Valmiki Ramayana mentions eastward flowing river Yamuna and river Yamuna merging with Ganga. It was a big river in its own right and Laxman had to build a raft in order to cross the river.
Valmiki Ramayana also mentions westward flowing Sutlej (Shatudri).
Thus it appears that significant portion of water flowing in rivers Yamuna and Sutlej was no longer feeding grand Sarasvati (as described in Rig-veda), during Ramayana times.
Journey of Messengers: From Ayodhya (eastern UP to NW (ancient) India)
(Valmiki Ramayana, Ayodhya Kanda, Sarga – 68)
They (messengers) reached river Malini (flowing between southern end of mountain Apartala and northern end of mountain Pralamba), crossed river Ganga near Hastinapura, proceeded westward to reach territory of Panchala through Kurujangala. They observed lakes with full blown flowers and rivers with lucid waters. They reached bank of river Sharadanda (translucent water and frequented by various birds). They reached a holy tree – Satyopayachan, on the west bank of river Sharadanda and then entered city of Kulinga. They emerged from town on Tejobhibhavana and then entered Abhikala (town), then crossed river Ikshumati, then reached mountain Sudama in the interior of Balhika kingdom. They also cross rivers Vipasa (modern river Beas/Bias), shalmali and other rivers. They eventually reached Girivraja, capital city of Kekaya.
No direct mention of river Sarasvati during this journey of messengers.
Let’s look at the journey of Bharata/Shatrughna from Girivraja (Kekaya) to Ayodhya
(Valmiki Ramayana, Ayodhya Kanda, Sarga – 71)
Bharata left from Rajagruha (Girivraja?), moved east, crossed river Sudama, another river Hradini and then crossed river Shatadru (modern Sutlej) whose stream was flowing west. He crossed another stream/river near Ailadhana and then reached the region of Aparaparvata where he crossed a river which petrified everything thrown into it. He moved further south-east to the region ‘shalyakarshana’. Then reached river Shilavaha, then crossed Mahashaila hills heading towards the forest –Chaitraratha. Then he reached river Sarasvati and branch of holy Ganga (close to their confluence), passed through northern part of Vira-matsya territory and entered the forest of Bharunda. He crossed river Kulinga, and river Yamuna. Then he reached great river Bhagirathi (Ganga) that was difficult to cross. He crossed it near town of Pragvata, he reached river Kutikoshtika and then arrived at town of Dharmavardhan. Then he passed through Jambuprastha, Torana and Varutha (towns and villages). Then he passed through (further east) Ujjihana, then Sarvatirtha and next day crossed river Uttanika and many other streams to reach town of Hastiprushtak, then crossed river Kutika. He crossed river Kapivati, near village Lohitya. He crossed river Sthanumati near Ekasala and crossed river Gomati near Vinata and then reached city of Kalinga and soon he reached Ayodhya.
Thus descriptions of Bharata’s journey mention westward moving river Sutlej (Shatudru), river Yamuna and also river Sarasvati. Also note that geographical locations of river Sarasvati (and also of rivers Sutej and Yamuna) corroborate well with their current stream or their paleo-courses (in case of Sarasvati).