Temporal variation characteristics of flow and water level in the Old Brahmaputra River

The principal objective of this paper is to assess the variability of water level and flow of the Old Brahmaputra River. The water level and Discharge trend of the river has been assessed based on historical data to find the variability over time. Average monthly discharge at a particular gauge station of the river has been plotted for the duration of 1975 to 2006 for each of the month to have an observed flow trend characteristics through time. The trend lines show that the river has undergone a considerable declination during the period. The wet period show large fluctuations of flow in a range of 2200 to 500 m 3 /s with a sharp decreasing pattern. The wet period flow of Old Brahmaputra River shows relatively lower discharge almost in every alternative year and its declination is radical compared to dry period flow. Alike discharge pattern water level also shows decreasing trend. Frequent fluctuations of water level are observed during the period from 1991-2008 when it shows more dispersive nature in the monsoon months. This declination of water level is observed to be faster during 1991-2008 than 1973-1990.