Water smartweed, or water knotweed. (Formerly this species was placed in the Polygonum genus (the knotweeds), hence the second name.
As the species name suggests this is an amphibious plant which can grow either on water or on damp land. When growing in water (up to 15ft deep) the leaves will float on the surface. When growing on land it sprawls with some stems reaching up about 3 or 4 feet.
I’ve seen this blooming at the west end of the wooden bridge over northern Lake Los Carneros in Goleta. I’ve seen it blooming in September but suspect it started long before then.
The inflorescence consists of many five-petalled pink flowers.