One man propitiates Indra, augmenting (his vigour) by sacrifice; another, who is insincere, worships (him) with mind averted (to worldly thoughts); (to the first) he is like the lake at the teertha to a thirsty (pilgrim), (to the other) like a long road which retards the end (of the journey). (Rig Veda, 1.173.11)
The sacrifice is prepared, poured are the allotted oblations: as to a teertha come the devout unto the Wondrous (Divine). May we obtain the happiness of heaven, may we become acquainted with immortals. (Rig Veda 10.32.3)
Day and night are the sea that absorbs everything and the twilights are the unfathomable teerthas (of this sea). (Sankhayana Brahmana, 2.9)
Just as some parts of the human body (e.g. the right hand or ear) are held to be purer (than others), so some localities on the earth are held to be holier than others. Teerthas are held to be holy on account of some wonderful natural characteristic of the locality or on account of the unique grandeur of the local waters or on account of the fact that some sage resorted to them (for austerity). (Skanda Purana, 4.6.43-4)
Gods and sages do come to bathe at Triveni (Allahabad). … If they do not come, the place can no longer be considered a teertha. (Swami Vijnanananda)