In order to show the stages of this plant, I've chosen photos that show the flowers which appear before the leaves and then one with the leaves. It is a plant found in many different locations throughout the park.
Amongst disturbed alluvial river terrace
Callitriche stagnalis, growing on bare ground.
I think it is this species though it is growing on bare soil which is dryer than its normal habitat.
Abundant self-sown seedlings in garden area, to the point of being a weed.
Self-sown. Fairly common. Established on bare ground before bark was laid down.
Lamiales is an order in the asterid group of dicotyledonous flowering plants. It includes approximately 24,000species divided into about 20 families. Well-known or economically important members of this order include lavender, lilac, olive, jasmine, the ash tree, teak, snapdragon, sesame, psyllium, garden sage, and a number of table herbs such as mint, basil, and rosemary.