Bill the little island has an Iris, Calla Lily, and 2 different kinds of Canna's. According to info that came with the plants the hardiest was the Iris to zone 5, I'm in zone 3 so they're probably history, but the grasses I transplanted to it should survive if the ice doesn't destroy the island.
I've measured the height of the natural islands from summer to winter and they only sink about 20 inches. I planted pickerel weed on them but the cattails, grasses and other plants were put there by nature and survive well. As you said the sinking probably helps protect the roots.
