[2] As currently configured in the state's official milepoint log, SH-51 terminates at the Business Loop 84 intersection with American Legion Boulevard, and is not otherwise officially concurrent/overlaid with BL-84 (where the signs at I-84's Exits 90 and 95 say "To SH-51/To SH-67") or the former section of US-20 that follows American Legion Boulevard to Interstate 84/U.S.

and the intersection with SH-67, the road crosses the Snake River into Owyhee County and passes through the towns of Bruneau, Grasmere, and Riddle.

About 11.5 miles (18.5 km) from the state border it enters the Duck Valley Indian Reservation.

The reservation's only major town, Owyhee is about 4 miles (6.4 km) south of the state border.

The basic route of today's SH-51 was in place as early as the 1930s, mostly as an all-weather gravel road from Mountain Home until it reached the northern boundary of the Duck Valley Indian Reservation, and as an unimproved road on through to the Nevada border and then-NV Route 11 as of the 1937 map.