Long Island Sound and the Atlantic Ocean are connected by what New Yorkers call the East River, which is actually a waterway called a strait, in this case having tidal flows between the bigger water bodies (so no, it's not actually a river). The East River passes the "mainland" (The Bronx) and numerous islands are touched by its saltwater flows (like Manhattan, and Long Island (Queens and Brooklyn are part of this island)). Other smaller islands, like South and North Brother Islands, which are not currently inhabited by people, and Randall's Island, are fully surrounded by straits*.
Randall's Island is really an amalgamation of terra firma. Its current state shows you just how much humans can impact land and water. We united apparently three formerly disconnected islands, including Ward's Island, Randall's Island, and the Sunken Meadow island. Looking at the Historical NYC Maps & Atlases, you can see three distinct islands up until 1922 (maps from at least 1839-1922 show 3 islands). While we had many impacts including deforestation by using the land itself , in the last hundred years, we also altered this area by joining the islands.