You just found water where it shouldn't be
You're standing in the basement or crawlspace access with a flashlight, and there's water where there shouldn't be. Maybe it's a slow drip off a copper joint, maybe it's a steady stream out of a fitting behind the washing machine, maybe an upstairs bathroom has turned the ceiling below it soft and discolored. Whatever it is, the instinct is right: shut the water off before you do anything else. But in a lot of Nixa homes, that first step trips people up more than the leak itself does.
Nixa has a mix of older homes near the historic downtown with original galvanized or early copper plumbing, and a lot of newer construction from the building boom of the last twenty years out toward the edges of town. Those two kinds of houses shut off water very differently, and knowing which one you have matters before the plumber's truck is even in the driveway.
Find the shutoff before you need it
Every house has a main shutoff valve, usually where the water line enters the house. In older Nixa homes with basements, that's often a gate valve near the water heater or where the line comes up through the slab. In newer homes on crawlspaces or slab foundations, it might be in a utility closet, the garage, or even outside near the meter pit at the edge of the yard.
Gate valves are the older style with a round wheel handle. If your house hasn't had plumbing work done in a while, that valve may not have been turned in years. It can be stiff, and if it's old enough, it can seize or even crack when forced. Turn it gently. If it won't budge with reasonable effort, stop. A cracked valve turns a manageable leak into a much bigger problem, and that's exactly the kind of thing that should wait for the plumber rather than get forced by hand.
Newer homes usually have a ball valve with a lever handle, a quarter turn and it's closed. These are more reliable, but they're not immune to problems either, especially if the house sat through a hard freeze at some point and the valve body took stress it wasn't built for.
More on this from Shutting The Water Off Before The Plumber Arrives.
What winter around here does to your pipes and valves
This part of Missouri gets real cold snaps, the kind where temperatures drop hard for a few days at a time, often after a warm stretch that lulls people into not thinking about it. Pipes running through unheated crawlspaces or along exterior walls in older Nixa homes are the ones that freeze first. A shutoff valve that sits in one of those cold spots, an unheated garage or a crawlspace with gaps in the skirting, can be just as brittle as the pipe you're trying to isolate.
If you're dealing with a leak during or right after a cold snap, treat the shutoff valve itself with some suspicion. Don't crank it hard expecting instant results. If it feels frozen or won't move smoothly, shutting off the water at the meter, or calling the plumber and describing the situation so they arrive ready for a stuck valve, is the safer move.
What you can reasonably do yourself
Locate the main shutoff now, before there's an emergency, and turn it once a year just to make sure it still moves. That alone saves a lot of stress later. If you find a leak, shutting off the main is usually within reach for most homeowners, along with shutting off individual fixture valves under sinks and toilets if the leak is isolated to one of those.
It's also worth knowing where the water meter is, usually near the street or the edge of the property line, with its own shutoff. If the house valve fails or you can't get to it, the meter shutoff is the backup. Some of these need a meter key or a wrench rather than bare hands, so it helps to know that ahead of time rather than discovering it during a flood.
Where it stops being a homeowner job
Once the water's off, resist the urge to start taking anything apart. Pipe joints, fittings, and valves in a house with any age to it are often more fragile than they look, and what seems like a simple fix, tightening a fitting or wrapping tape around a joint, can turn a contained leak into a pipe that lets go entirely once the plumber turns the water back on to test it.
If the shutoff valve itself is leaking, won't fully close, or feels like it's about to break in your hand, stop and wait. That's a sign the valve needs replacing, not just operating, and that's plumbing work, not a homeowner project. Same goes for any leak involving galvanized pipe, which is still common in Nixa's older housing stock. Galvanized fittings corrode from the inside, and what looks like a small leak at a joint is often a sign the pipe nearby is thinning out and ready to fail somewhere else too. That's worth a professional look at the whole run, not just a patch at the spot you can see.