When you get a lawyer, and he/she contacts the authorities about the wetlands, get everything in writing, especially if it’s in your favor.
And if it’s flowing constantly, it’s not a sump pump. I’ve had sump pump issues and so has my neighbor. They periodically go WHOOSH.
Normally, you do not have the right to discharge water from your property in a manner that imposes upon your neighbor. Basic property rights law. As a general manner, his drainage should discharge upon his own property.
Then, if it makes its way over the surface onto your property, that’s another matter.
Couple of examples:
In our past house, the neighbor redirected his downspout from the front of his house to the back, discharging right at the property line nearest our house, such that it ended up in our foundation drainage system. With the village’s assistance, he moved/extended the pipe 10’ further back where it discharged harmlessly to the pack of the properties.
Our current neighbors had the front downspout directed right at our house (maybe 20’ wall to wall.) The city code directs that it should be aimed to the front or back, and not at the neighbor. All it took was a frigging different elbow on the downspout, such that the topography carried it harmlessly to the street. Again, it took city involvement to have the neighbor do such a basic, commonsense, neighborly, freaking legally required thing. :smack:
Good luck! I think I’m going to build my next home on stilts! 
OK, so I’m an environmental engineer here in Connecticut. However, while I am an engineer, I’m not your engineer. Also, while I am licensed in the State of Connecticut, I can’t make any formal recommendations or provide specific professional guidance without seeing the property and studying your specific situation. (I always wanted to write a disclaimer like this, too.) Anyway, here are some general thoughts on this situation, based on the OP’s description of the issue:
[ul]
[li]As others have stated, the flow is almost certainly groundwater infiltration coming from your neighbor’s footer drain (or possibly from a basement sump pump). This flow should never have been directed to create a surface water flow across an abutting property.[/li][li]That said, if the flow has existed long to create a well-established stream and wetlands area, you may be stuck. As Enginerd stated, artificially-created wetlands, once they’re established, are regulated the same way as natural ones. In Connecticut, these regulations are strictly enforced. If your Town or the State finds out that you have eliminated a wetlands area without permission, you can be subjected to significant penalties. (For those who are wondering, there is no county-level government in Connecticut.)[/li][li]So if this long-existing stream has created a wetlands area, then before any work is conducted either in the wetlands area, or in the wetlands buffer zone, or that might potentially affect the wetlands area (like eliminating the source of the flow of water), then you (or your neighbor) will likely have to go before your Town’s Inland Wetlands Commission. This is a municipal commission that is regulated by CT DEEP. See here for more info.[/li][li]It is possible (but certainly not guaranteed) that your Town’s Inland Wetlands Commission might allow the discharge pipe and stream to be diverted so that it remains on your neighbor’s property. A commission, if they were inclined to grant permission for this, would typically require that an equal area of new wetlands be created to replace any that is filled in. You would likely have to hire an environmental professional to apply to the commission on your behalf and oversee this work.[/li][li]The fact that your property is listed as having wetlands greatly limits what you can do on your property. As I and others have indicated, not only is work within the wetlands area regulated, but so is work in the wetlands buffer zone (which can be up to 100 feet from the wetlands area). This may require that you go before your Town’s Inland Wetlands Commission before you do any work in the wetlands buffer zone, such as installing a deck or a patio.[/li][li]To find out if there is an existing easement for this drainage pipe and stream, you can simply go to your Town Hall and look up the land records for your property. An attorney or environmental professional can also do this for you.[/li][li]To address this situation, you are going to have to talk to a lawyer who is experienced in this field of law, an engineer, and possibly an environmental professional (such as a wetlands scientist). There are several issues to consider: (1) the legal issues, such who is responsible (for which you need a lawyer); (2) the drainage issues (for which you need an engineer); and (3) the environmental issues (for which you need an environmental engineer or other environmental professional). Many local engineering firms have the necessary staff to cover all of the bases for the latter two issues. Unless you are willing to foot the bill for all of this, though, you are probably going to have to talk to the lawyer first. [/li][li]Good luck! :)[/li][/ul]
Personally, I think robby knows his shit, and tells it like it is. 
With allowance for state-to-state differences, as a Realtor, I run across situations like this in real estate all the time. Heed his advice!
Seconded. And I’ll echo robby’s “good luck!” as well.
Now that’s the straight dope, and why I love this place. Thanks, robby, and the rest of you as well.
Let us know how it works out!
plug the pipe and see what happens
not really of course but hmmm…
I was just going to mention Adverse Possession:
If it has been running like that for X years, whoever is responsible for it may have established a legal right to do as it is doing.
Somebody replaced the cheap fence the builder installed with a nice redwood fence.
They also moved the lot line a couple of feet.
Of course, the folks on the other side may have grabber a couple of feet from this lot.
You know, pushing/packing some rags in then some quickset cement would likely bring matters to a head.
In my book, a “water feature” is a BIG plus!
I would work with the neighbor to make it attractive for both properties. (If possible.)
Other than that, standing water can be a nuisance. Attract insects, etc. In that case, lawyer.
Just curious - any update here?
In CT what sort of issues could there be with selling a property with a wetland on it? Our property has a fairly large area of wetland on it (at least 15’ x 20’) and the only extra hassle my parents had when building the house was meeting certain state-dictated setbacks. But we’re in NH, not CT.