Federal court cases provide answers for several categories of inquiry including more importantly here; "what is a discharge?", "what is a water of the United States?", and "what is an impact to waters of the United States". A forth, related category deals with "does this action comply with NEPA?" The following offers some insight into these issues.
Solid Waste Agency of Northern Cook County v. US Army Corps of Engineers
On January 9, 2001, the US Supreme Court limited the Corps of Engineers' Clean Water Act jurisdiction over "isolated" wetlands in SWANCC v. US Army Corps of Engineers 99-1178. The overturned decision rendered by the 7th Circuit Court of Appeals (98-2277) is also good reading on this topic. This decision refutes the use of the Migratory Bird Rule used under authority of the Commerce Clause to establish jurisdiction over wetlands which are not adjacent to waters of the US. EPA and COE issued a memorandum offering guidance on the SWANCC decision. Memo dated 19 Jan 2001
The US District Court for the District of Columbia's Jan 23, 1997 decision in American Mining Congress v. US Army Corps of Engineers, provides an excellent review of the definition of "discharge". Following the District Court's Opinion declaring the 1993 Tulloch Rule "invalid and set aside", EPA and the COE issued Joint Guidance on de minimis discharges COE/EPA , 11 April 1997. This case was affirmed on appeal in National Mining Association v. US Army Corps of Engineers on June 19, 1998 by the US Court of Appeals for the District of Columbia Circuit. A revised definition of "discharge of dredge material" was issued by the Corps and EPA in the Federal Register May 10, 1999. (Link back not provided).
A December 23, 1997 4th Circuit Court of Appeals Case, United States v. Wilson, provides a good review of both topics. We offer this only as a PDF document US v. Wilson (119 KB PDF file). The original decision may be seen at US v. Wilson at Emory University Law Library. Guidance was issued by the Corps and EPA in response to this decision during May and June 1998 but was withdrawn during January 2001 due to the US Supreme Court's SWANCC decision.
See also Avoyelles Sportsmen's League v. Marsh, 1983 (137 KB html) for some history on the issue of de minimis discharges, or for a more recent discussion, Save Our Community v. EPA by the Court of Appeals Fifth Circuit, Sept. 14, 1992.
A decision concerning the extent of Waters of the United States is in US v. Eidson, civ. ac. No. 94-2330, United States Court of Appeals, Eleventh Circuit, March 31, 1997.