Question: W.H. Walker and Brothers was a narrowboat builder based in a small town approximately 20 mi northwest of what?
Sentence (from "Rickmansworth"): Rickmansworth is the administrative seat of the Three Rivers District Council; the local authority is named from the confluence of three rivers within Rickmansworth's borders; the River Gade and the Grand Union Canal join the upper River Colne near Rickmansworth's eastern boundary and are joined by the River Chess near the town centre from where the enlarged Colne flows south to form a major tributary of the River Thames.
Relevant evidence: No