The Mianus River Park features the simple and unadorned natural beauty of ridges and valleys left behind by glacial movement thousands of years ago. Intermittent streams and perched Red Maple swamps are scattered throughout.. A network of trails passes through Oak and Tulip dominated uplands, areas with young Ash and Cherry trees, old stands of Hemlock and Beech trees, an old field, rock outcrops and Mountain Laurel patches. Among the Black Birch, Tulip Tree, Red Ash and Tupelo can be found many types of fern including: Ladyfern, Sensitive Fern, Cinnamon Fern, Maidenhair, Royal Fern and Bracken.

The park’s wetlands are prime breeding pools for many types of salamanders and frogs. The variety of vegetation provides resting and nesting habitats for wildlife, such as ruffed grouse, pheasant, mallards, cranes, heron, songbirds, woodpeckers, squirrels, deer, raccoons, fox, coyote, weasels, mink and river otter, and other small mammals.

The park’s forest is made up of hardwoods such as maples, elms, oaks, tulips, ashes, hickories, cherries, birches, sassafras, tupelos, hackberries, and beeches. Conifers include hemlock and white pines. Squirrels, raccoons, possums, screech owls, tree swallows, and woodpeckers nest in holes in standing dead trees ("snags"). Hawks like the tops of tall trees; robins prefer the lower branches; owls often choose conifers.

Where the understory is undisturbed, the shrub layer is equally diverse, including sweet peppers (Clethra), spicebushes, winterberries, witchhazels, shads (serviceberries), mountain laurels, maple-leaf viburnums, arrowwood viburnums, blueberries, and tree saplings. Many songbirds nest in the shrub layer.

At ground level, there are ferns, grasses, toadstools, and wildflowers. Grouse, thrashers and rabbits nest under the shrubs; foxes, skunks, and tiny rodents dig burrows. Box turtles bury their eggs in loose soil. Chipmunks and snakes often make their homes in the old stone walls.

Along the water, look for cottonwoods, willows and sycamores; shrubs include elderberries, alders, and silky dogwoods. Blue herons and Baltimore orioles often nest in tall trees near water. Weasels, minks, and muskrats burrow in the riverbank. Mallards nest and painted and snapping turtles bury eggs along the riverbank. Salamanders, toads, and frogs lay eggs in the wetland pools. Fish eggs, fingerling fish, and crayfish are found in the nooks and crannies of the riverbed.

Over 150 resident and migratory birds have been spotted in Mianus River Park and around 70 are recorded as breeding in the park. The Friends group has worked with the Audubon Society to create an "Urban Oasis" in the park, with native, bird friendly plants, to provide a resting place for the many birds which migrate through this area.

Around 15 different mammals have been recorded in the park, ranging from white tailed deer to white-footed mice. Many are shy and can most easily be seen early in the morning or later in the day, especially away from the busiest sections of the park.

Of the amphibians, several types of frog and toad have been recorded in the park, namely; the Gray Tree Frog, Pickerel Frog, Wood Frog, American Bullfrog, Spring Peeper, and American Toad. Spotted Salamanders, Red-backed Salamanders, and Eastern Newts are also park residents.

A wonderful resource for further research and printable lists of birds and animals in the area can be found at Audubon Greenwich.