Spatterdock:

Spatterdock, also known as the Cow Lily, is a large aquatic plant with floating or emerged heart-shaped leaves.  It blooms yellow flowers from June through September that appear to be “half-opened”.  Spatterdock can grow in lakes, ponds, and sluggish streams where it establishes its large leaves, thick stems, and flowers.  Spatterdock spreads easily through pollination by insects and can quickly take over the surface of a body of water.  This aquatic plant can be treated with a glyphosate herbicide such as AquaPro mixed with Cide Kick-II.