You might compare it to Desmodium triflorum. Native to warm, high rainfall areas. Not sure if it grows IN water, though.
hortster

To me it looks a lot like some sort of trefoil such as one of the Medicagos. M lupulina looks similar. I don't know what would be likely in your region.

I suspect that it's a terrestrial plant that became established during a dry period. Medicago lupulina is a definite possibility.

I'd go along with the Lesser yellow trefoil.

I worked through a few possibilities, and no golden tickets yet. Desmodium triflorum has leaflets with the right shape, but the plants are too hairy and the leaflets don't have those lateral veins. And online sources suggest that it needs a warmer, wetter climate than available here. Medicago lupulina leaves have much shorter petioles, and the plants are too hairy. Medicago polymorpha is interesting, since it is a less hairy plant and has those long petioles. But the leaves should be more heavily toothed toward their tips. Trifolium campestre and Trifolium dubium seem wrong - too hairy and leaves on fairly short petioles.

Lycopus makes a good point - the plant may have been established during our summer drought, so may not be an aquatic species.

When I cropped the photos, I did see that leaflets had about 5 pairs of lateral veins. The leaves also had leafy bracts at their base, which were not yet fully developed. Finally, all plant parts appeared to be glabrous.

If you have more ideas, I'll happily check them out. But I'm starting to suspect that these may be developing plants that are not yet in their final form.

Thanks for the input.

If I saw it growing here, I would think Medicago lupulina.

I was on a hike this past weekend where the focus was on the changes to the ecology of an area immediately surrounding a reservoir pond (or lake in some parts of the country!) that had been partially drained to repair damage to a retaining wall. The plants that opportunistically moved into the very fertile areas that used to be under water were quite amazing. Plants do find a way!

FataMorgana