When they originally were planning on building up south Florida, the Everglades got in the way, so they designed an impressive canal drainage system, Unfortunately, their engineering was good but the science was bad, they need those wetlands and dumping valuable fresh water into the ocean is just not practical in these days of big droughts and fires and increasing human demand. In order to help bring the ecosystem there back into better balance, a huge earthen reservoir is being built, to store water and release it as needed.