Cadaver of newborn infant submerged two weeks (top) and four weeks (bottom) in running freshwater
Long before there were large-scale body farms (most famously the UT-Knoxville Forensic Anthropology Center, aka “The Body Farm”) to systematically test and observe the effects of various conditions on deceased bodies found in nature (or, for that matter, in unnatural settings), forensic pathology pioneers would sometimes replicate outdoor conditions on cadavers that were found and considered unidentifiable, and learn how different conditions affected the rates and modes of decomposition.
This newborn was found two weeks after death (determined by the growth rate of local algaes at that time of year), but was left in place for two more weeks in order to provide accurate depiction of a body submerged for one month in cold running water.
Note the algae forming a mostly-uniform coat on the body, but amassing much larger growths in the curves and open spaces, such as behind the knee, and in the crook of the elbow. Though the skin eventually decomposes and obscures the natural angles of those regions, the algal blooms will obscure such angles much earlier on in the decomposition process. From afar, this can make the cadaver appear to have been deceased for much longer than it actually has been.
Atlas of Legal Medicine. Dr. Eduard von Hofmann, 1898.