“l’“, wich refer to the river is here the complement d’objet direct of “comparée“.

If you prefer, you could replace “l'” by “rivière” (I had to teaks verbs tense to pass to live speech):

Les eaux de cette rivière sont si pures, ses rivages si enchanteurs
que cette rivière a souvent été comparée au Rhin ou au Rhône.

The past participle comparée agrees with l’ because l’ is a direct complement which is placed before. That’s the role you’re referring to. It’s a direct complement since there is no preposition before it. The pronoun l’ is feminine since it refers to rivière.

In the past tense when the verb is conjugated with the auxiliary “avoir” the past participle of the verb does not agree neither in genera nor in number with the subject but in particular case it agrees with the direct object complement when this one is placed before the verb. in the case of this sentence the “L” which replaces the direct object complement is placed before the verb “L’a souvent comparé” and seen that “eau” is feminine so we add the “e” to the verb “comparer” .

Well, the « COD » (Complement d’objet direct) is before the verb (a). So we need to change the adjectives after it. We put comparée because the COD is referring to river, which is a feminine word.