14-mile Island is an alluvial island, meaning it formed by sediment carried downriver and deposited. This glacially-derived sediment is picked up where the river flows quickly and deposited where the river flows slowly.
Sand bars, called shoals, can be found nearby in shallow areas of the river and are often flooded. The islands are heavily covered in vegetation and remain undeveloped, due in part to regular flooding of the islands.
The lower island of Allegheny Islands State Park is the second largest and most natural of the river islands in the Allegheny or Ohio rivers within Allegheny County. The island's shoreline and the river shoals around it provide important habitat for numerous wildlife species.
Wildlife species often seen at the island, along with an occasional deer, include:
- Beaver
- Raccoon
- Great blue heron
- Belted kingfisher
- Wood duck
- Mallard
- Northern flicker
- Various species of swallowtail butterflies
Numerous plants can be found as well, including:
- Sycamore
- Cottonwood
- Maple
- Willow
- Silky dogwood
- Cardinal flower
- Aster
- Lily of the valley
- Tall sunflower
- Ironweed
Unfortunately, Japanese knotweed, garlic mustard, mile-a-minute, and other invasive plants are also found on the island.