EdLoach
Reactivating as I found the original note and comment interesting enough to look up the history of the reservoir which was built in the 1830s. Finding a map from before it was built proved a bit tricky, but I did find these two (where the river doesn't seem to follow the exact course where the boundary runs, but it presumably did at some point). http://biblio.unibe.ch/web-apps/maps/zoomify.php?pic=Ryh_1813_12.jpg&col=ryh http://www.bl.uk/onlinegallery/onlineex/crace/a/zoomify88781.html