The Bhagirathi is one of the most important and largest river of the Garhwal hills. The phytoplanktonic community in Bhagirathi was represented mainly by Bacillariophyceae (86%), Chlorophyceae (10%) and Myxophyceae (4%). The most common blue-green algae were Fragilaria species followed by Diatoms species frequent in the mountainous stretch and its degree of occurrence decrease in the lower region. The Ulothrix species of Chlorophyceae algae were moderately frequent at the upper part of Bhagirathi but less frequent in the lower region. Phormidium species was less frequent at upper region but moderately frequent at downstream of Bhagirathi. The highest resemblance between phytoplanktonic communities of two Zones, upstream and downstream of Tehri reservoir was observed. Zone 1 showed 60% resemblance with these two zones. The diatoms density shows an inverse relationship with turbidity, despite the fact that high turbidity indicates the possibility of high silicate level in the water, which is essential for diatoms growth. The present study revealed that the combined impact of DO, TDS, the velocity of water, turbidity and water temperature influenced the diversity of phytoplankton population dwelling Bhagirathi river.