## 3 Results
### Hydrological Control
#### 3.2.4 Groundwater-Surface Water Relationships  
The relationship of declining basin elevation with unsaturated zone thickness, borehole depths and saturated groundwater thicknesses available is shown in Figure 6. Boreholes average 43 \(\pm\) 9.0 m in depth (10 m deeper than the Garson and Walshaw [31] historical dataset) and are estimated to screen the lower 15 m of borehole (average screen length of approximately one hundred borehole camera inspections by CJF in Malawi). Unsaturated zone thickness increases mid-reach to near lake. Assuming river levels occur approximately 5-10 m below regional ground surface, consistent with typical river incising into terrace deposits up to _c._ 10 m [5], it is reasonably inferred from Figure 6 that river stage occur above observed water tables and hence rivers potentially losing over their mid-reach to near-lake approach. This accords with recent and historical visual observations of most of the tributary rivers of Lake Chilwa although being perennial in their upper reaches, gradually losing their flows/water as they approach the lake because of increased porosity of the fine-grained, but still porous lacustrine deposits and storage availability in surrounding wetlands [13], i.e., the rivers become influent and 'disappear' into the lowland lake terrace deposit plains on approach to the lake.  
Baseflow discharges at higher elevations (down to approximately 690 m) is inferred from observed water tables above or close to stream elevations (assumed 5 m (or even 10 m in more incised valleys) below regional ground surface (Figure 6)) that are enhanced by steep hydraulic gradients (Figure 5a). This is consistent with perennial upland streams continuing to flow in the dry season. Geological deposit elevations (Figure 6) suggest most baseflow emanates from the colluvium or perhaps highest elevation high-level deposits. Baseflow is expected to decline with Basin elevation as increased low-permeability lacustrine deposits may confine or semi-confine deeper groundwater.  
Groundwater heads regionally converge on the lake, indicative of general groundwater discharge to the lake or its contributing rivers. This is consistent with groundwater flux in the Basin water balance [12] with hysteresis associated with temporal groundwater storage, resulting in a delayed lake level relationship to precipitation that fell several years earlier [4]. Near-lake groundwater discharge  
Figure 6: Plot of ground elevation versus borehole (n = 48) depths, estimated variation of unsaturated zone, saturated zones plain cased and screened and potentially available aquifer estimated from the deeper boreholes; and, a lower profile of borehole and Lake Chilwa sampled TDS water quality.  
could be via gradual seepages over diffuse area driven by upward gradients near the dynamic lake interface (but hindered by the low permeability of lake bed sediments), or focused into areas of permeable sediment pathways or near-lake reaches where rivers have incised and carved deep pools into soft sediments. For instance, into the 3 to 5 m depth holes observed in the larger river deltaic land, including the Phalombe [4; 22], especially where holes are intersected by permeable gravels or perhaps (weathered) basement. Such holes form a critical refuge for fish and ecology during Lake Chilwa drying and provide a key mechanism aiding rapid lake fishery recovery following drought, but are increasingly at siltation risk from near-river agricultural and deforestation activity [4; 22; 55].