Nasturtium officinale

Watercress. Grows in streams. Common in the Ventura River. Small patches show up on various creek.

It is difficult to get a good feel for when it blooms since winter floods tend to scour it out of our creeks, but in the winter 2011-2012 we did not get a heavy enough rain, and it looks as though watercress starts blooming in late March and finishes in mid-December.

Formerly this was known as Rorippa nasturtium-aquaticum. Calflora claims it is indigenous to California, while Wikipedia implies it is only native to Europe and Asia.