Storage (also called retention) involves temporarily retaining water, typically in designated reservoirs upstream, during high water levels to help reduce peak flows. The water is then released at a rate that will avoid flooding downstream.
Diversion and storage can be undertaken together to redirect and retain greater amounts of water.
For example, after extensive flooding in 2013, the City of Calgary and the Province of Alberta have worked on plans to reduce spring flood risks. These include: 1) renovating and managing capacity at Glenmore Dam, a drinking water reservoir, to reduce downstream flows on the Elbow River, and 2) building a diversion channel to connect the Elbow River to a new off-stream reservoir at Springbank. The reservoir would be dry most of the year and filled as needed for flood control.
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In BC/Lower Mainland
In BC, the Nechako Reservoir (Kenney Dam) and Bridge River Reservoir system were used to mitigate flood risk during the 1972 freshet. In that case, the storage of water in the reservoirs was calculated to have reduced the peak flow of the Fraser River at Hope by approximately 1,100 m³/s, which resulted in a 30-cm drop in water level at Mission. This step helped considerably to reduce flooding that year.
Managing the facilities to minimize flooding for all neighbouring and downstream communities can be challenging. In 2015, for example, water that was spilled from the Nechako Reservoir resulted in flooding in Vanderhoof and lands of Cheslatta Carrier First Nation, including a community cemetery.