Horseshoe Lake, Alexander County, Illinois, is a shallow floodplain lake that lies at the northern range boundary of Taxodium distichum var. distichum (L.) L.C. Rich (bald cypress) in the Mississippi River Valley. About 35% of the lake surface is occupied by a flooded forest of Taxodium distichum and Nyssa aquatica L. (water tupelo). Pollen in two sediment cores was examined to determine the time of arrival of the forest at the site. Taxodium and Nyssa pollen increased in sediment deposited in 1850. The Modern Analogue Technique (MAT) showed that samples dating from before the arrival of Taxodium were most similar to sites in a variety of locations with high Quercus (oak) and Carya (hickory) and low Pinus (pine) percentages without Taxodium. For sediment deposited after the increase in Taxodium, the most similar modern analogues came from Taxodium/Nyssa swamps in the lower Mississippi River Valley and Virginia. Ring analysis of extant trees suggested an increase in reproduction and growth after 1850. The MAT results were consistent with the hypothesis that the Taxodium/Nyssa forest first colonized the lake around 1850 when American farmers developed the upland loess soils of the watershed for agriculture. We suggest that agricultural development on the uplands surrounding the lake encouraged the growth of the Taxodium/Nyssa forest by increasing sedimentation rate and producing large shallow areas of seasonally flooded eroded material.