The percentage of major rivers and streams that are classified as bad or very bad. The rivers and streams referred to include: Ho Chung River, Kam Tin River, Lam Tsuen River, Mui Wo River, River Beas, River Ganges, River Indus, Sam Dip Tam Stream, Shing Mun River, Tai Po River, Tuen Mun River and Yuen Long Creek. The quality of these rivers are measured by the Water Quality Index compiled by the Environmental Protection Department. Water Quality Index refers to annual average index of the stream sampled at a specific location based on measurements of dissolved oxygen, 5-day biochemical oxygen demand and anmoniacal nitrogen. Rivers with water quality indices of 3.0-4.5 are classified as of excellent quality, 4.6-7.5 as good, 7.6-10.5 as fair, 10.6-13.5 as bad and 13.6-15.0 as very bad.