Find the thermocline
on a $300 fish finder.

What the thermocline is

In summer, lake water stratifies. Warm oxygenated water sits on top (the epilimnion). Cold low-oxygen water sits on the bottom (the hypolimnion). Between them is a thin band — usually 8-20 feet thick — where temperature drops sharply. That's the thermocline. Every salmonid lives within 5 feet of it from mid-June through September. Find the thermocline, find the fish.

Two ways to see it on sonar

Most fishing sonars show the thermocline as a horizontal band of "noise" between two cleaner layers. It looks like a fuzzy gray cloud spanning the full screen at a specific depth.

• Method 1: turn sensitivity up to 80-90%. The thermocline shows as a band of speckle around 30-60 feet depending on the lake.
• Method 2: drop a thermometer on a downrigger ball to find where temperature drops 1°F per 2 feet. That's the top of the thermocline.

Lake-by-lake thermocline depths (Western US, July)

Rough start points. Check on the day of your trip — they shift with sun, wind, and inflow.

• Tahoe: 60-90 ft (the deepest in the West)
• Pend Oreille: 50-75 ft
• Coeur d'Alene: 35-55 ft
• Stampede: 30-50 ft
• Wallowa: 25-45 ft
• Flaming Gorge: 40-65 ft

Set the downrigger to 5 ft above the band

Kokanee and chinook hold at or just above the thermocline because that's where oxygen, baitfish, and the right temperature meet. Drop your downrigger ball to 5 feet above the top of the band. Your spoon at 28" leader sits about 4 feet down from the ball — putting it 1 foot above the band, in the strike zone.

Hashmarks are fish — read them right

A horizontal hashmark at thermocline depth is a fish moving across your beam. A vertical streak is a fish chasing your lure. A cluster of hashmarks 2-3 feet apart is a school. When you see a school below your spoon, troll OVER it (don't adjust depth), because salmonids look up to feed.