What native plant found on shorelines has thin flat leaf blades and a flap of tissue at the top of the leaf sheath?

The description provided in the question accurately matches the characteristics of Southern watergrass. This native plant is typically found on shorelines and is recognized by its thin flat leaf blades and a distinctive flap of tissue at the apex of the leaf sheath.

Southern watergrass thrives in wetland environments, which makes it well adapted to shoreline habitats. Its morphology of flat blades not only helps in photosynthesis by maximizing light capture but also aids in water dispersion and nutrient absorption. The flap of tissue on the leaf sheath is a unique feature that distinguishes it from other similar species, serving a functional purpose in protecting and supporting the emerging leaf blades.

The other plants listed do not have the specific combination of features described in the question. For instance, mosquito fern is a small floating aquatic plant with finely divided fronds, which do not resemble flat blades and lack the leaf sheath characteristic. Soft rush has a different leaf structure and stiff form, while slender spikerush or road-grass typically has a much slimmer profile and does not exhibit the same flap of tissue on its leaf sheaths. Thus, based on the specific traits mentioned, Southern watergrass stands out as the correct answer.