Watercress (Nasturtium officinale) is ready to harvest in our neck of the woods, and we will be out today cutting some.
We have a favorite trout stream we go back to each year. Water quality is a big factor in locating a stream (we try to avoid runoff from livestock and fertilizer/pesticides -- trout are a good omen). And if on private land it is essential to obtain the landowner's permission.
http://www.wildedible.com/wild-food-guide/watercress