Streams in Taiwan are short with small drainage basins. Classified -by the Water Conserving Bureau, 19 are regarded as major. The Tam Shui River is one of them and the longest in northern Taiwan.
Arrangement of the drainage network of the Tam Shui River system is of trellis pattern. Four major physiographical divisions may be divided according to elevations, surface reliefs and geological structures. The upper reaches of the stream are flowing down from the northern foothill zone of the Central Mountain Range with numerous gorges and incised meanders. Subsequent valleys are dominant. Tablelands composed of Pleistocene deposits can be found in the middle section of the river. The area drained by its lower reach is the Taipei Basin which was formed by recent faults. Volcanoes are numerous around the river mouth.
Main factors controlling the climatic conditions of the Tam Shui River Basin are monsoons and typhoons. Natures, intensities and frequencies of the winter monsoon are different from those of the summer monsoon. The southwest summer monsoon lasts about five months from early May to late September. Precipitation is high during the period. Typhoons occur frequently in July through September. As they are often accompanied by severe cloudbursts, some windward slopes may receive as much as 1,200 mm of rain within 24 hours which is the highest record in the world. The ensuing floods cause Serious damages for the Taipei Basin. Warm thundershowers also occur in the long summer months.
The northeast winter monsoon prevails from early October to late March, bringing heavy rainfall to the northeast sectors where are the catchment areas of the Hsintien Chi and the Keelung River, the main tributaries of the Tam Shui River.
As measured by more than 70 gauge stations in this river basin for the last 67 years from 1897 to 1963, it had an average annual rainfall of 3,010 mm. Maximum annual rainfall of 8,408, which is the highest figure in the Republic of China, was also recorded.
