1740 Ecological Applications, 12(6), 2002, pp. 1740–1754  2002 by the Ecological Society of America ASSESSING CUMULATIVE IMPACTS OF LEVEES AND DAMS ON FLOODPLAIN PONDS: A NEUTRAL-TERRAIN MODEL APPROACH SARAH E. GERGEL 1 Zoology Department/Center for Limnology, University of Wisconsin, Madison, Wisconsin 53706 USA Abstract. Nearly all large rivers and their floodplains in the Northern Hemisphere are subject to multiple disturbances such as levees, impoundments, channelization, dams, and changes in land use. Isolating the relative impact of different disturbances is difficult when the combined effects are nonadditive. I developed a ‘‘neutral-terrain model’’ to examine the cumulative impacts of levees and dams on the hydroperiod of ponds and wetlands that form throughout floodplains after flood events. Using simulated floods of different mag- nitude, I address two major questions: (1) What is the relative influence of levees vs. dams on the duration and abundance of temporary ponds and wetlands? (2) Are the cumulative effects of levees and dams additive, synergistic, or antagonistic? Levees decreased the area occupied by temporary ponds (as compared to the natural scenario), until the levee was breached by large-magnitude events. When the levee was breached, no differences were found between the natural- and levee-floodplain scenarios. A simulated upstream dam decreased, increased, or had no effect on the area occupied by temporary ponds, depending on flood magnitude. Synergistic interactions between levees and dams were apparent for larger floods, where the reduction in flood stage caused by an upstream dam prevented levee breaching. Lastly, I introduce a qualitative framework for understanding the impacts of levees and dams on the duration of floodplain ponds. I also discuss the applications of this new three-dimensional simulation modeling technique, neutral-terrain modeling, as a tool for addressing spatial aspects of watershed and floodplain questions. Key words: dams; flood magnitude; floodplain; levees; neutral-terrain model; temporary pond; wetlands; Wisconsin River. INTRODUCTION The ecological impacts of single disturbances and stresses have long been of interest to ecologists. How- ever, less attention has been paid to the effects of mul- tiple disturbances (Bedford and Preston 1988, Paine et al. 1998, Breitburg et al. 1999) although many eco- systems and organisms are impacted by multiple dis- turbances and stressors (Schindler at al. 1996, Yan et al. 1996, Paine et al. 1998). In addition, because an- thropogenic disruptions of ecosystems are pervasive and generally increasing in frequency and magnitude (Holling and Sanderson 1996, Vitousek et al. 1997), multiple perturbations to ecosystems will likely be- come more common (Paine et al. 1998). Isolating the relative impact of different types of concurrent distur- bances is difficult. Furthermore, predicting the effects of several disturbances is especially challenging when the combined effects are interactive or nonadditive. Nonadditive effects of disturbances can be synergistic (have an impact greater than the summed individual disturbance effects) or antagonistic (produce less dis- ruption than the summed individual disturbance ef- fects). Manuscript received 30 October 2000; revised 28 September 2001; accepted 11 March 2002. 1 Present address: National Center for Ecological Analysis and Synthesis, 735 State St., Suite 300, Santa Barbara, Cal- ifornia 93101 USA. Large rivers and their floodplains are often subject to multiple disturbances. In the northern hemisphere, nearly all major rivers have been altered by levees, impoundments, channelization, dams, and/or upland changes in land use (Potter 1991, Dynesius and Nilsson 1994, Power et al. 1995, Nilsson et al. 1997, Vitousek et al. 1997, Fitzpatrick et al. 1999). In the U.S., 98% of rivers are regulated (Vitousek et al. 1997), and 60% of the world’s stream flow was expected to be regulated by the year 2000 (Petts 1989). There are 17 000 km of levees in the upper Mississippi River basin (Tobin 1995) and an estimated 40 000 km of levees, flood- walls, embankments, and dikes in the USA (Johnston Associates 1989). By reducing peak flows, dams can reduce the frequency and amount of overbank flooding. Dams also interrupt downstream sediment transport and upstream fish migration (Sparks 1995). Levees pre- vent the lateral flow of sediment, nutrients, and organ- isms between rivers and their floodplains and can also increase flood heights (i.e., stage) for a given discharge (Sparks 1995). Levees and dams are ubiquitous and co- occur in many regions (e.g., the Upper Mississippi, USA; the Yellow River, China). Here, I examine the cumulative impacts of levees and dams on temporary ponds and wetlands that form throughout floodplains after flood events. Floodplain ponds and wetlands are important habitat for many taxa. The range in duration of these habitats, from high-