20/12: **UPDATE**
The National Park team has confirmed that the bridge at Allt Rostan (aka Boundary Bridge, about 2.5 miles north of Inversnaid) is passable with care. At times of heavy rain or – as in this case – fast snow melt, the volume of water surging down the burn can make it impassable. If you are out walking this section over winter, please be aware that the situation can change fairly rapidly due to weather conditions, so please avoid if the forecast is bad. At this time of year, there is no alternative route (the waterbus does not run) and accommodation in the area is likely to be closed.
19/12:
We have received some reports of an impassable bridge on the West Highland Way between Inversnaid and Doune Bothy. Weather dependent, National Park Rangers will investigate tomorrow (Tuesday 20th) and it is advised not to attempt this section until this can be properly assessed.