The chart comes from the Water Survey of Canada which maintains five automated stations within the Cowichan Watershed. The Cowichan Watershed Board provides a portal to the WSC on the Discharge Charts page.
This chart has four main areas, which are described here:
In the centre, is the 18 month chart showing the seasonal rise and fall of Cowichan Lake. The line is made up of the daily average of measurements from the automated station. The level is given in metres above sea level. 162.37 metres is the top of the weir.
On the left, is a photo of the gauge mounted on the weir. It uses the Imperial scale in feet, with the zero at 162.37 metres above sea level.