If anyone finds these next few posts even remotely fascinating, I'll give them $100. Okay, so I'm lying, but I do think that these will be rather tedious. You've been warned.
It's probably appropriate to begin by explaining what agencies are primarily involved and generally why. This post will deal with the U.S. Army Corps of Engineers involvement. I happen to refer to the Army Corps of Engineers as "the Corps," but I've also seen reference to ACE, ACOE and USACE. Alphabet soup.
Anyway, you have the Clean Water Act (CWA), or perhaps more specifically, Section 404 of the CWA. This section requires that anyone interested in depositing dredged or fill material into "waters of the United States, including wetlands," must receive authorization for such activities. The Corps is responsible for administering the Section 404 permitting process.
The Corps (and the EPA) define wetlands as follows (40 CFR 232.2), "Those areas that are innundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs and similar areas."