go upstream, retrace the past
溯 centers on moving against a current to return to an origin: going upstream or retracing the past to find where something began.
溯 combines 氵 (water) with 朔, which likely contributes the sound and the idea of going against or returning. The character expresses moving against a water current.
Water 氵 on the left and 朔 on the right: imagine a person pushing a boat upstream against the flow, retracing the river's path back to its source.
For ソ, picture a salmon swimming upstream: the salmon's 'so' sound (like the beginning of 'salmon') cues ソ, and the fish retraces its path against the current.