The Carmans River is one of the four major rivers on Long Island. The 10 mile river starts as a freshwater stream in Yapank and runs south through Upper and Lower Lakes, South Haven Pond and into the Great South Bay.  The river gets its water from groundwater and for the first 8 miles consists of fresh water, however it becomes brackish in the last two miles as it mixes with the salt water of the Great South Bay.

​

The Carmans River is ten miles long and has a drainage basin of seventy-one square miles. Parkland and underdeveloped private property as well as a few residential and agricultural areas, border most of the river.