The Wilson Creek Weir is a small weir in front of the tunnel under the railway near Helensburgh station. When you go down from the road to river level on the right hand side you will see a waterfall and on the left hand side you will see the tunnel. You can walk through the tunnel towards the weir and go for a swim!
Don't forget to get the clue for the GC815RA "A Tourist in Helensburgh" cache!
Oruvaq gur fgbar jnyy.