Crowe Valley Watershed
The Crowe Valley Waters
The Crowe Valley watershed is one of the most pristine watersheds in southern Ontario. Residents and visitors use, enjoy and respect this watershed, the lakes, rivers, streams and wetlands that make up this ecosystem. From a water management perspective, in the Crowe Valley watershed, there are three primary subwatersheds that the Crowe Valley Conservation Authority (CVCA) operates and controls levels and flow. Kasshabog Lake is the major lake situated in the North River subwatershed.
So, what is this slice of heaven called Kasshabog Lake and how and why does the Crowe Valley Conservation Authority control the lake level? Before I delve into that answer, here are some basics facts about Kosh.
At over 8 kilometres long and approximately 1.5 kms at its widest point, it is the third largest lake in the entire Crowe River watershed. It has a multitude of bays and inlets with over 100 islands and its deepest fishing hole is 24 metres. The lake has a summer surface area of approximately 2000 acres to enjoy (or think of it as about 1,500 football fields!).
The storage capacity of Kasshabog Lake is 38,240,000 cubic metres (another visual aid for you – this equals about 15,300 Olympic size swimming pools) and is quite large in proportion to the catchment area which is 79 square kilometres.
The dam, which obviously controls the lake level, is owned by the Ministry of Natural Resources and operated by CVCA staff. The operating agreement has been in place for over 30 years (renewed annually now). The dam was originally built in 1948 and repaired in 1956. Further modifications have been made to the dam for improved and safer stop log adjustments.
It is a single-bay concrete gravity structure with seven stop logs. The dam is in fair condition, but there is some concrete spalling, pattern cracking over the surface of the piers and weir, no reinforcing steel visible in spalled areas and the expansion joint filler has reached its useful life span.
At the time of the assessment (a few years ago now), the total cost to replace the existing dam was estimated to be approximately $1.1 million dollars.
In terms of dam operations, the CVCA adheres to our engineered study which was completed to determine summer and winter lake level elevations. These elevations guide the CVCA’s stop log adjustments at the Kasshabog dam.
Primarily, every year the CVCA staff lowers the lake level after Thanksgiving to a level that creates a flood storage capacity for the following spring freshet (runoff). Generally speaking, the amount of the drawdown is approximately 30 to 40cm from the summer lake level elevation in any given year. Of course, this may vary depending on how high the lake level is in the fall, precipitation received during the drawdown period when we begin drawdown, etcetera. At the end of the day, staff always aim to acheive the winter level setting before winter truly takes a firm grip on the lake.
Just to let you folks know, often there is a January thaw. If this happens the CVCA must adjust logs to allow for the passage of the excess water, especially if there is rain associated with the warmer temperatures. The CVCA does not have the luxury to retain this water since it would defeat the purpose of the flood storage capacity that we created through the drawdown. Once staff has removed stop logs and water levels receded to the winter target, logs are replaced and we wait for the spring freshet.
Every spring is the same, but unique at the same time. Snow melts, spring rains arrive and Kasshabog Lake fills to its summer elevation. In terms of stop log adjustments in the critical days and weeks of a spring freshet, staff must consider 1) current stop log settings, 2) use years of experience, 3) the current weather conditions, 4) long-range forecasts, 5) amount of the snowpack recorded (CVCA snow surveys), 6) rainfall amounts, 7) when precipitation is received, 8) how frequently rainfall events occur, 9) temperatures, 10) whether five-day averages are above or below freezing for both daytime and nighttime and well, the list seems to go on and on.
Eventually, the lake is filled to the summer elevation (this too can be quite variable when it occurs as there is no predetermined timeline). Further stop log adjustments will likely occur until the weather patterns stabilize, which then gives the CVCA an opportunity to minimize the fluctuations. For example, there have been many times when stop log adjustments are completed and then a significant rainfall event necessitates the removal of one or more of the stop logs.
After the summer level is set, the CVCA’s intent is to keep the lake level within a summer range. Once again, there are factors beyond our control such as extended dry periods with little or no rain. Although no one wants a wet cool summer (definitely puts a damper on the cottage experience), at least average rainfall amounts in June, July and August help the Authority maintain the summer water level.
In summary, I would like to say CVCA staff are committed to maintaining levels as close as possible to the targeted levels all year long. Sometimes though, we wish mother nature would be just as committed to helping us do our job!
By Tim Pidduck, General Manager/Secretary-Treasurer
Crowe Valley Conservation Authority: https://www.crowevalley.com/our-dams\