From The Cahokia and Surrounding Mounds by David Bushnell (1904) is this stunning set of photographs of the Native American mounds originally located in Forest Park. The mound shown above, 3.5 feet high and 55 feet in diameter and noted as Mound F on the map below, was one of several demolished during the preparations for the World's Fair. Evidence of human remains was found in several (but not all of the mounds). Given the lack of vegetation, it seems likely that slumping occurred to a number of the mounds (as most were under 5 feet high).
Bushnell writes that the photographs were taken at the ridge of the bluff in the park, near a bend in the River Des Peres. According to a map of the area from 1897 and 1903, that bend became the western corner of the waterway formed as part of Post-Dispatch Lake (the pool of water at the bottom of Art Hill). Bushnell stated that a set of small, low-lying mounds dotted the landscape near the river, while a set of taller mounds stood atop Art Hill. See map below for Bushnell's sketch of the locations of the mounds: