The sound that gets your attention
Most people in Nixa find out they don't know where their main shutoff is at the worst possible moment. A supply line lets go under a sink, or a frozen pipe in a crawlspace splits after a hard January freeze, and suddenly there's water spreading across the floor and nobody in the house can say where to turn it off. That scramble, checking the garage, then the water heater closet, then calling someone who might know, is the whole problem. It costs minutes you don't have.
Why Nixa homes make this trickier than it should be
The housing stock around Nixa is a mix. You've got older homes near the historic part of town with shutoffs in odd spots, sometimes behind water heaters or tucked into additions that were built on later. Then there are the newer subdivisions off Gregg Road and further out toward Highlandville, where builders have been more consistent about putting the shutoff near the front hose bib or in the garage, but not universally so. If your house was built or renovated by more than one contractor over the years, don't assume there's only one shutoff, or that it's where the house next door has theirs.
Crawlspace foundations are common in this area, and that's where a lot of the main line runs before it comes up into the house. That means the shutoff itself might be inside the crawlspace access, not in a finished room. If you've never been down there, that's worth fixing before winter, not during it.
More on this from Finding The Main Shutoff Before You Need It.
What a hard freeze does here
Southwest Missouri doesn't get the brutal, sustained cold of states further north, but that's part of what makes it dangerous. Homes here aren't always built with the same margin for deep freezes, and a lot of plumbing runs through crawlspaces or exterior walls that assume a milder winter. When an Ozarks cold snap does drop temperatures into the single digits for a few days, which happens most winters at least once, that's exactly when pipes split and exactly when you'll want the main shutoff found fast, not searched for.
Spring brings its own version of the same problem. Heavy rain and quick thaws push groundwater up, and older shutoff valves that haven't been touched in years can seize or leak once you finally need to turn them.
What you can check this weekend
Walk the perimeter of your house and look for where the water line enters, usually near the front, close to where the meter sits at the street or property line. Follow it inward. In a lot of Nixa homes that means heading into the garage or down into the crawlspace access. If you find a valve, turn it a quarter turn and back, just to see that it moves freely rather than waiting to discover it's frozen solid the one day you need it.
Check your water heater closet too. Sometimes a secondary shutoff lives there, and it's useful even if it's not the true main. Label what you find with tape and a marker so anyone else in the house, a spouse, a teenager, a house sitter, can find it without you there to point.
When it stops being a weekend project
If you turn the valve and it doesn't move, don't force it. Old gate valves especially can shear off internally, and then you've turned a slow leak into a geyser with no way to stop it yourself. That's the point to call in a plumber who can replace the valve on a non-emergency basis, which is cheaper and calmer than doing it during a flood.
If you can't find a shutoff at all after a reasonable search, or if your crawlspace access is blocked, overgrown, or you're not confident navigating it, that's also worth getting a professional out for. They can locate it, tag it clearly, and tell you whether your particular setup needs anything else, like a second shutoff for an addition or a detached garage. Better to have that visit on a calm Tuesday than to be making the call at midnight with water already on the floor.