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 
Which other texts talk about Saraswati as a major river and when does it start getting described as "dried up"?