Canto 2 -
Ayodhyā-kāṇḍa
Chapter 105: Rāma Consoles Bharata With Spiritual Instructions
Text 2.105.19

अत्येति रजनी या तु सा न प्रतिनिवर्तते।
यात्येव यमुना पूर्णा समुद्रमुदकाकुलम्॥

atyeti rajanī yā tu sā na pratinivartate
yāty eva yamunā pūrṇā samudram udakākulam

atyeti = that has gone by; rajanī tu = the night; na = does not; pratinivartate = return; yāti eva = goes straight; yamunā = Yamunā; pūrṇā = [when] filled [with water]; samudram udaka-ākulam = to the water ocean.

The night that has gone by does not return. Yamunā, [when] filled [with water], goes straight to the water ocean.1

Yamunā cannot be restrained by any means from going to the ocean.

1 Why water ocean? Because we learn from the scriptures that there are different types of oceans in the universe within the sense perceptual range of higher living entities. Rāmāyaṇa-bhāva-dīpa: udakārṇavam iti dadhy-ādi-samudra-vyāvṛtty-artham.