Understanding Spring Turnover by Brian Chan

More and more western stillwaters are finally shedding the last of the winter ice. The first week or so after ice off can provide some exceptional shallow water fly fishing opportunities. However, these nutrient rich lakes will eventually undergo spring turnover. This limnological event is an important ecological process that refreshes the water chemistry of the lake for the spring and early summer months.

So what is turnover? Basically it is the mixing of all the water in the lake. The end result is the entire water column becoming saturated with oxygen. What was clear water one day will, over a few hours, become quite turbid and have detritus suspended throughout. To the angler this means fishing success will be very poor during a turnover event as the water chemistry during the mixing process is poor.

When the ice comes off the lake the water temperature of the surface or upper layers of water are colder than that of the water closer to the lake bottom. Water resists mixing when there is a distinct temperature gradient. During the first few days to about 10 days after ice off the surface waters slowly warm and become similar in temperature to the deeper water. Water is most dense when it is 4 degrees Celsius (39 degrees F). So when the entire water column from surface to bottom becomes close to 4C the surface waters will sink to the bottom. Add a strong wind and the entire water column will “roll over” or mix. This mixing will sweep detritus or decomposing plant matter up off the bottom areas into the entire water column. Methane and hydrogen sulphide gases which are by-products of plant decomposition that has occurred all winter long are mixed throughout the water which add to the poor overall water chemistry during the turnover event.

It can take up to a week for the water chemistry to stabilize and the water to clear up. The end of spring turnover signals the start of the first major aquatic insect emergences, namely chironomids and the real good fly fishing begins.