What is Lake Turnover and Why Is It a Prime Fly Fishing Opportunity in the Spring?
Towards the end of winter, fly fishers become antsy for warmer weather and the chance to return to their favorite waters. While much of fly fishing in the spring can be predictably good, there is one event that you can monitor and then capitalize on for one of the first prime fishing opportunity of the year. Lake turnover is an event that happens on certain bodies of water and helps to jumpstart the fish and their feeding habits. Knowing what lake turnover is and being able to time it can provide you with success while out on the water.
What is Lake Turnover?
Simply put, turnover is a mixing of water in lakes that occurs during two main periods: once during the spring and again in the fall (for lakes that freeze over in winter). In this article, we will be focusing on spring turnover.
Lake turnover in the spring does not occur on all lakes, it mainly happens on bodies of water that have depths of 20 feet or more (and we will focus on lakes that freeze over in the winter). Sunlight penetrates shallow lakes more easily which warms and mixes the water quicker and more uniformly than lakes that are deeper.
To understand turnover, it is also important to understand the different temperature and density layers of a lake. A lake is divided into three layers: the surface layer is called the epilimnion, the narrow middle layer is the thermocline, and the bottom is the hypolimnion. Water in a lake is separated into these layers as it cools or warms and thus becomes more or less dense.
Without getting too technical, water is at its densest at 39°F and as the water temperature rises above or below 39°F it becomes less dense. So, if we fill a bucket with 33°F water and 37°F water, the 37°F will sink to the bottom. If we fill the bucket with 42°F and 55°F, the 42°F will sink to the bottom.
Lake Stratification in Winter
In winter, the surface freezes and becomes a protective sheet of ice. The epilimnion layer is just below that ice and sits at a temperature below 39°F. The bottom of the lake (the hypolimnion) is warmer and is around 39°F (because it is denser than that colder water above it). Between the two is the thermocline which is a narrow layer that has a quick transition in temperature.
The protective sheet of ice helps to keep the rest of the lake from plummeting into freezing temperatures, but it also prevents any new oxygen from entering the lake. What oxygen there is in the lake when it freezes over is what the fish and other organisms have until the lake thaws.
At the bottom of the lake is organic matter that has piled up and bacteria feast on this throughout the winter which continues to drive oxygen levels down. This process also releases heat into the lake.
In this environment, cold water fish (like trout) will be a little more active than warm water fish (bass, bluegill, crappie, etc.). Warm water fish will also seek out warmer water which is nearer the bottom.
Lake Turnover in Spring
As spring progresses, the sun and warmer temps melt the ice and the top layer of the lake begins to warm. Winds also start to churn the upper layer of the lake. The rest of the lake remains at around 39°F and the water is relatively clear. The warm water fish are most likely still located near the bottom (dormant and in survival mode), but the trout could already be on the move in shallower water. The trout fishing will typically be good for about a week to a week and a half after the ice melts.
Within a week or two, the top layer of the lake continues to warm to 39°F which causes it to sink. As this water sinks it pushes the lower layer of water up towards the surface, and with the help of the winds, a mixing process fully takes over (i.e. turnover). The lake is now around 39°F from top to bottom and it has lost those stratified layers (epilimnion, thermocline, and hypolimnion).
If you were to return to the lake at this time, you would notice that it is off-color, turbid, and might have a strong odor to it. All of that decomposed organic matter that was on the lake bottom has now been mixed into the rest of the lake and the particles create a murky environment. Although things look pretty poor, oxygen is entering the lake which will help to revitalize it.
Fish are now on the move, but they may not be feeding due to the low visibility and a lack of insect hatches. Some larger, more predatory fish could take advantage of this moment to feed on minnows and smaller game fish if enough oxygen is entering the lake. So, if you drive out to the lake and find this situation (and choose to fish it), a streamer pattern would be the fly pattern selection to make. However, while the fishing may not be the best right now, but things will get better.
Within a few more days to another week, the oxygen levels will be near their highest amount and the water will become clearer. Insect activity will also pick up and the fish will cue in on this. After surviving the winter and now finally getting a real meal, these fish (and particularly trout) will go on their first feeding binge on the season.
If you can time it, you could have some great fishing. Not only are these fish really starting to feed, they have gone months without seeing a fly, lure, or bait (except for maybe the occasional ice fisher). This prime fishing opportunity is at its best for a couple weeks with hungry trout that have not wised up yet to anglers.
Eventually, the trout will grow more accustomed to angling pressure, but the remainder of spring will provide productive fishing until we get closer to summer. As we approach summer, the lake has been warming and stratifying back into its original layers: epilimnion (now the warmest layer of the lake), thermocline, and hypolimnion (the coldest). By summer, these layers are now well in-place and trout and other game fish will begin to seek cooler, deeper, more oxygenated parts of the lake which is typically down near the thermocline.
Throughout summer, oxygen levels near the bottom will start to decrease since the lake is no longer turning over and mixing. The oxygen levels in the lake continue to decrease until a second turnover occurs in the fall which will again saturate the lake with oxygen before icing over.
Timing Turnover for Fly Fishing
You can get super technical with timing lake turnover, but there is a simple approach that anyone can use (and this is primarily geared at trout fishing). First, select a few lakes (that have depths of at least 20 feet) that are at various elevations or that typically receive warmer weather sooner in the season. Lakes lower in elevation, or that receive warmer weather sooner, will ice-off and turnover earlier in spring. Keep your eye on these lakes for when they melt.
After the ice melts you can fish for trout with some success, but once turnover begins, and the lake becomes murky, fish activity will settle down. Wait a few days to a week and then head out and fish it again. The prime window of opportunity and potentially fantastic fly fishing is when the lake begins to clear after turnover. By then (if enough oxygen has been mixing into the lake) the insects and fish should be active and feeding.
If you miss timing one of these lakes, just head higher in elevation or into an area where the lakes receive colder weather longer and look for the next lake ready to ice-off and turnover. Or, if you do not have another lake in mind, you will at least know that the trout fishing will be productive for the remainder of spring until it sets into its summer stratification layers.
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Lastly, if you live in or are visiting Spokane, I provide guided fly fishing trips and instructional lessons around Spokane to include float trips on the Spokane River and stillwater trips across various Eastern Washington lakes. I guide with Fly Fish Spokane, and more information/trip rates can be found at the website: FlyFishSpokane.com