Tehri hydroelectric project of 2,400 MW capacity, the Asia's highest dam constructed on the Bhagirathi river in Uttarakhand is one of the most debatable dams of the country. The fluvial system of Bhagirathi and Bhilagana has been converted into a huge lentic water reservoir. Therefore, the information on periphytonic diversity before impoundment is very important. The preimpoundment study over one year (September 2004-August 2005) was carried out monthly to analyse the periphytonic diversity in a lotic environment of Bhagirathi, one of the parent streams of Ganges.