More importantly, the floodplains contain a pattern of low mound sites that vary in size, form, and height. The arrangement and location of the mounds is also variable. They occur in isolation, in groups of two, three, four or five around or near the ancient and present river channels. In some cases, the mounds are located next to or separated by low depressions that also vary in depth and size. This floodplain comprises at least 23 square kilometers of San Lorenzo's inner hinterland.
Natural boundaries define the northern alluvial floodplain.
The Chiquito River defines the eastern boundary. In 1993, Cyphers investigated one of the low mound sites within the northern floodplain and uncovered seven fire pits within a six square meter area. In 1998, I carried out an initial survey in the northern floodplain and determined that number of mounds is closer to 100. I also carried out test excavations at 15 of those low mound sites from February 28 to May 29, 1998. This archaeological research was carried out under the auspices of the San Lorenzo Tenochtitlan Archaeological Project with the permission of the Consejo Nacional de Arqueologia of the Instituto Nacional de Antropologia e Historia in Mexico.