It [Whetstone River] enters the Minnesota River at Ortonville, Minnesota, about 0.25 mi (0.4 km) after crossing the state line and just downstream of the Minnesota’s emergence from Big Stone Lake.- Big Stone Lake (Wikipedia)
Big Stone Lake (Dakota: Íŋyaŋ Tháŋka Bdé) is a long, narrow freshwater lake and reservoir on the border between western Minnesota and northeastern South Dakota in the United States.