Why Ottawa bombs its frozen rivers
At the end of each winter, explosives experts lay dynamite in the icy river that runs through the heart of Ottawa, Canada. Why? And what does this do to the fish?
This weekend, Ottawa's residents must stay a safe distance from the river - it's the annual ice-blasting to prevent melt-water flooding the city.
Since mid-February, specialist workers have been using buzz saws and amphibious ice breakers to cut channels and long grooves in the ice.
The next step is to drill holes, pack these holes with sticks of dynamite, and blow apart the sheets of ice. These mini-icebergs can then float safely down river, avoiding an icy bottleneck around a low bridge.
Filmed for the BBC's Human Planet, ice cutting and blasting will continue each weekend into March, while water levels are at their peak.
The risk of flooding in this 9km-stretch of river is high without this dramatic intervention, as water could back up behind trapped ice and overspill the river banks. While the river does not freeze solid, the work takes place at the base of a waterfall where the ice is about 3m thick (almost 10 feet).
Clearance work to eliminate ice jams has been carried out since the 1880s, says City of Ottawa spokeswoman Jocelyne Turner. "Historical records indicate that numerous floods have occurred because of, or were aggravated by, ice jams."
In the early years, ice jams were only cleared when they occurred, says Bruce Reid, of the Rideau Valley Conservation Authority, which works with the local authority on ice clearance.
"By the 1960s and 70s, it evolved to be a flood prevention measure. It's unique to Ottawa in terms of the actual methods used."
Similar work - minus the explosives - is carried out on Manitoba's Red River, which is also historically prone to ice jamming.
"Ice management work is also done to protect other Canadian communities, but tends to be done only in reaction to ice jams when they occur, and where rivers discharge into Great Lakes," says Reid.
Ice-blasting does have its disadvantages. Explosives experts and amphibious craft do not come cheap, and the blasting must be done with care to avoid scarring the riverbed and killing wildlife.
"The river does remain open in fast moving reaches, so the fish stay in the deep pools and the water remains oxygenated by exchange with the atmosphere in rapids. Harm to them during ice removal has been a concern, hence the increasing use of an amphibious excavator instead of explosives," says Reid.
In 1992, the local authority began using an eco-friendly ice-breaking craft to help reduce the reliance on explosives.
Today's methods now involve much more ice cutting and much less dynamite. Before 1992, up to 8,000kg (17,636lb) of explosives would be used in a season. Today, it is between 700 and 1,500kg (1,543 to 3,000lb). This benefits wildlife habitats and also reduces the seismic impact on nearby bridges and other structures.
It costs the city $460,000 a year to clear the river ice, with 18 staff involved. But it's money well spent, says Turner. Without this work, some 900 buildings and other structures would be at risk of flood damage - and the clean-up costs would be far higher.