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 of 2.62x10(9) m3 capacity. 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. Periphytons are the important primary producers of metarhithronic stretch of Bhagirathi. 40 taxa of periphytons, represented by the families of Bacillariophyceae (25 taxa), Chlorophyceae (11 taxa) and Myxophyceae (4 taxa), were recorded from the Bhagirathi river before impoundment. The Shannon-Weiner diversity index ranged between 3.404 to 4.360 for periphytons, which indicates a good health of the ecosystem. The present study revealed that the combined impact of velocity of the water, hydromedian depth (HMD), turbidity, dissolved oxygen, total dissolved solids (TDS), and water temperature influenced the diversity of periphytons dwelling Bhagirathi.