Naturalization of the flood regime in regulated rivers: The case of the UMR Sparks, R. E., J. C. Nelson, and Y. Yin. 1998. Naturalization of the flood regime in regulated rivers: The case of the Upper Mississippi River. BioScience 48(9):706 720. Reprinted by U.S. Geological Survey, Environmental Management Technical Center, Onalaska, Wisconsin, November 1998. LTRMP 98-R016. 15 pp. (NTIS #PB99-123630) ABSTRACT: Annual floods and occasional great floods structure landscapes, habitats, and communities in large river-floodplain ecosystems such as the upper Mississippi and Illinois rivers. Infrequent, great floods will continue to occur in these rivers because levees have design limits, because navigation dams do not stop floods (in contrast to high dams for water storage), and because unusually heavy and protracted rains will come again. Levees do exclude smaller, annual floods from about half the floodplain of the upper Mississippi; navigation dams alter the temporal-spatial pattern of flooding over about a fourth of the length of the river; and human alterations of the watershed increase the rate of delivery of water to the mainstem rivers, causing unnatural water level fluctuations in the floodplains and backwaters that diminish productivity and species richness. In the watershed, several long-term programs to restore water retention capacity are just beginning. Along the mainstem rivers, there are two current approaches to recreating more natural water level regimes. One involves manipulation of water levels within portions of the floodplain isolated from the river by either low or high levees. The other involves modification of the operation of the navigation dams to more closely mimic the presettlement flood regime. Naturalization of the flood regime involves social consent to some release of constraints to achieve a natural range of variability. Maintenance of a functioning floodplain-river ecosystem and its full complement of species requires some degree of floodplain-river connectivity, seasonal flooding, and topographic diversity. Determining just how much, when, and where is both a key area of research in floodplain-river ecology and a focus of public debate. KEYWORDS: Adaptive management, alteration, basin, biodiversity, birds, bottomland, channelization, community, diversity, drainage, drawdown, ducks, fish, flood, flooding, floodplain, floodplain-river ecosystem, floodpulse, forest, flow, fluvial geomorphology, impoundment, levee, manipulation, moist-soil, navigation dam, perirheic zone, policy, population, precipitation, productivity, recovery, regulation, rehabilitation, restoration, river, runoff, scale, sediment regime, trend, vegetation, waterfowl, water level, water regime, watershed, weather, wildlife