[AFPhys]

I suspect "to the river" simply means "as far as the river", not meaning there is necessarily an exit there. I will believe this is simply a transport tunnel toward central Baghdad until otherwise reported.

The important tunnels are not going to be this easy to access, especially with all the rubble of those "Palaces" blocking the entrances.

[Cboldt]

I just assumed "the river" was a reference to the Euphrates, away from the city, but looking again at the map, the Tigris is indeed closer.

I agree with you, it is unlikely to be part of any "secret" or otherwise militrily/regime valuable network. The WDC subway system, etc. is not connected to the military/political bunker and tunnel system "nearby."