34 
Ganl. Bull. Singapore 70 (Suppl. 1) 2018 
of Stamford Raffles in 1819 and the turn of the 21 st century, nearly all forests had been 
converted to other land covers. Today, only about 0.2% of the total area (719 km 2 ) of 
Singapore is considered to be primary forest (Brook et al., 2003). 
The 7.55 km 2 Nee Soon area, in the heart of Singapore’s Central Catchment 
Nature Reserve (Fig. 1), contains virtually the last freshwater swamp forest in 
Singapore. Nee Soon freshwater swamp forest catchment occupies 4.8 km 2 (boundary 
in Fig. 1). The catchment, which is bounded by the Upper Seletar Reservoir to the 
northeast and the Upper and Lower Pierce Reservoirs to the south, is recognised for 
its conservation value (e.g. Ng & Lim, 1992; Wee & Ng, 1994; Briffett & Ho, 1999; 
O’Dempsey & Chew, 2013; Li et al., 2016; Clews et al., 2018), yet very little has been 
written about its physical nature—which is the goal of this paper. 
Herein we draw from the results of a recently conducted project entitled “Nee 
Soon Swamp Forest Biodiversity and Hydrology Baseline Studies” Phase II (Tropical 
Marine Science Institute, 2016). The background and objectives of the project, to 
guide future management of the catchment in anticipation of increasing urbanisation, 
are described by Davison et al. (2018). We present initial findings related to the hydro- 
geomorphological status of the catchment. The interpretations of the findings may 
change as new data are collected. 
Geology 
Nearly all of the Nee Soon catchment is underlain by the Triassic Bukit Timah Granite 
Formation, which varies from granite through adamellite, granodirite and several 
hybrid granitoids (Ives, 1977). Few core stones or outcrops are present in Nee Soon 
catchment, except on the hilltops in the southwest, and therefore variations in the 
bedrock can only be inferred from scattered outcrops outside the Nee Soon catchment. 
A ground penetrating radar survey, conducted along a publicly restricted walking trail, 
the Woodcutter’s Trail, indicated the maximum depth to bedrock is about 9 m (Tropical 
Marine Science Institute, 2016). However, because of the difficulty in distinguishing 
the interface between the residual soil and the moderately weathered granite, solid 
unweathered bedrock may be as deep as 20 m to 70 m in some locations. 
A granite rock sample we tested from the upper catchment has medium- 
to-coarse grains and contains about 76% SiCf, 13% A1 2 0 3 , and 1.5% Fe 2 0. It also 
contains substantial Ba (797 ppm), Mn (197 ppm), and Sr (83 ppm), relative to other 
minor elements. Aluminum (6.3%), Na (2.9%) and K (3.5%) are the most abundant 
major elements. 
Soils 
Ives (1977) identified two dominant soil types within the catchment (Fig. 2a): (1) 
Rengam Series, developed on igneous rock; and (2) Tengah Series developed on 
alluvium. The Rengam Series is generally a clayey, kaolinitic, isohyperthermic, Typic 
