EPA:

The following aquatic areas are generally not protected by the Clean Water Act:

-Wet areas that are not tributaries or open waters and do not meet the agencies’ regulatory definition of “wetlands”
-Waters excluded from coverage under the CWA by existing regulations
-Waters that lack a “significant nexus” where one is required for a water to be protected by the CWA
-Artificially irrigated areas that would revert to upland should irrigation cease
-Artificial lakes or ponds created by excavating and/or diking dry land and used exclusively for such purposes as stock watering, irrigation, settling basins, or rice growing
-Artificial reflecting pools or swimming pools created by excavating and/or diking dry land
-Small ornamental waters created by excavating and/or diking dry land for primarily aesthetic reasons
-Water-filled depressions created incidental to construction activity
-Groundwater drained through subsurface drainage systems and
-Erosional features (gullies and rills), and swales and ditches that are not tributaries or wetlands
http://water.epa.gov/lawsregs/guidance/wetlands/CWAwaters_guidesum.cfm