My friend has a house that dates back to the early 1900's. It's in NY state. There is a 12' x 12' section of floor in one corner where there is no concrete on the floor. The area was walled off with wall board and a wooden door. According to neighbors who have been there 60ish years the area is not prone to flooding. Or it wasn't till my friend got her house. 2 out of the 3 springs since she got it she had to pump it out when the spring thaw and heavy rain hit at the same time. I theorize that the bare earth section was for a root cellar. The plan this winter is to dig a hole, put in a layer of gravel, insert a perforated sump pump liner, surround that with gravel and install a sump pump. The bare earth area is not level. I think we need to bring it down to the same level as the concrete floor, or slightly below. Then I think we need to put a layer of concrete over all the bare spot except the hole for the sump pump. Is this plan sound? What would cause the water table in an area to change so radically?

Your plan seems sound. I would recommend removing the dirt to a level far enough below the finished floor to allow for a layer of gravel under the concrete, which should be level with the current concrete floor. This gravel should be clean, no fines, which will allow any ground water that comes up under the new slab to drain to the sump.
As to the cause of the change in water patterns, the most common reason is a change in landscaping around the house, or a change in the rainwater drainage from the house. (check gutters and downspouts for proper operation) However, there are any number of possible causes, including everything from minor earthquakes to your neighbor 3 houses down changing his landscaping.

I concur. This issue should be dealt with sooner than later. The water may be traveling down the foundation wall, through a crack into the basement. Check the backfill around the house and ensure there is still a positive slope moving the water away from the foundation. If not it may be time to fill in any negative slopes that are draining water into the foundation walls. The sump pump is a must have and should help deal with any water in the basement, but if you can figure out where the water is coming from and fix that problem, you will have better long term results.

Good luck.

We've never been there when the basement was filling, just after it's gotten flooded. All the neighbors had the same thing happen and all are old-timers who've been there decades and they never saw anything like it until 3 years ago.

The land around the house is pretty flat, though the main road out front is a bit higher. Right after you cross the road behind the house the land drops off to a creek which rose quite a bit, (again more than the neighbors remember it ever doing), but it didn't get near high enough to cross the road.

It could just be that for a few years running the snow built up and melted just as the spring rains hit and the water table rose higher than normal.

We never saw signs that the water rose above ground level and the neighbors never mentioned that happening. It seems that the basements were not built to be water-proof way back when.

i agree with polycore, it is better if you try to find out first from where the water is flowing inside, then the after doing the modifications try installing the sump pump.

I doubt that it's coming through the windows. The previous owner painted the walls and there does not seem to be any breaks in the paint. The floor is getting crumbly. My best guess is it's seeping up through the floor, especially the bare spot. Since we can't induce the flooding to see where it's coming in and we know the spring is usually wet, we want to get the pump in place before then.

I used to own a home in Portland, OR built on a crawl space that had a flowing spring in the crawl space. The builder had bored a hole at the low point in the foundation and ran a water drain line down to the street. Whenever the spring flowed, water ran into the street!

My grammar school in Brooklyn, NY was built over a spring. The building super allowed the water to flow through a channel. It came in through one side of the foundation and out the other. He carved out a small pond that was fed by the spring. He had screening at the inlet and the outlet to keep the goldfish in. He built a small bridge over the pond. The pond and channel were both concrete lined and painted with that blue paint they use for cement swimming pools.

Another reason to get concrete over that earthen floor is radon gas. Even small holes in the concrete can be sources of radon gas infiltration let alone a whole dirt floor.

This website has some tips for reducing radon buidup and construction methods to prevent the infiltration of the gas.
http://www.epa.gov/radon/rrnc/index.html

Good advice. Before she bought the house a radon test was required. Not sure if it was the state, county or the mortgage company that required it. It passed, so I'm not worried about that.