. SR 21 originated as the
Marble Mount Road that extended from the north end of the
Keller Ferry to
Republic and was established in 1899. The Marble Mount Road was later numbered in 1905 and renamed to the
Sans Poil-Loomis Road in 1907. An extension of State Road 4 from Republic to the
Canada–US border existed from 1915 until 1923. When the
Primary and secondary highway system was established in 1937, (PSH 4) replaced State Road 4 and was extended south from the Keller Ferry to
Wilbur. The Canada–US border branch of State Road 4 that was deleted in 1923 was re-added as Secondary State Highway 4A (SSH 4A) and a highway extending south from Wilbur to
Lind became SSH 2B. All three roadways were later combined as SR 21 in a
highway renumbering in 1964. In 1983, SR 21 was extended south to in
Kahlotus. The
speed limit between
Curlew Lake State Park and
the community of the same name was temporarily lowered on March 9, 2009 to due to cracks in the pavement. The speed limit was restored to on March 30, 2009.
Keller Ferry history route from September 9, 1948-July 7, 2013, just two months short of 65 years. The Keller Ferry connects SR 21 between
Lincoln and
Ferry counties, which are separated by
Franklin D. Roosevelt Lake. The ferry originated as an oar-propelled canoe that was used by Native Americans prior to the late 19th century. In the early 1890s a four-car
cable ferry, owned and operated by Todd Clark and William Robertson, was established. After the town of
Keller was established north / upriver on the
Sanpoil River in 1898, J.C. Keller, the founder of the town, purchased the cable ferry in 1899. In 1925, Lincoln and Ferry counties jointly purchased Keller's ferry and in 1929 replaced the original ferry with an eight-car cable ferry that was later named
Keller of Seattle. Between 1944 and 1948, the
Ann of Wilbur, a
tug boat, tugged the
Sanpoil barge that was used as a temporary replacement for the
McLeod. On September 9, 1948, the
Martha S. was launched and has been in continual operation since. Since the
Martha S. is over seventy years old, the
United States Coast Guard requires that the ferry undergo a full drydock inspection every five years. The Keller Ferry was repaired on February 15, 2007 to expand its lifespan; WSDOT detoured traffic onto . The
Martha S. had a leak that was discovered in October 2009 and is suspended. Traffic has been detoured onto other highways. Ferry service resumed on October 19, but one of the two engines overheated on October 26, only one week later, needing to be replaced, thus shutting down the ferry a second time during the same month. Due to the prohibitive cost of having to specially manufacture many replacement parts, the Martha S was retired on July 7, 2013. It was replaced with the new M/V Sanpoil, which made its maiden run on August 14, 2013. ==Major intersections==