Final answer:
A semicolon is needed to separate the two independent clauses in the given sentence.
Step-by-step explanation:
In the given sentence, a semicolon is needed to separate the two independent clauses.
The revised sentence would be: On the Pont des Arts, a pedestrian bridge in Paris, couples have expressed their love by clipping padlocks to the river's railings and throwing the keys into the; however, the weight of so many locks has caused the railings to collapse in some places.