Ecologically speaking, Any natural structure or depression or cavity on the surface, wherein Earth is in bottom, Water on top of it and open to Air (atmosphere) is called a Waterbody. Ecology doesn’t differentiate between river, pond, lake, rivulet, drain, canal, or reservoir, as long as the above definition is met, provided human being don’t interfere with bottom lamination or top cover.
And all three come in contact in the same order in a natural Waterbody. Thus A Waterbody is natural biggest pool of habitat for microorganisms, whose metabolic activities help in regeneration of natural resources on this planet.