Sungai Kayan berada Kecamatan Tanjung Selor Kabupaten Bulungan Provinsi Kalimantan Utara. Sungai ini merupakan salah satu sungai terbesar yang memiliki peran penting bagi masyarakat sekitarnya baik dari segi ekonomi, sosial, maupun ekologi. Pemanfaatannya seperti sarana transportasi, perikanan, pertanian, sumber air bersih, pariwisata dan rekreasi, dari kegiatan masyarakat tersebut berpotensi menyebabkan terjadinya pencemaran yang berdampak pada penurunan kualitas air Sungai Kayan. Tujuan dari penelitian ini adalah untuk menentukan Indeks Pencemaran (IP) Sungai Kayan berdasarkan parameter fisika, kimia dan biologi berdasarkan Peraturan Pemerintah No. 22 tahun 2021. Metode yang digunakan pada penelitian ini deskriptif kuantitatif. Lokasi pengambilan sampel dilakukan pada empat stasiun mengunakan metode purposive sampling dan metode grab sampling lokasi Stasiun 1 terletak di Tanjung Palas, Stasiun 2 terletak di Tugu Cinta Damai, Stasiun 3 terletak di kawasan Instalasi PDAM dan Stasiun 4 di Selimau. Pengambilan sampel dilakukan pada waktu pagi dan sore hari dengan parameter fisika (suhu, kekeruhan, TSS dan TDS), kimia (pH, DO, BOD, COD, nitrat, fosfat, dan MBAS) dan biologi (E.coli). Berdasarkan hasil penelitian yang telah dilakukan menunjukkan bahwa aktivitas masyarakat di sepanjang Sungai Kayan berpengaruh terhadap kualitas air Sungai Kayan hal ini berdasarkan pada nilai indeks pencemaran menunjukkan bahwa kualitas air di pengambilan pagi dan sore kategori cemar ringan dan kategori cemar sedang . Nilai Indeks Kualitas Air (IKA) yaitu 47,5 kondisi tersebut termasuk kategori kurang. Berdasarkan hasil penelitian perlu dilakukan pengelolaan dan monitoring secara berkelanjutan terhadap Sungai Kayan agar pemanfaatannya tidak menimbulkan resiko.

The Kayan River is situated in the Kayan area of Tanjung Selor District, Bulungan Regency, North Kalimantan Province. It is one of the largest rivers in the region and plays a vital role for the surrounding community in economic, social, and ecological aspects. The river is used for various purposes, such as transportation, fisheries, agriculture, clean water sources, tourism, and recreation. These community activities have the potential to cause pollution, which can lead to a decline in the quality of the water. This study aims to analyze the water quality of the Kayan River that is influenced by community activities using the Water Quality Index (WQl) approach based on Minister of Environment and Forestry Regulation No. 27 of 2021 and Government Regulation No. 22 of 2021. The research method was quantitative descriptive. Sampling was conducted at four stations using purposive sampling: Station 1 (Tanjung Palas), Station 2 (Tugu Cinta Damai), Station 3 (near the PDAM installation), and Station 4 (Selimau). Water samples were collected twice a day, in the morning and afternoon, and analyzed for physical parameters (temperature, turbidity, TSS, and TDS), chemical parameters (pH, DO, BOD, COD, nitrate, phosphate, and MBAS), and biological parameter (E. coli). The research findings indicate that community activities along the Kayan River significantly affect its water quality. This is evidenced by the Pollution Index values that show the water at the four stations to be classified from lightly to moderately polluted. Meanwhile, the Water Quality Index (WQI) score of 47.5 categorizes the water as "poor," indicating it does not meet quality standards. Based on these results, sustainable management and regular monitoring of the Kayan River are necessary to ensure its continued use does not pose environmental and public health risks.