The GRCA owns seven large multi-purpose dams and reservoirs to reduce flood damages and maintain river flows during the summer and fall.
Water from the spring runoff is stored in the reservoirs to reduce flood peaks. The stored water is then released gradually during dry months to maintain flows.
Gauges at the reservoirs monitor water levels. The information is relayed to the GRCA head office and used to update the charts shown in this section of the website. They are updated every hour.
These gauges are an important part of our flood forecasting and warning systems. They give GRCA flood managers real-time information on changes in water levels so they know how much room is available to hold additional water during high flows. They can manage the levels, releasing water or holding it in as needed to prevent or reduce flooding downstream of the dam.
The gauges have automatic alarms that are triggered by a sudden change in water levels.
The water level charts are also useful to recreational users such as anglers, boaters and others.
The GRCA also owns several smaller dams built in the 1800s and 1900s for water supplies, water power and other purposes. These dams, such as Wellesley, New Dundee and Damascus, are not part of the flood control system.
The reservoir charts show changes in water levels over the course of the year.
1. This line shows the current water level.
2 and 3. The green lines, called the "rule curves," show the desired highest and lowest water levels over the course of the year. Reservoir levels are lowest in the winter, to prepare for the spring runoff. The water level is highest in late May and early June so there is enough water available in the spring and fall. Water levels decline gradually during the course of the year until they return to winter lows.
4. This is the maximum allowable water level. If water levels get higher, there is a risk that water may flow over the top of the dam, damaging it and possibly causing a dam failure.