name of a river
沘 is a rare kanji used primarily as a proper noun for a river name. Its standalone meaning is limited to that specific river reference.
沘 combines 氵 (water) with 比, which likely contributes the sound. It is a phono-semantic compound used for a river name.
The water radical 氵 beside 比 suggests a river whose name sounds like 比. Picture a river named after the 比 component flowing beside it.
For ヒ, imagine the river named 沘 is called the 'He' river: He -> ヒ, and the water 氵 flows beside the 比 sound marker.
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