Watercress, Nasturtium officinale
Brassicaceae or Mustard Family
 

Spreading plant with compound leaves and dense clusters of white flowers, purplish inside.  Flowers have four petals in a cross arrangement, and are purplish inside.  Grows in water or mud and blooms in late spring. 

Very common in the marshy areas near the Park entrance.

Easily identified by its white flower clusters and very wet habitat.