This female California Darner (Rhionaeschna californica) was ovipositing (laying eggs) in the leaves of the submerged vegetation. At this stage, she has already mated with a male, so the eggs are fertilized.
A male California Darner was hovering nearby. It's likely he was the last male to mate with this female. The male was actively guarding the female, defending her against visits by other males. (There's a good reason for doing so, as it's thought that dragonflies can remove the sperm from previous matings. By chasing away other males, he protects his investment.)
Here's a close-up of the male in flight:
The eggs that the female deposited will eventually hatch into larvae (dragonfly nymphs). In large species such as this, the larval stage will often last for at least a year or more. Then the nymph will undergo metamorphosis, crawl up a plant stem into the air, and break out of its nymphal skin to emerge as a flying dragonfly. The adult dragonfly may live for a month or two.