Modern Glacial Lakes
In modern glacial environments we can find equivalents of the glacial lakes that formed in Glen Roy and neighbouring glens, for example the Märjelensee at the side of the great Aletsch Glacier in Switzerland and lakes alongside the margins of glaciers in Greenland, Iceland and the Patagonian icefields in South America. These modern equivalents help us to understand the formation and drainage of the Parallel Roads lakes.
A marginal lake, formed where a stream from a side valley is dammed by a glacier, is unstable. This is partly because cracks in the ice are continually opening and closing as the glacier slowly moves down its valley, allowing leakage to take place. Moreover, because ice is less dense than water, once the lake reaches a certain size the ice tends to float, allowing water to discharge, under or alongside the glacier, sometimes within a few hours.
This very rapid drainage is known as a glacier burst or by the Icelandic term, 'jökulhlaup'. However, where water from the lake is able to escape over a low barrier into another valley, this acts as a control on its level, and discharge under or along the margin of the glacier is less likely.