THE ENVIRONMENT
388
below the present level, leaving the site of present-day Hong Kong as much as 130 kilometres from the coast. At that time, when glaciers covered northern China, the flat areas between what are now the islands of Hong Kong were part of the flood plain of the Pearl River. In interglacial periods, such as at the present time, the global sea level rose to its present level and higher, while marine sediments were deposited around the territory.
Information about Hong Kong's terrain is contained in 55 maps and 12 reports of the Geotechnical Area Studies Programme. Onshore and offshore geology is described in a series of 15 geological maps published at a scale of 1:20 000 and five memoirs produced by the Hong Kong Geological Survey, a part of the Geotechnical Engineering Office.
Hydrography and Oceanography
Hong Kong's waters are characterised by the interaction of oceanic and fresh water masses that vary in relative effect throughout the year. The discharge of fresh water from the Pearl River is the single most important factor influencing Hong Kong waters, especially in the summer. Three different ocean currents with different characteristics affect the territory.
During the summer, a north-easterly oceanic flow brings water of high salinity from the Hainan current into Hong Kong waters. This warm water mass interacts with fresh water from the Pearl River and divides the territory into three distinct sections. In the west, where the fresh water influence is greatest, the environment is estuarine and the water brackish. In the east, the water is dominantly oceanic, with relatively minor dilution from direct monsoonal rainfall and run-off from small streams. The limits of the central transitional zone vary, depending upon the relative influence of Pearl River flood waters and marine currents.
In winter, the Kuroshio oceanic current brings warm water, also of high salinity, from the Pacific through the Luzon Straits. The fresh waters of the Pearl River have a lesser influence than in summer and water salinity is more uniform across the territory. Besides the Kuroshio current, the coastal Taiwan current brings cold water down the South China coast. This affects inland waters, making the surface water cold during the winter.
The normal tidal range in Hong Kong waters is between one and two metres. The tidal pattern is complex due to the relative effects of the diurnal and semi-diurnal components. The basic pattern during flood-tides is for oceanic water to flow north into Mirs Bay, west through Victoria Harbour and north through Kap Shui Mun and the Ma Wan Channel. Maximum tidal currents range from 0.5 to 1.5 knots. This flow is reversed during the ebb tide, though currents in some parts may be stronger due to the influence of the Pearl River.
Flora
Hong Kong is near the northern limit of the distribution of tropical Asian flora and has an estimated 2 800 species of vascular plants, both native and exotic.
Implementation of various conservation measures have transformed formerly bare hillsides and slopes into areas with trees of local and introduced species. Besides greening and beautifying the countryside, woodlands are important habitats for