waterpipe, flume
筧 centers on a conduit for water: a pipe or flume, often made of bamboo, used to channel water.
筧 combines 竹 (bamboo) with 見, which likely contributes the sound. The character originally referred to a bamboo water pipe.
Bamboo 竹 is hollow and can be used to make a pipe. The bottom part 見 suggests seeing water flow through it. Picture a bamboo flume carrying water where you can see the stream.
For ケン, imagine a ken (a traditional Japanese unit of length) of bamboo pipe channeling water: ken -> ケン, and the bamboo flume measures exactly one ken.
open water pipe; conduit