Complex sentence: Port Isabel Slough was found by Captain W. H. Pierson of the schooner "Isabel" in 1865, that allowed for an anchorage, sheltered from the tidal bore, permitting safe transfer of cargoes and a few miles farther up a slough ideal for a shipyard.
Split into simpler sentences: Port Isabel Slough was found by Captain W. H. Pierson of the schooner "Isabel" in 1865. This slough was an anchorage, sheltered from the tidal bore, permitting safe transfer of cargoes and a few miles farther up if was another slough ideal for a shipyard.