What you're probably noticing
The shower that used to run hot for fifteen minutes now turns lukewarm after eight. There's a low rumbling or popping sound coming from the water heater in the closet or garage, especially right after it kicks on. Maybe you've noticed the recovery time is slower than it used to be, or the pilot light area smells faintly metallic. None of this is dramatic. It's the kind of thing you notice, shrug off, and notice again a week later.
In Nixa and the rest of the Ozarks, this pattern shows up a lot, and it almost always points to the same culprit: sediment building up in the bottom of the tank.
Why it happens here
Water in this part of Missouri comes out of the ground with a fair amount of dissolved mineral content, mostly calcium and magnesium carbonates from the limestone and dolomite bedrock that runs under Christian County. That's just what Ozark groundwater is like. It's not dangerous, but it's hard, and hard water leaves mineral deposits behind wherever it sits and heats up.
Every time your water heater cycles, a thin layer of that mineral content settles to the bottom of the tank. Over months and years it builds into a crust. That crust does two things: it insulates the burner or lower heating element from the water above it, so heating takes longer and less efficiently, and it rattles and pops as trapped water boils underneath it, which is your rumbling noise.
Homes built in the newer subdivisions around Nixa, and the older farmhouses and ranch homes scattered through the county, all draw from the same regional water supply, so this isn't a matter of one neighborhood being unlucky. Age of the house doesn't matter much either. What matters is age of the water heater and whether it's ever been flushed.
More on this from Reading List Sediment, Rumbling And A Shorter Hot Shower.
What the seasons add to it
Summer in southwest Missouri means more demand on the water heater from showers after yard work, kids in and out of the pool, extra laundry. That's more heating cycles, which means sediment builds faster during the warmer months even though you'd think winter would be harder on the unit.
Winter matters in a different way. Incoming groundwater temperature drops, so the heater works harder to bring water up to temperature, and it takes longer to recover between uses. A tank that's already lost a third of its capacity to sediment will show its limits fastest on a cold January morning when everyone wants a hot shower back to back.
What you can check yourself
Start with the drain valve at the bottom of the tank. If you haven't flushed the heater in the last year or two, that's step one. Turn off power or gas to the unit, connect a garden hose to the drain valve, run it to a floor drain or outside, and let several gallons run out before you look at the color. Cloudy, gritty, or grey-tinted water confirms sediment. Clear water after a gallon or two means you're in decent shape.
Listen for when the rumbling happens. If it's right when the burner or element kicks on, that's consistent with sediment. If it happens randomly or gets louder over weeks, that's worth paying closer attention to.
Check the age of the unit. Most tank water heaters are stamped with a manufacture date near the top, sometimes buried in the serial number. A unit past ten years old that's rumbling and running short is often past the point where a flush fixes things for good.
Where it stops being a DIY job
A simple flush is reasonable for most homeowners to attempt, but there are a few points where it makes sense to stop and call someone. If the drain valve won't open, or opens and won't fully close again, don't force it. Cheap plastic drain valves are common on older tanks and they fail easily.
If you flush the tank and the noise doesn't improve, or the hot water still runs out fast, the sediment layer may already be thick enough that flushing alone can't fully break it up, or the heating element or burner itself may be damaged from the insulation effect. That's a job for someone with the tools to inspect the element or descale the tank properly.
And if you smell gas, see water pooling at the base of the unit, or notice rust-colored water rather than grey sediment, stop troubleshooting and get it looked at. Rust in the water usually means the tank itself is corroding from the inside, which isn't something a flush or a part swap fixes. At that point you're deciding on repair versus replacement, and that decision is easier with someone who can actually look at the tank in person.