Rig Veda talks about Saraswati as a mighty river (from Quora)
2.41.6 - Sarasvati is described as naditama, ambitama, devitama or the best river, best mother and best Goddess
6.52.6 - Sarasvati is made swollen by many rivers (pinvamana sindhubhiH)
7.95.2 - Sarasvati flows from mountains to the ocean (giribhyaH A samudrAt)
10.177 - Mandala 10 is the last portion of Rig Veda in terms of chronology. Even this book has hymns which pray to Sarasvati for continuation of sustenance and good fortune [9]
Which other texts talk about Saraswati as a major river and when does it start getting described as "dried up"?