下游
xiàyóu · HSK 7
lower reaches
🌊 下游 = 下 (lower) + 游 (flow/swim) = the lower flow of a river
下 — horizontal line 一 + mark below
下 is an indicative character: a long horizontal line represents a reference level, and a shorter stroke placed beneath it points to the area below. The original meaning 'below, down' comes directly from this spatial diagram, and it later extended to verbs like 'descend' and 'lower'.
游 — water radical 氵 + 斿 (banner/streamer)
游 combines the water radical 氵 with 斿, which depicts a banner streaming in the wind. The original sense was a flag fluttering or floating, extended to drifting on water, swimming, and wandering. Today it commonly means to swim, travel, or roam.