This is the Ballona Lagoon in 2,000, BC. As you can see, it was much more expansive than today, and at that point, it was abutting against the Ballona Bluffs (El Segundo Sand Hills) to the south. Two creeks--Centinela and Ballona--once emptied into it.
This is the lagoon in 1861. Siltation and erosion accounted for its shrinkage, and tidal marshes eventually encircled it.