There have probably been islands in Hillsborough Bay for almost as long as the bay has existed. The original delta islands were created as sediment, and other material flowed out of the Hillsborough River and into the bay. Plants eventually took root, and more material accumulated on the growing islands, more on the southerly island than on the one closer to the river's mouth.
Small sandbars extended out from the two islands, further demonstrating the shallow nature of Hillsborough Bay. It was these two islands, and the surrounding tidal area, that formed the nucleus for Seddon Island (now Harbour Island), created in 1904 as part of Tampa's growing port, and Davis Islands, developed by David P. Davis from 1924 to 1926 during Florida's land boom. Read the full Tampa Tribune article here.