Bearspaw Treatment plant in 1972 on the Bow River to supplement the city's water supply. 2005 flood Although the dam usually provides effective flood protection, a major flood in June 2005 caused the reservoir to exceed its capacity. The excess spilled over the dam and into the river. The flow downstream increased from its normal average of 20-30 cubic metres per second up to 350 cubic metres per second. As a result, some roads were closed and 2,000 Calgarians who lived downstream were evacuated. The Glenmore water treatment plant had difficulty treating the heavily silted water, which caused the municipal government to issue water restrictions. Environment Canada noted the 2005 Alberta floods were a 200-year flood occurrence. 2013 flood In June 2013, heavy rainfall west of the city caused the reservoir to exceed its capacity. As it did in 2005, excess water spilled over the dam and into the Elbow River, with downstream flows up to 544 cubic metres per second. 75,000 people from 26 neighbourhoods in the vicinity of the Bow and Elbow rivers were placed under a mandatory evacuation order as the rivers spilled over their banks and flooded neighbourhoods. City officials urged Calgarians, particularly the 350,000 people who worked downtown, to stay home and limit non-essential travel. Unlike the 2005 flood, the Glenmore water treatment facility had no difficulty treating water. City officials did, however, implement municipal outdoor watering restrictions to ensure water quality remained high throughout the incident. Government officials called the flooding the worst in Alberta's history. After the flood, studies commissioned by the city and province investigated the construction of a $457 million tunnel to divert water from future flooding. Features The Glenmore Dam is a gravity dam which uses the downward force (weight) of the structure to resist the horizontal pressure of the water within the dam. These massive dams resist the thrust of water entirely by their own weight. The Glenmore Water Treatment Plant, constructed in three phases in 1933, 1957 and 1965, is a conventional treatment plant that gets its water from the Elbow River. The Glenmore plant supplies drinking water to south Calgary. Flood control The reservoir is maintained at a level that, depending on the flow rate of the Elbow River, minimizes the risk of flooding around the reservoir and downstream of the dam to the greatest degree possible. During periods when the rate flow of the Elbow River