The CPCB network monitors water quality at 4,484 locations across the country. Biochemical oxygen demand (BOD) exceeding 3 milligrams per litre (mg/l) is identified as polluted locations. Two or more polluted locations identified on a river in a continuous sequence are considered as a “polluted river stretch.” A BOD less than 3 mg/l means the river stretch is fit for “outdoor bathing.” Further, stretches with BOD exceeding 30 mg/l are considered “Priority 1” (P1), meaning, the most polluted and thus needing the most urgent remediation. There are five such categories with “Priority 2” (P2) indicating a BOD of 20-30 mg/l and “Priority 5” (P5) indicating 3-6 mg/l. The success of river-cleaning programmes are measured by the number of stretches moving from 1 to 2, 2 to 3 until those in 5 (requiring the least action) too reduce. Gujarat and Uttar Pradesh had the highest number of “Priority 1” river stretches (6), Maharashtra had the most polluted river stretches of 55, followed by Madhya Pradesh (19), Bihar (18), Kerala (18), Karnataka (17) and Uttar Pradesh (17).