All summer long Belton and Stillhouse have been stratified into three “layers” of water. There is a relatively cold and oxygen-poor layer of water nearest the bottom, a relatively warm, oxygen-rich layer of water nearest the top and a layer of water separating these two.
Technically speaking, this bottom layer is called the hypolimnion, the top layer is called the epilimnion, and the middle layer is called the thermocline.
Currently, because we have only experienced two mild, short-lived cold fronts thus far that did little more than change our wind directions, these three layers are still firmly in place. The middle layer (the thermocline) is found right at around 36-40 feet on Stillhouse, and a bit deeper on Belton, at around 40-44 feet.
As days get shorter, as solar input decreases, and as cold fronts arrive more frequently, the surface layer, which has warmed into the low- to mid-80s by now, will begin to cool. The cooler the water, the more dense it becomes. When the “lighter” surface water cools to the point where it is as cool or cooler than the bottom layer of water (the hypolimnion), it will begin to sink toward that bottom layer.
This downward movement of the top layer of water displaces the bottom layer, forcing it upward and allowing much mixing of the water as it moves. This, in effect, is a turning over of the lake’s waters, referred to by anglers as “turnover.”
Once turnover is complete, the lake will have fairly uniform temperature and oxygen levels from top to bottom.
In the summer, fish are found only above the thermocline, as this is where the oxygen levels are sufficient to support life. After turnover, fish can and do go anywhere they please because oxygen levels are sufficient at all depths.
Turnover is rarely an angler-friendly event. Fish typically scatter and turn off for a week or more as this process is underway. Fortunately, we have two reservoirs here in Central Texas, and can fish whichever one is not in the turnover process at the time, assuming they do not cycle at the same time. Typically, Stillhouse turns over first and more dramatically due to its smaller size. Belton turns over later, and has large portions in the upper Cowhouse Creek arm and upper Leon River arm that never stratify due to shallow depths, and therefore go unaffected by this change.
The deep, clear basin areas nearest the dams on both reservoirs are the areas most heavily impacted.
About the only thing an angler can do, short of switching lakes, is lower expectations of being able to pattern fish in any meaningful way during this time, and expect to have to hunt for fish using sonar, each trip out during this time, as fish typically will not hang in the same places for very long.
The negative trend during this event, typically taking place in the last two weeks of October and into the first week of November, is significant enough that I typically put my guide business on hold during this time and take a bit of a break to go fish down at the coast and do some home-building mission work with my church down in Mexico. When I return in early November, I typically return to a whole new ballgame.
Fish tend to shift to the deepest waters they will inhabit all year. Fish congregate heavily. Fish movement becomes more limited as cooling water lowers their metabolism, and fish become more susceptible to vertical presentations such as jigging with slab spoons.
Don’t get me wrong. One can stay on Belton and/or Stillhouse from now through turnover, right into the great bite often experienced in November and continue to catch fish, but consistency in catching those fish will be negatively impacted as nature forces a re-grouping via this annual event called turnover.