I'm working with a similar problem, except neighbor dammed the creek and it's now flooding my property.
A lot of your options will depend on what your state reg's are. I should have started by reading the Texas Water Code, would have saved a few phone calls; see if your state has something similar.
In my case none of the seemingly appropriate State agencies would help, nor would the Federal Soil Conservation folks. I do have a promise of assistance from the County Commissioner, assuming he wins the election at the end of this month.
Is this a year-round creek or a seasonal creek - one that dries up unless it's raining? If it's a year-round creek, that means there should be water entering his dam year round, and once the dam is full the overflow should head your way, which would re-create your wetlands.
Does he have any kind of overflow system on the dam? Sluice gates, overflow pipes etc? Does his dam meet all of the requirements of all applicable state rules & regs?
Are permits and/or inspections required and did he pull the permits and/or have the inspections performed? The Texas Dam folks will come out and inspect any dam for any reason - even a complaint/request from a neighbor. If the dam isn't constructed properly, and a lot of home-built dams aren't, it could pose a flooding problem if/when something goes wrong with it. The most common problem with dam construction is that the builder just piles the dam dirt on top of the existing dirt, not mixing the two together by tilling or similar method. This creates a loose joint between the two, which can cause problems ranging from minor seepage at the base of the dam to the entire dam giving way - with all the water coming towards you at one time.
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