cleanse, river in Gansu province
洮 centers on cleansing with water, and by extension a specific river in Gansu province. The river name is a proper-noun application of the cleansing-water idea.
洮 combines 氵 (water) with 兆, which likely contributes the sound. The character originally referred to a river name, and the cleansing meaning likely derives from the water association.
Water 氵 beside 兆 suggests a river washing things clean. Picture the river in Gansu province as a natural cleanser flowing through the land.
For トウ, imagine the river's cleansing water flowing into a toe: toe -> トウ, and the water washes the toe clean.