the high land along a river
皋 refers to the high land along a river, specifically a raised bank or marshy shoreland. The meaning is narrow and topographical.
皋 is composed of 白 (white) and 夲 (a variant of 本, root/origin). The exact historical development is uncertain, but it likely originally depicted a white, exposed highland or a landmark on a riverbank.
The top 白 (white) suggests a bright, open space, and the bottom 夲 (root-like) suggests the ground. Picture a white, root-covered high bank rising above a river.
For コウ, imagine a cow (コウ) standing on the high land along a river, grazing peacefully. The cow's moo echoes across the bank: cow -> コウ.