Lake Levels – High Water Mark

One of the biggest concerns I hear are to do with lake levels, particularly in the winter at peak water levels or often referred to as the High Water Mark (HWM).

As is turns out the Government of Canada has been recording lake levels and river flows for decades. This can be found on their website at http://www.wateroffice.gc.ca and look for station 08HA009 where it will show you current water levels and you can search historical data as well. The graph is referenced as: Real-Time Hydrometric Data Graph for COWICHAN LAKE NEAR LAKE COWICHAN (08HA009) [BC]

This data shows us that before the existing weir was constructed in 1957 the HWM averaged 164.16m from 1953 to 1956 and has since averaged 164.01m for the 61 years from 1957 to 2018. The lowest HWM was 162.33m in 1978 and the highest HWM was 165.39m in 1968.

The current weir crest height is at 162.37m. With an average HWM of 164.01m you can see that the weir is actually submerged (by 1.6 meters or 5 feet) during the high water season. I took a few photos a couple of weeks ago and the flow over the weir is hardly noticeable as the water flows completely over it, across the weir island and around the south side. The flow control shifts downstream to the natural river valley close to the Greendale Trestle and it is the river valley that then controls the HWM as the valley narrows and the river bottom rises. Similarly the new weir (at 163.07m) will be submerged during the high water season and shouldn’t impact annual High Water levels.

With longer, hotter and dryer summers becoming the new norm it will be important to hold back some more of these winter waters for release through the summer months.

Will it or Won’t it?

Will the new weir raise the winter high water levels or won’t it?

One of the more common questions I have heard is regarding lakefront owners concern for flood levels. Many of the older structures were built before the current development rules (to build above the 200 year flood level) came into place and as such some of those buildings see flood conditions on occasion. They don’t want to see an increase in frequency or higher flood elevations if possible.

This brings up the question on whether or not the new weir will impact flooding conditions. Well, this is exactly one of the concerns we are trying to determine within our current projects (Weir Design and Shoreline Assessment). The shape, location and operability of the weir along with modelling historical, current and future hydrologic conditions will help us determine the impact the new weir would have during every part of the year. The answer, at this point, is we do not know yet. The weir is in the middle of it’s design process and has to account for a number of design inputs with the priority being a structure that optimizes environmental (fish) flows and fish passage from river to lake and vise versa.

The current weir is submerged throughout the winter months and the gates are laying flat on the river bottom. In the winter high water condition you can’t even tell a weir exists as it is submerged and the water easily passes overtop. That is because the weir itself is flooded due to the restriction at the Greendale Trestle where the river canyon narrows and the river bed rises. After the weir is taken off of control in early November the river flow, and therefore the elevation of the lake, is controlled by other restrictions such as the narrow valley at the Greendale Trestle. The new weir will also be submerged during the wetter winter months – as such it should become the narrow valley that continues to control flooding conditions. Our analyses will help us determine whether or not that is true.