Neither, just silting up. The Tiber has been filling in that area with sediments for thousands of years. The harbour was just a tiny dent in that natural proces

Neither, just silting up. The Tiber has been filling in that area with sediments for thousands of years. The harbour was just a tiny dent in that natural process. Here's a poster with some progress drawings over the millennia: https://www.academia.edu/29407769/The_effect_of_the_Tiber_delta_development_in_the_Central_Tyrrhenian_Sea https://www.academia.edu/29407769/The_effect_of_the_Tiber_de...