Rorippa sessiliflora (Nutt.) Hitchc.
Family: Brassicaceae
southern yellow-cress, stalkless yellow-cress
[Nasturtium sessiliflorum Nutt.,  more...]
Rorippa sessiliflora image
from USDA Plants website  
Etymology: Rorippa: an Anglo-Saxon word rorippen whose meaning has been lost
Plants: usually erect annual, 8"-16" tall forb, smooth, usually branched
Leaves: stalked, lower leaves often pinnately-divided below the middle
Flowers: yellow, 4-parted, no petals, mature stalk less than 1/8" long; inflorescence a cluster (raceme) of stalked flowers; blooms May-Oct.
Fruits: thick pods
Habitat: wet; marshes, shallows
Conservation Status: Native