Unless you intentionally install a vertical waterfall on your basement wall, you should not have water running down it. This could create some major water damage for everything that is within six feet of the wall, and everything that is on the floor. Clearly, it is a plumbing issue, and you should call a plumber. However, there are some things you can do before the plumber arrives.
1. Find the Source of Your "Waterfall"
If the water is running down the wall and does not seem to be dripping from anywhere else, look above this area. Check for pipes that are spraying water or are broken entirely. This will be the go-to area for the plumber, and you can point it out to him/her when he/she arrives.
2. Put Your Finger in the "Stream"
It helps to know if this issue is continuous, or if it is a trickle that ebbs after a few minutes. If the problem is continuous, it means that a main water line has the leak. If it ebbs and dries up, it means that you only have a problem when you turn on a faucet or appliance overhead. You can test the latter, too. Have someone turn on the faucet and run the water in the sink.
If you have a double sink, run water down both of the drains. Be sure to take a five-minute break in between running water in one sink and then running it in the other. This will help pinpoint which sink or appliance is contributing to the water on the wall.
3. Find a Water Shut-off Valve
Every plumbing fixture has a water shut-off valve. Find the one that leads to the plumbing that leaks. you may have to backtrack the plumbing from the leak to a valve. Turn it off. This may take some trial and error tests before you can figure out which direction to turn the valve to shut off the water. Since your plumber will need to turn this off to work on the plumbing anyway, you just did this part of the job for him/her.
4. Refrain from Using Water in That Room
Whether the leak comes from a bathroom or from your kitchen, refrain from using the water in this room until the plumber shows up. If you need to remind others in the house, post a sign. The repair may take a few hours after the plumber arrives, so plan accordingly.