What plant is that?

The April Mystery Plant is a perennial herb found along lake and river shores and in wet meadows. It has opposite, simple and entire (untoothed), elliptical (oval) leaves. The yellow flowers are 1/8” – 1/3” wide, are borne singly on elongated pedicels, and have 5 separate petals and sepals and 5-12 stamens per flower. The fruit capsule is 2-3.5 mm in length.