The upper catchment is jungle and patches of agriculture. A high proportion of urban development borders on the Brunei river. Where a tributary enters the Brunei river stands the capital Bandar Seri Begawan. The Brunei river falls into the Brunei Bay beyond which is the Port of Muara. The upper reaches of the river are a major freshwater source particularly for the western part of the country.
Climatic variations result because of the influence of the monsoon winds.
The northeast monsoon blows from December to March while the southeast monsoon occurs around June to October. Two inter-monsoon periods prevail from April to May and November to December. The annual rainfall is generally high. In 1994 the annual total rainfall was 2,653.5 mm. February to April is virtually a drought period.