Leyden lies along the eastern edge of the Berkshires, with the town mostly lying along the valleys of the Green River (which also forms the western border) and the branches of Glen Brook. The majority of the town away from the brooks is forested, and the town has a small state forest and a wildlife management area within its borders. The confluence of the main and east branches of Glen Brook is dammed to form the Greenfield Reservoir, before the brook flows southward into Greenfield and meets the Green River.