Great Lakes wetlands have lost much of their historical extent, structure and function. Their transformation was influenced by a number of factors acting over a period of decades including modifications in the basin's hydrology (watershed drainage, dikes, lake levels), biology (exotic species), geology (sediment transport and composition), and chemistry (water quality). The relative importance of each of these catalysts likely varied from region to region and depended on pre-settlement conditions and natural variability in the marshes, both generally unknown. We applied Geographic Information Systems (GIS) technology to a 120-year record (1872-1991) of images of a 2000-ha marsh system along the southwestern shore of Lake Erie, Ohio, USA. Long-term variability in aggregate characteristics of wetland vegetation was linked with environmental changes and human impact in three regions representing (1) a naturally existing open marsh with the lowest relative topograhical elevation, (2) an open marsh with a protected exposure to the lake and higher elevation, and (3) a diked marsh with manipulated water levels. The deep, open marsh lost half of its emergents, and a third of its patchiness and edge habitat in the early 1900s, when severe watershed degradations accompanied relatively low Lake Erie water levels. Nearly all remaining emergents were eliminated between 1940 and 1991 in this open marsh, following progressively higher lake levels. In the protected open marsh, the extent of emergents fluctuated with lake levels until 1977, and declined severely since then during sustained high lake levels. Habitat parameters varied little until recent decades, but declined markedly thereafter. The diked marsh maintained pre-1900 conditions for emergent plants, patchiness, and habitat edge; variability was linked to breached dikes and the presence/absence of marsh management. No landward re-establishment of the entire marsh complex since 1873 was evident on the 10 images studied. We propose the interaction of three forcing functions as the principal mechanism controlling the historical and current distribution of aquatic plants in southwestern Lake Erie marshes. These factors, collectively referred to as the Landward Advance Paradigm (LAP), include (1) the restricted ability of the marshes to advance landward, (2) sustained above-average lake levels, and (3) the presence of carp ( Cyprinus carpio) in wetlands with silt and clay sediments. Management focused on enhancing the role of Lake Erie wetlands should consider these landscape-level alterations and attempt to maximize wetland functions within the context of the LAP.