After the death of Ieyasu, the second shogun, Hidetada TOKUGAWA ordered the the diversion of the Hirakawa River, which flowed southwards from west side of the Koishigawa-mon gate, to flow eastwards to secure a defensive barrier for Northeastern Edo..
家康の死後、二代将軍徳川秀忠は、江戸の北東の守りを確保するため、小石川門の西から南に流れていた平川をまっすぐ東に通す改修を行った。