In October 2005, heavy flooding in the New Hampshire area forced the closure of NH 123 in two different locations.

The first was at the intersection of NH 123 and Cold River Road near Drewsville, where a bridge was destroyed from high water in the Cold River, approximately 5 miles (8 km) east of North Walpole.

The second closure was approximately 3 miles (5 km) south of the junction with New Hampshire Route 123A in the town of Alstead, where a section of roadway was completely washed away by the water.

The southern segment of NH 123A is a 4.216-mile-long (6.785 km) secondary rural road that runs from the town of New Ipswich to the Massachusetts border.

The northern segment of NH 123A is a 10.070-mile-long (16.206 km) secondary rural east–west highway in western New Hampshire, running between the towns of Alstead and Marlow.