These 4" wide river sections allow wider rivers and, with the transition pieces, are modular with the 3" river pieces. The Straight/Curve package contains a 8" long winding river and a 5 1/2" long 45° curve section. The 6" transition pieces allow the use of 3" rivers and 4" rivers in the same river system. The two pieces can also be used as a wide spot on a 3" wide river or a narrow crossing in a 4" wide river.