seasonal expected fluctuation of approximately 1 m, Lake Chilwa levels regularly fluctuate by 2 to 3 m, resulting in partial or complete lake desiccation during successive droughs, as inflows become low relative to lake evaporation [4; 6]. Chilwa is reported to have dried up completely on nine occasions during the last century (1903, 1913-1916, 1922, 1934, 1943-1944, 1967, 1973, 1975, and 1995-1996). The lake is also believed to have dried in 1882, not that long after [PERSON]'s visit [3; 6]. Continuing lake dynamics, with complete desiccation in 2018, followed by flood in 2019, motivate the on-going need to understand the complexity of natural and anthropogenic dynamic factors involved [4; 10].  
Figure 1: Lake Chilwa basin map showing water features and former (relict) lake shorelines (collated from literature cited in Section 3.1). Small-scale water features such as stream networks and dambo wetland are not shown.