dam
埧 centers on an earthen barrier that holds back water: a dam. The given meaning is a single concrete object.
埧 combines 土 (earth) with 具, which likely contributes the sound. The character is a phono-semantic compound for an earthen dam.
The left side is earth 土, and the right side 具 looks like a tool or structure. Picture a massive earthen tool holding back a river: 埧 is a dam.
For ク, imagine a cool, calm lake behind the earthen dam: cool -> ク, and the dam keeps the water still.