The Stream Doctor

August 11th, 2003

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Q. Question: From Flieguy: Dr. Cushing, I have a question about fall turnover in lakes in general. At what temperature does it occur? What exactly takes place? Is the chemical composition at all toxic? How does it affect the tailwater fishery? Does it put fish down? Thank you very much

A. This doesn't have much to do with streams, but I think I can handle 4 out of the 5 questions. Before answering the first question, we need to set up the physical and chemical status of the lake or reservoir prior to fall turnover. We're assuming a typical, temperate body of water for this scenario. In late summer, the lake will be stratified with a warm layer of water (epilimnion) overlying the cold water at the bottom of the lake (hypolimnion). The two layers are separated by the thermocline, a layer where water temperatures and usually oxygen concentrations decrease rapidly with depth until it reaches those found in the hypolimnion, usually around 4 degrees C. The epilimnion, being less dense because of its higher temperature is easily circulated by the wind. Now as cooler air temperatures in the fall began to lower the epilimnion temperatures, water temperatures here and in the thermocline gradually decrease until they are the same as those in the hypolimnion. At this time, there are no density differences and the winds can mix the entire water column, resulting in uniform temperatures and oxygen conditions throughout the lake. This is fall overturn, and it usually take place at about 4 degrees C. It is unlikely that toxic chemical conditions will occur at this time; the wind-generated mixing rapidly incorporates dissolved oxygen. Toxic conditions are more likely to occur when decaying plants use up the oxygen in the water under ice cover in shallow lakes, resulting in fish winter-kill. It is unlikely that fall turnover would adversely affect tailwater fisheries, because water temperatures and oxygen conditions at this time are usually adequate for fish. It also wouldn't make much different whether you had a surface or deep discharge because the water is uniformly mixed; this wouldn't hold true during summer stratification. I don't know if this would put fish down. I imagine it would be related to what they were acclimated to prior to turnover (cold, low-oxygenated water from a deep withdrawal, or warmer, well-oxygenated water from a surface withdrawal), and how rapid the change occurred. Most shocks result in some kind of behavioral change and it may well put fish down temporarily. That's the best I can do for this part of your question, but four-out-of-five isn't bad. Let me know if you want to pursue this further.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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