According to lake researcher Sergi Pla, of the Paleoecological Environmental Assessment and Research Laboratory at Queen's University in Kingston, Ont., it's more normal for lakes to retain their characteristic colouring year-round, with little seasonal change. The factors that determine a lake's colour include the topography and vegetation of the lake's catchment area, its volume and depth, and the size and structure of the inlets that feed it as well as the outlets that drain it. Very generally, lakes with wetlands in their watersheds will exhibit a "tea-stained" colour - the more wetlands, the deeper the colour - while lakes with no wetlands are clear all year round.

That said, your theory does partly explain the changes you've observed at Otter Lake. In the fall, organic material from decomposing plants would go into the lake, producing the dissolved organic compounds that give the water its brown colour. In winter, ice and snow cover would tend to prolong this condition, by preventing ultraviolet (UV) rays from breaking the compounds down into their colourless chemical components and also slowing the rate of runoff, thus trapping the compounds in the lake. When the ice and snow melt in spring, a rapid reversal could occur. UV rays would act to oxidize and break down the organic compounds. At the same time, melting snow and ice would add huge amounts of clean, uncoloured water to the lake, producing what Pla calls a "washing-up effect."

Pla speculates that your lake may be one that - perhaps because of an unusual pattern of runoff and drainage - experiences a greater-than-normal washing-up effect, with a near-complete change of water during spring breakup and snow melt.