Entlang describes that its along the river. But why was "an" used here? "entlang" is more related to movement along something ... while "an .... entlang" is more related to existence without movement. So I wonder why did he put "an" while the sentence is talking about active movement towards the city of Base.
Your observation is wrong, there is no such connection with movement or simple presence. An+Dativ+entlang is common for both:
Sie ging am Ufer entlang, als …
Ein Weg führte an der Mauer entlang.
Entlang+Genitiv is a bit old-fashioned or even poetic:
Sie ging entlang des Ufers, als …
Entlang der Mauer führte ein Weg.
A third option is
Sie ging das Ufer entlang.
Ein Weg führte die Mauer entlang.
This uses entlang as a bare adverb. It changes the meaning of the verb directly. Some even think entlanggehen and entlangführen are separable verbs taking the thing passed as an accusative object.
I think your confusion comes from the fact an ist one of the dual-way prepositions. That's true. But entlang required the genitive in the past, and using an+Dativ is a more recent development. So, this particular an-entlang isn't dual-way but a strictly dative preposition.
Similarly, an-heran, auf-drauf, and in-hinein are strictly accusative prepositions, though bare an, auf, and in are dual-way.