Photo attached; growing near the edge of a small lake in Raleigh, NC; my guess is one of the primroses.
yes, it is in the evening primrose family, Onagraceae. It is a nice picture but I really need to see more of the plant. Based on the picture it may be Ludwigia uruguayensis, which can grow in large mats on ponds. It is certainly a Ludwigia, anyway. One needs to know if the stems are erect or prostrate and more about its habitat. In other areas Ludwigia peploides is more common, a similar species.
Dr. Steven Hill, Botanist, SCNPS