A Geographic Information System was used to assess the representation of ecological land units (ELUs) both within and external to the current preserve network of the Lower Twin Creek Watershed (LTCW), southwestern Ohio. ELUs were classified using multivariate and cluster analyses on forest canopy tree species and seven physiographic and soil variables derived from digital elevation models and a soil series map. Cluster analysis of the five most significant variables (landform, soil drainage, hillshade, curvature, and percent slope) influencing vegetation distribution resulted in nine discrete ELUs. They included uplands dominated by Fagus grandifolia-Acer saccharum, dry slopes dominated by Quercus spp.-Carya ovata, mesic slopes dominated by a mixed mesophytic community, and wet floodplains dominated by Platanus occidentalis-Populus deltoldes. The area of forest in each ELU in the preserve network was compared to the area of forest in each ELU in the watershed as a whole and the potential natural area of forest cover determined by topographic position. Well-drained upland and poorly-drained upland ELUs were most underrepresented in the preserve network followed by wet broad floodplains. Results of the gap analysis based on current vegetation distribution were influenced by the pattern of deforestation in the LTCW, which was, in turn, related to topographic position: slopes were more likely to be forested and incorporated into a preserve system than more level lands suitable for agriculture. Protecting and restoring forest in upland ELUs will be necessary to capture a full range of ecosystem diversity in the LTCW.