The Common Garter Snake (Thamnophis sirtalis) is one of the most abundant and widespread of North America’s snakes. In Canada they are mainly found in the far south close to the USA border but a few out-lying populations occur in the northern Rocky Mountains and extreme southern North West Territory.

It is a very variable snake and many subspecies have been described but even within a single population colour and patterning are very diverse. The most common forms may have yellow stripes, or are brown and green blotched with an average length of 55 cm, although specimens exceeding one metre have been recorded. Although their saliva is mildly toxic it is of no threat to man. Their success can be ascribed largely to their very wide dietary range, preying on everything from earthworms to amphibians, small mammals to birds.

There is one aspect of their lifestyle that has brought them to the attention of scientists and those interested in natural history. And that is that before the onset of winter in the north of their range large numbers of garter snakes gather at denning sites, known as hibernacula. One of the largest such sites is located 6 km from the town of Narcisse, Manitoba, and known as the Narcisse Snake Pits. At one stage it was estimated that some 70 000 and more garter snakes gather here before the cold winter weather sets in, where they shelter in subterranean caverns. Severe weather conditions have been known to kill thousands of snakes, with the last such die-off being recorded in 1999.

In our temporary home of Clearwater, British Columbia, we discovered such a hibernaculum purely by chance on 31 May 2012 whilst watching rising flood waters of the North Thompson River. Their den is amongst rocks that have been used to prevent erosion by the river waters, over a frontage of some 50 metres.  Over the following days we estimated that between 400 and 500 snakes were calling this home. Several of the snakes had already started to slough (shed their skins) but this was to continue until 9 July when just a few snakes were still present by which time the majority had dispersed into the surrounding countryside.

We will be watching from mid-September when the snakes will be returning to this newly discovered hibernaculum.