Geomorphology and hydrology of Nee Soon 
41 
but this is expected, as other acid-related constituents, sea-spray, dust, and aerosols 
from biomass burning all contribute to acid rain in Singapore (Balasubramanian et al., 
2001 ). 
The ranges of specific conductivity values determined at five sites (n=64 samples) 
tend to be lower in the upper catchment streams (19-30 pS cm 1 ), springs (16-38 pS 
cm 1 ), and groundwater (23-73 pS cm 1 ), than in the lower catchment main channel 
(16^-04 pS cm 4 ), streams (27-207 pS cm 4 ), and ground water (25-149 pS cm 4 ). 
The lower catchment waters tend to have higher concentrations of Cl (6 vs 2 ppm) 
S0 4 2 (6 vs 1 ppm), Na + (5 vs 2 ppm), and Ca 2+ (4 vs 2 ppm). Values vary depending 
on rainfall conditions (with respect to depth and acid rain associated with the urban 
environment). As with the case of pH, however, insufficient data have been collected 
to determine controls, natural versus anthropogenic, of the differences among streams 
in the catchment. 
Hydrological resilience 
The Nee Soon freshwater swamp forest appears to be somewhat resilient to weather/ 
climatic fluctuations, including the 2014-2016 drought, which contained the second 
driest year recorded for Singapore (Meteorological Service Singapore, 2015). In 
the simulated water balance for the 3-year study period, mean annual water storage 
loss in the catchment was an estimated 70 mm (Sun et al., 2018), a depth that surely 
taxed groundwater reserves feeding the swamp. Nevertheless, while many Singapore 
streams dried during the drought, much of the swamp area in Nee Soon remained wet 
or moist (observations by the authors). 
Evidence of long-term resilience can be gleaned from our preliminary pollen 
analyses of a 1.95 m core taken from the lower swamp at Nee Soon. For example, old 
growth forest pollen was only about 20% at about 600 BCE, increasing to about 60% 
by 1000 CE. During this period, increasing numbers of fern spores and palm pollen 
provide evidence of increasing rainfall, because the other key variable for vegetation, 
temperature, can be assumed to have varied little at the equator in this time period. An 
abundance of rainfall is also suggested by the low values of charcoal found in the cores 
(i.e., few fires). Pollen and spores within a third sediment layer suggests gradually 
increasing cover of old growth forest and ferns, and decreasing grass. This period 
may be associated with increasing rainfall following the Little Ice Age (-1300-1870 
CE). In recent times (the last 70 to 100 years), little change in the vegetation can be 
observed. Given that we have dated the deposits to more than 15,000 years, our initial 
pollen analyses indicate the swamp has remained despite the vegetation being quite 
different (e.g. from grasslands to forest) in response to changing climates. 
Another form of resilience is associated with the persistence of the swamp 
despite inundation of the lower stream system when excess water is released over the 
slipway of the Upper Seletar Reservoir and released water flows backwards up the 
main stream channel, flooding the lower part of the swamp by several centimetres. 
The reverse flow transports sediments, nutrients, and biota into the swamp and up the 
