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Reservoir fishing and the Thermocline

Judy Van Put
Posted 7/26/22

Despite the heavy deluges of rain that fell over the last week or two, our rivers and streams continue to be at a lower flow. On Sunday afternoon, the Beaverkill at Cooks Falls registered 98.1 cubic …

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Reservoir fishing and the Thermocline


Despite the heavy deluges of rain that fell over the last week or two, our rivers and streams continue to be at a lower flow. On Sunday afternoon, the Beaverkill at Cooks Falls registered 98.1 cubic feet per second, below the Median average flow on that date of 161 cfs and Mean average of 396 cfs over 108 years of record-keeping. However, after several days of high 80s to 90 degrees and above, water temperatures have shot up higher than we like to see – ranging from a low of 69 degrees last Tuesday morning to a high of 83 degrees Fahrenheit on Saturday and Sunday afternoons. Most trout fishers have headed to the cooler tailwater fisheries or switched their fishing to lakes, ponds and reservoirs.

Trout that live in our free-flowing streams and rivers are extremely susceptible to stresses in the heat of summer from water temperatures that are too warm. Sadly, even the cool headwater and tributary streams that early anglers, from John Burroughs to Rube Cross, Herm Christian and Roy Steenrod, enjoyed fishing during the heat of summer are now experiencing higher than normal temperatures. And as we know, trout are cold-blooded and their body temperatures will match the temperature of the water they are living in; these are truly stressful times for trout.

One of the most important requirements for trout survival is the amount of oxygen in their environment - which goes hand-in-hand with water temperature. Trout can live and function in water temperatures above 68 degrees Fahrenheit only if the water is well saturated with oxygen – but as the water temperature rises, the amount of oxygen in the water decreases.

In order to survive, trout will seek the cooler waters, and in these situations, should not be disturbed, as they are vulnerable and are already in a stressed condition.

Some may believe that it’s okay to fish in warm water as long as the fish is released quickly and handled as little as possible; however the trout are already in a state of stress under warmer water and lower oxygen than they require, and will become so exhausted that they may not survive despite the misguided angler’s best intents.

Why not consider fishing on one of our Catskill reservoirs - these offer fantastic trout fishing during the heat of summer, thanks to the thermocline which offers respite to the trout.

Large bodies of water, such as a lake or reservoir (even the ocean) are made up of temperature-controlled layers; the surface (top layer) is the warmest layer, called the epipelagic zone. This layer is less dense, and will ‘sit’ on top of the colder, denser water. It interacts with sunlight, wind and resulting waves, mixing the water and distributing the warmth. Below this layer is the metalimnion or thermocline, which is the middle or transitional layer between the warmest top layer and the coolest deep layer below. The thermocline will be noticeably cooler – and if you send down a thermometer into the water, once it reaches the thermocline it will suddenly show a rapid decrease in temperature as it continues toward the bottom. Reservoirs stratify, or layer, and will develop a thermal barrier, or thermocline, above the deepest zone which is anoxic (water without oxygen) on the bottom. During the spring and summer, this lowest zone collects organic matter from plants and animals that sink to the bottom. The decomposition that occurs over the course of the summer uses up all the available dissolved oxygen that may have been present and becomes essentially a ‘dead zone’ - as without any dissolved oxygen, fish will not survive.

During autumn, as the temperatures cool, the upper layer of warm water will eventually become cooler and reach a temperature similar to that of the water below. This will result in a ‘turnover’ of the lake or reservoir, as the entire water column will mix and the cooler water will sink down to the bottom. Turnover generally occurs twice a year, in spring and fall, and usually begins when the water temperature is in the mid to low-50s.

Many veteran reservoir anglers depend on sonar “fish finders” to find the thermocline and locate fish. Lacking a fish finder, you can still find the thermocline zone with the use of a temperature gauge. When you first lower the gauge in the water, the water temperature will remain unchanged from the surface down until it reaches the thermocline layer. At this point, the gauge will show a dramatic change in the water’s temperature – which is generally at a depth of from 10 to 40 feet below the surface, depending on the depth of the reservoir. (Websites and local sports shops can be helpful in providing information on where the thermocline is at any given time during the summer.) As the gauge continues down, the water temperature will continue to lower until it levels out and stops decreasing – this occurs at the bottom of the thermocline. Your best fishing success should be in the upper layer of the thermocline to about five feet above the top layer of the thermocline, which is where the fish will most likely be. Casting sinking lures/weighted wet flies or streamers and trolling through this depth of water can be productive.

Don’t pass up the opportunity to try your hand at fishing the Catskill Reservoirs when our free-flowing streams are warm and low; be sure to visit the NYC Environmental Protection Recreation website for information on obtaining an access permit and special boating permit to store your boat on the shoreline in designated areas. Both access and boating permits are free of charge.


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