But as autumn approaches, a lake's surface water quickly cools. Almost overnight, warmwater species like bass vanish. As the temperature difference between the surface water and thermocline shrinks, the thermocline recedes into an increasingly narrow band until it disappears completely. Icy water from the lake's bottom then mixes with the warmer water above, until the lake's temperature is uniform at every depth. When this occurs, the lake is said to have "turned over." Walleyes then disperse throughout the lake, and finding them becomes a matter of luck.