 Yoshiaki supported Tokugawa Ieyasu when the Battle of Sekigahara broke out in 1600, and defended his territory against the attack of Uesugi clan’s large army, and robbed the Uesugi clan of the Shōnai area.As a reward, he was given 570,000 "koku" by Ieyasu after the Battle of Sekigahara ended, and the Mogami fief became the fifth largest in Japan, excluding the lands held by the Tokugawa.Afterwards, Mogami Yoshiaki developed flood control of the Mogami River in his territory, which allowed for safer navigation of the river and also for irrigation to increase rice farming, and this was one of his great successes.