What would cause water to change colour in the winter, such as going from clear to a yellow-brown shade?

It’s more normal for lakes to retain their 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 year-round.

In the fall, organic materials from fallen leaves and decomposing plants go into the lake, producing the dissolved compounds that give the water its brown colour. In winter, ice and snow cover tend to prolong this condition in two ways: First, by preventing ultraviolet (UV) rays from breaking the compounds down into their colourless chemical components. Second, by slowing the rate of runoff and trapping the compounds in the lake. When ice and snow melt in spring, a rapid reversal could occur. UV rays would 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.