I believe this is Yellow Nutsedge (cyperus esculentus)?
There is a lot of this growing in the mud flats, that are under water in Summer. I can get more pictures if there is something else need for identification. This was one of the largest plants.
I think the photo of the base of plant distinguishes this from Cyperus odoratus.
In the main park now. I dug one up and there are tubers on the stolons. It was hard to measure the flower scales without a dissecting scope and stage micrometer. They seemed around 4 to 5 mm.