river in Shansi province
涑 is a rare kanji whose only clear meaning is a specific river name in Shansi province. Its standalone meaning is otherwise unclear.
涑 combines 氵 (water) with 束, which likely contributes the sound. The exact historical development is uncertain.
The water radical 氵 beside 束 suggests a river tied to the sound of 束. Picture a river whose name sounds like 'soku' flowing through Shansi.
For ソウ, imagine the river's name sounding like 'sow' (as in planting seeds) along its banks: sow -> ソウ, and the water flows past.