Sungai Pamusian yang berlokasi di Pulau Tarakan dapat dipandang sebagai system alami yang menjadi tempat berlangsungnya proses-proses biofisik hidrologis maupun kegiatan sosial-ekonomi dan budaya masyarakat yang kompleks. Penelitian ini dilaksanakan selama 4 bulan yaitu dari bulan Maret hingga Juni 2016. Tujuan penelitian ini adalah untuk mengetahui kondisi kualitas air di Sungai Pamusian yang ditetapkan berdasarkan Peraturan Pemerintah No. 82 Tahun 2001 kelas I dan menentukan Indeks Pencemaran pada hulu, tengah dan hilir Sungai Pamusian. Hasil penelitian menunjukkan bahwa kualitas air dari tiga stasiun pada Sungai Pamusian, terdapat 8 variabel kualitas air yang nilainya cenderung tidak memenuhi kriteria baku mutu air, variabel tersebut diantaranya: TSS, TDS, pH, DO, BOD, COD, Fospat dan Nitrit serta status mutu Sungai Pamusian berdasarkan Metode Indeks Pencemaran menunjukkan bahwa semua stasiun tergolong tercemar sedang dan telah melampaui baku mutu yang telah di tetapkan yaitu menurut Menteri Lingkungan Hidup nomor 115 tahun 2003 tentang pedoman penentuan status mutu air.
Kata Kunci: Sungai Pamusian, Kualitas Air, Pencemaran Air, Indeks Pencemaran

A River Pamusian is located in Tarakan island can be viewed as the natural system where the processes hydrological biophysical and socioeconomic activities and culture of society complex. This study was conducted in four months from March to June 2016. The purpose of this research is to know the condition of water quality of River Pamusian Tarakan Island set based on a government regulation no. 82 2001 about class water number I and determine index pollution water in upstream, central and downstream a side of Pamusian River. The research results show that the water quality at three stations in River Pamusian based on observation and analysis of eight indicators of water quality parameters including TSS, TDS, pH, DO, BOD, COD, PHOSPHATE and NITRITE showed that not tendency to the criteria for water quality standarts. Based on these facts, it’s can be concluded that Pamusian River water at every station in this studies are being polluted is according to environment minister no. 115 2003 on guidelines the determination of water quality status. Keyword: A River Pamusian, The Quality Of Water, Water Pollution, Index Pollution