It's been labelled “the deadliest body of water in the USA”: the All-American Canal on the southern US border is a classic, and tragic, example of how linear infrastructure can harm both humans and wildlife.
Roads are the most prevalent of man-made linear structures criss-crossing the earth's surface, but there are others – canals, pipelines, fences, power lines – that can also increase mortality risk and impede movement for wildlife, and often for humans too.
An especially poignant example of how linear barriers can harm both humans and animals was cited by Melanie Bucci from the Department of Ecology and Evolutionary Biology, University of Arizona, in her presentation at the International Wildlife Management Conference in Durban this week:
The All-American Canal, which runs parallel to the Mexican-US border in southeastern California, supplies water from the Colorado River to the San Diego area.
The Canal impedes wildlife movement, and, running as it does through a sandy, arid area, it also attracts animals like mule deer who wouldn't normally be in such habitat but are drawn by the water source – which puts them at risk of drowning should they slip down the sides of the Canal.
There was increased concern from conservationists when cement lining of the Canal began in order to prevent water losses via seepage. Previous cases of canal lining had been associated with increased wildlife mortality rates: cement sides are smooth and denuded of riparian vegetation, so raise the risk of animals slipping into the current and not being able to gain purchase on the banks to pull themselves out.
550 lives lost to the Canal's waters
The Canal also attracts people for a very different reason – illegal immigrants from Mexico who risk this wide, deep, fast-flowing channel to cross the border into the US. In 2010, a media expose of the situation revealed that over 550 people had drowned in the Canal.
Until then, no money had been allocated to prevent human fatalities, the rationale from authorities being that increased safety measures would encourage illegal crossings. There were ladders at intervals on the Canal banks, but these were not effective in preventing drownings – not least because most crossings occur at night when the ladders are not clearly visible to a swimmer struggling mid-channel.
In 2011, $1.12 million was spent to install Spanish-English warning signs and 108 buoy lines along 131 km of Canal to give those crossing the water a way to exit safely. Since then, the Canal has claimed only three lives, but Bucci's research team feels that the effectiveness of the safety measures remains questionable.
Mitigation measures for preventing deer and other large mammal mortality in canals includes ridging the cement banks at intervals to allow animals to be able to better grip the sides, and fencing the canal off altogether – the latter measure is however prohibitively expensive.
In the case of the All-American Canal, a non-deliberate structural change has been helpful: expansion joints in the cement have formed rough gravel areas that offer better grip for deer.
- Olivia Rose-Innes, EnviroHealth Editor, Health24, July 2012
Elephant tunnels, possum bridges.