The Valley is named after a sacred water spring called “Tirth” which originates at Hanskund Peak (4800m) and flows down, through deep forested areas. The feeder channel to Tirthan River is fed by deposits of ice, almost all round the year. After covering a distance of 70Kms from the source, it joins Palchan Nallah at Gushaini. The Tirthan River forms the Valley in its origin at Tirth and continues till Larji, where Tirthan and Sainj streams merge with the Beas River. Some of its tributaries rise from aquifers and come out, on the surface, as springs.
Tirthan River flows along a deep V- shaped valley in its upper reaches. The Valley is fairly open and wide near its confluence with the Beas River. The main geomorphic features of the Tirthan valley are incised meanders, river built and river-cut terraces, cliffs, and steep slopes. The entire catchment of this river is covered by alpine, sub-alpine, temperate and sub-tropical coniferous and deciduous forests. Human habitation is there in the form of small villages that exist along the river. The Valley is scenic, full of secluded hamlets, waterfalls, cedrus and pine forests. The valley covers an area of 754 Sq. Kms and the same constitutes Tirthan Wildlife Sanctuary and part of GHNP.